witnext2/

abi.rs

use crate::{
    Function, Int, Interface, Record, RecordKind, ResourceId, Type, TypeDefKind, TypeId, Variant,
};
use std::mem;

/// A raw WebAssembly signature with params and results.
#[derive(Clone, Debug, Hash, Eq, PartialEq, PartialOrd, Ord)]
pub struct WasmSignature {
    /// The WebAssembly parameters of this function.
    pub params: Vec<WasmType>,
    /// The WebAssembly results of this function.
    pub results: Vec<WasmType>,
    /// The raw types, if needed, returned through return pointer located in
    /// `params`.
    pub retptr: Option<Vec<WasmType>>,
}

/// Enumerates wasm types used by interface types when lowering/lifting.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub enum WasmType {
    I32,
    I64,
    F32,
    F64,
    // NOTE: we don't lower interface types to any other Wasm type,
    // e.g. externref, so we don't need to define them here.
}

fn unify(a: WasmType, b: WasmType) -> WasmType {
    use WasmType::*;

    match (a, b) {
        (I64, _) | (_, I64) | (I32, F64) | (F64, I32) => I64,

        (I32, I32) | (I32, F32) | (F32, I32) => I32,

        (F32, F32) => F32,
        (F64, F64) | (F32, F64) | (F64, F32) => F64,
    }
}

impl From<Int> for WasmType {
    fn from(i: Int) -> WasmType {
        match i {
            Int::U8 | Int::U16 | Int::U32 => WasmType::I32,
            Int::U64 => WasmType::I64,
        }
    }
}

/// Possible ABIs for interface functions to have.
///
/// Note that this is a stopgap until we have more of interface types. Interface
/// types functions do not have ABIs, they have APIs. For the meantime, however,
/// we mandate ABIs to ensure we can all talk to each other.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum Abi {
    /// Only stable ABI currently, and is the historical WASI ABI since it was
    /// first created.
    ///
    /// Note that this ABI is limited notably in its return values where it can
    /// only return 0 results or one `Result<T, enum>` lookalike.
    Preview1,

    /// In-progress "canonical ABI" as proposed for interface types.
    Canonical,
}

// Helper macro for defining instructions without having to have tons of
// exhaustive `match` statements to update
macro_rules! def_instruction {
    (
        $( #[$enum_attr:meta] )*
        pub enum $name:ident<'a> {
            $(
                $( #[$attr:meta] )*
                $variant:ident $( {
                    $($field:ident : $field_ty:ty $(,)* )*
                } )?
                    :
                [$num_popped:expr] => [$num_pushed:expr],
            )*
        }
    ) => {
        $( #[$enum_attr] )*
        pub enum $name<'a> {
            $(
                $( #[$attr] )*
                $variant $( {
                    $(
                        $field : $field_ty,
                    )*
                } )? ,
            )*
        }

        impl $name<'_> {
            /// How many operands does this instruction pop from the stack?
            #[allow(unused_variables)]
            pub fn operands_len(&self) -> usize {
                match self {
                    $(
                        Self::$variant $( {
                            $(
                                $field,
                            )*
                        } )? => $num_popped,
                    )*
                }
            }

            /// How many results does this instruction push onto the stack?
            #[allow(unused_variables)]
            pub fn results_len(&self) -> usize {
                match self {
                    $(
                        Self::$variant $( {
                            $(
                                $field,
                            )*
                        } )? => $num_pushed,
                    )*
                }
            }
        }
    };
}

def_instruction! {
    #[derive(Debug)]
    pub enum Instruction<'a> {
        /// Acquires the specified parameter and places it on the stack.
        /// Depending on the context this may refer to wasm parameters or
        /// interface types parameters.
        GetArg { nth: usize } : [0] => [1],

        // Integer const/manipulation instructions

        /// Pushes the constant `val` onto the stack.
        I32Const { val: i32 } : [0] => [1],
        /// Casts the top N items on the stack using the `Bitcast` enum
        /// provided. Consumes the same number of operands that this produces.
        Bitcasts { casts: &'a [Bitcast] } : [casts.len()] => [casts.len()],
        /// Pushes a number of constant zeros for each wasm type on the stack.
        ConstZero { tys: &'a [WasmType] } : [0] => [tys.len()],

        // Memory load/store instructions

        /// Pops an `i32` from the stack and loads a little-endian `i32` from
        /// it, using the specified constant offset.
        I32Load { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `i8` from
        /// it, using the specified constant offset. The value loaded is the
        /// zero-extended to 32-bits
        I32Load8U { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `i8` from
        /// it, using the specified constant offset. The value loaded is the
        /// sign-extended to 32-bits
        I32Load8S { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `i16` from
        /// it, using the specified constant offset. The value loaded is the
        /// zero-extended to 32-bits
        I32Load16U { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `i16` from
        /// it, using the specified constant offset. The value loaded is the
        /// sign-extended to 32-bits
        I32Load16S { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `i64` from
        /// it, using the specified constant offset.
        I64Load { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `f32` from
        /// it, using the specified constant offset.
        F32Load { offset: i32 } : [1] => [1],
        /// Pops an `i32` from the stack and loads a little-endian `f64` from
        /// it, using the specified constant offset.
        F64Load { offset: i32 } : [1] => [1],

        /// Pops an `i32` address from the stack and then an `i32` value.
        /// Stores the value in little-endian at the pointer specified plus the
        /// constant `offset`.
        I32Store { offset: i32 } : [2] => [0],
        /// Pops an `i32` address from the stack and then an `i32` value.
        /// Stores the low 8 bits of the value in little-endian at the pointer
        /// specified plus the constant `offset`.
        I32Store8 { offset: i32 } : [2] => [0],
        /// Pops an `i32` address from the stack and then an `i32` value.
        /// Stores the low 16 bits of the value in little-endian at the pointer
        /// specified plus the constant `offset`.
        I32Store16 { offset: i32 } : [2] => [0],
        /// Pops an `i32` address from the stack and then an `i64` value.
        /// Stores the value in little-endian at the pointer specified plus the
        /// constant `offset`.
        I64Store { offset: i32 } : [2] => [0],
        /// Pops an `i32` address from the stack and then an `f32` value.
        /// Stores the value in little-endian at the pointer specified plus the
        /// constant `offset`.
        F32Store { offset: i32 } : [2] => [0],
        /// Pops an `i32` address from the stack and then an `f64` value.
        /// Stores the value in little-endian at the pointer specified plus the
        /// constant `offset`.
        F64Store { offset: i32 } : [2] => [0],

        // Scalar lifting/lowering

        /// Converts an interface type `char` value to a 32-bit integer
        /// representing the unicode scalar value.
        I32FromChar : [1] => [1],
        /// Converts an interface type `u64` value to a wasm `i64`.
        I64FromU64 : [1] => [1],
        /// Converts an interface type `s64` value to a wasm `i64`.
        I64FromS64 : [1] => [1],
        /// Converts an interface type `u32` value to a wasm `i32`.
        I32FromU32 : [1] => [1],
        /// Converts an interface type `s32` value to a wasm `i32`.
        I32FromS32 : [1] => [1],
        /// Converts an interface type `u16` value to a wasm `i32`.
        I32FromU16 : [1] => [1],
        /// Converts an interface type `s16` value to a wasm `i32`.
        I32FromS16 : [1] => [1],
        /// Converts an interface type `u8` value to a wasm `i32`.
        I32FromU8 : [1] => [1],
        /// Converts an interface type `s8` value to a wasm `i32`.
        I32FromS8 : [1] => [1],
        /// Converts a language-specific `usize` value to a wasm `i32`.
        I32FromUsize : [1] => [1],
        /// Converts a language-specific C `char` value to a wasm `i32`.
        I32FromChar8 : [1] => [1],
        /// Conversion an interface type `f32` value to a wasm `f32`.
        ///
        /// This may be a noop for some implementations, but it's here in case the
        /// native language representation of `f32` is different than the wasm
        /// representation of `f32`.
        F32FromIf32 : [1] => [1],
        /// Conversion an interface type `f64` value to a wasm `f64`.
        ///
        /// This may be a noop for some implementations, but it's here in case the
        /// native language representation of `f64` is different than the wasm
        /// representation of `f64`.
        F64FromIf64 : [1] => [1],

        /// Converts a native wasm `i32` to an interface type `s8`.
        ///
        /// This will truncate the upper bits of the `i32`.
        S8FromI32 : [1] => [1],
        /// Converts a native wasm `i32` to an interface type `u8`.
        ///
        /// This will truncate the upper bits of the `i32`.
        U8FromI32 : [1] => [1],
        /// Converts a native wasm `i32` to an interface type `s16`.
        ///
        /// This will truncate the upper bits of the `i32`.
        S16FromI32 : [1] => [1],
        /// Converts a native wasm `i32` to an interface type `u16`.
        ///
        /// This will truncate the upper bits of the `i32`.
        U16FromI32 : [1] => [1],
        /// Converts a native wasm `i32` to an interface type `s32`.
        S32FromI32 : [1] => [1],
        /// Converts a native wasm `i32` to an interface type `u32`.
        U32FromI32 : [1] => [1],
        /// Converts a native wasm `i64` to an interface type `s64`.
        S64FromI64 : [1] => [1],
        /// Converts a native wasm `i64` to an interface type `u64`.
        U64FromI64 : [1] => [1],
        /// Converts a native wasm `i32` to an interface type `char`.
        ///
        /// It's safe to assume that the `i32` is indeed a valid unicode code point.
        CharFromI32 : [1] => [1],
        /// Converts a native wasm `f32` to an interface type `f32`.
        If32FromF32 : [1] => [1],
        /// Converts a native wasm `f64` to an interface type `f64`.
        If64FromF64 : [1] => [1],
        /// Converts a native wasm `i32` to a language-specific C `char`.
        ///
        /// This will truncate the upper bits of the `i32`.
        Char8FromI32 : [1] => [1],
        /// Converts a native wasm `i32` to a language-specific `usize`.
        UsizeFromI32 : [1] => [1],

        // Handles

        /// Converts a "borrowed" handle into a wasm `i32` value.
        ///
        /// > **Note**: this documentation is outdated and does not reflect the
        /// > current implementation of the canonical ABI. This needs to be
        /// > updated.
        ///
        /// A "borrowed" handle in this case means one where ownership is not
        /// being relinquished. This is only used for lowering interface types
        /// parameters.
        ///
        /// Situations that this is used are:
        ///
        /// * A wasm exported function receives, as a parameter, handles defined
        ///   by the wasm module itself. This is effectively proof of ownership
        ///   by an external caller (be it host or wasm module) and the
        ///   ownership of the handle still lies with the caller. The wasm
        ///   module is only receiving a reference to the resource.
        ///
        /// * A wasm module is calling an import with a handle defined by the
        ///   import's module. Sort of the converse of the previous case this
        ///   means that the wasm module is handing out a reference to a
        ///   resource that it owns. The type in the wasm module, for example,
        ///   needs to reflect this.
        ///
        /// This instruction is not used for return values in either
        /// export/import positions.
        I32FromBorrowedHandle { ty: ResourceId } : [1] => [1],

        /// Converts an "owned" handle into a wasm `i32` value.
        ///
        /// > **Note**: this documentation is outdated and does not reflect the
        /// > current implementation of the canonical ABI. This needs to be
        /// > updated.
        ///
        /// This conversion is used for handle values which are crossing a
        /// module boundary for perhaps the first time. Some example cases of
        /// when this conversion is used are:
        ///
        /// * When a host defines a function to be imported, returned handles
        ///   use this instruction. Handles being returned to wasm a granting a
        ///   capability, which means that this new capability is typically
        ///   wrapped up in a new integer descriptor.
        ///
        /// * When a wasm module calls an imported function with a type defined
        ///   by itself, then it's granting a capability to the callee. This
        ///   means that the wasm module's type is being granted for the first
        ///   time, possibly, so it needs to be an owned value that's consumed.
        ///   Note that this doesn't actually happen with `*.witx` today due to
        ///   the lack of handle type imports.
        ///
        /// * When a wasm module export returns a handle defined within the
        ///   module, then it's similar to calling an imported function with
        ///   that handle. The capability is being granted to the caller of the
        ///   export, so the owned value is wrapped up in an `i32`.
        ///
        /// * When a host is calling a wasm module with a capability defined by
        ///   the host, its' similar to the host import returning a capability.
        ///   This would be granting the wasm module with the capability so an
        ///   owned version with a fresh handle is passed to the wasm module.
        ///   Note that this doesn't happen today with `*.witx` due to the lack
        ///   of handle type imports.
        ///
        /// Basically this instruction is used for handle->wasm conversions
        /// depending on the calling context and where the handle type in
        /// question was defined.
        I32FromOwnedHandle { ty: ResourceId } : [1] => [1],

        /// Converts a native wasm `i32` into an owned handle value.
        ///
        /// > **Note**: this documentation is outdated and does not reflect the
        /// > current implementation of the canonical ABI. This needs to be
        /// > updated.
        ///
        /// This is the converse of `I32FromOwnedHandle` and is used in similar
        /// situations:
        ///
        /// * A host definition of an import receives a handle defined in the
        ///   module itself.
        /// * A wasm module calling an import receives a handle defined by the
        ///   import.
        /// * A wasm module's export receives a handle defined by an external
        ///   module.
        /// * A host calling a wasm export receives a handle defined in the
        ///   module.
        ///
        /// Note that like `I32FromOwnedHandle` the first and third bullets
        /// above don't happen today because witx can't express type imports
        /// just yet.
        HandleOwnedFromI32 { ty: ResourceId } : [1] => [1],

        /// Converts a native wasm `i32` into a borrowedhandle value.
        ///
        /// > **Note**: this documentation is outdated and does not reflect the
        /// > current implementation of the canonical ABI. This needs to be
        /// > updated.
        ///
        /// This is the converse of `I32FromBorrowedHandle` and is used in similar
        /// situations:
        ///
        /// * An exported wasm function receives, as a parameter, a handle that
        ///   is defined by the wasm module.
        /// * An host-defined imported function is receiving a handle, as a
        ///   parameter, that is defined by the host itself.
        HandleBorrowedFromI32 { ty: ResourceId } : [1] => [1],

        // lists

        /// Lowers a list where the element's layout in the native language is
        /// expected to match the canonical ABI definition of interface types.
        ///
        /// Pops a list value from the stack and pushes the pointer/length onto
        /// the stack. If `realloc` is set to `Some` then this is expected to
        /// *consume* the list which means that the data needs to be copied. An
        /// allocation/copy is expected when:
        ///
        /// * A host is calling a wasm export with a list (it needs to copy the
        ///   list in to the callee's module, allocating space with `realloc`)
        /// * A wasm export is returning a list (it's expected to use `realloc`
        ///   to give ownership of the list to the caller.
        /// * A host is returning a list in a import definition, meaning that
        ///   space needs to be allocated in the caller with `realloc`).
        ///
        /// A copy does not happen (e.g. `realloc` is `None`) when:
        ///
        /// * A wasm module calls an import with the list. In this situation
        ///   it's expected the caller will know how to access this module's
        ///   memory (e.g. the host has raw access or wasm-to-wasm communication
        ///   would copy the list).
        ///
        /// If `realloc` is `Some` then the adapter is not responsible for
        /// cleaning up this list because the other end is receiving the
        /// allocation. If `realloc` is `None` then the adapter is responsible
        /// for cleaning up any temporary allocation it created, if any.
        ListCanonLower {
            element: &'a Type,
            realloc: Option<&'a str>,
        } : [1] => [2],

        /// Lowers a list where the element's layout in the native language is
        /// not expected to match the canonical ABI definition of interface
        /// types.
        ///
        /// Pops a list value from the stack and pushes the pointer/length onto
        /// the stack. This operation also pops a block from the block stack
        /// which is used as the iteration body of writing each element of the
        /// list consumed.
        ///
        /// The `realloc` field here behaves the same way as `ListCanonLower`.
        /// It's only set to `None` when a wasm module calls a declared import.
        /// Otherwise lowering in other contexts requires allocating memory for
        /// the receiver to own.
        ListLower {
            element: &'a Type,
            realloc: Option<&'a str>,
        } : [1] => [2],

        /// Lifts a list which has a canonical representation into an interface
        /// types value.
        ///
        /// The term "canonical" representation here means that the
        /// representation of the interface types value in the native language
        /// exactly matches the canonical ABI definition of the type.
        ///
        /// This will consume two `i32` values from the stack, a pointer and a
        /// length, and then produces an interface value list. If the `free`
        /// field is set to `Some` then the pointer/length should be considered
        /// an owned allocation and need to be deallocated by the receiver. If
        /// it is set to `None` then a view is provided but it does not need to
        /// be deallocated.
        ///
        /// The `free` field is set to `Some` in similar situations as described
        /// by `ListCanonLower`. If `free` is `Some` then the memory must be
        /// deallocated after the lifted list is done being consumed. If it is
        /// `None` then the receiver of the lifted list does not own the memory
        /// and must leave the memory as-is.
        ListCanonLift {
            element: &'a Type,
            free: Option<&'a str>,
            ty: TypeId,
        } : [2] => [1],

        /// Lifts a list which into an interface types value.
        ///
        /// This will consume two `i32` values from the stack, a pointer and a
        /// length, and then produces an interface value list. Note that the
        /// pointer/length popped are **owned** and need to be deallocated with
        /// the wasm `free` function when the list is no longer needed.
        ///
        /// This will also pop a block from the block stack which is how to
        /// read each individual element from the list.
        ListLift {
            element: &'a Type,
            free: Option<&'a str>,
            ty: TypeId,
        } : [2] => [1],

        /// Pushes an operand onto the stack representing the list item from
        /// each iteration of the list.
        ///
        /// This is only used inside of blocks related to lowering lists.
        IterElem { element: &'a Type } : [0] => [1],

        /// Pushes an operand onto the stack representing the base pointer of
        /// the next element in a list.
        ///
        /// This is used for both lifting and lowering lists.
        IterBasePointer : [0] => [1],

        // buffers

        /// Pops a buffer value, pushes the pointer/length of where it points
        /// to in memory.
        BufferLowerPtrLen { push: bool, ty: &'a Type } : [1] => [3],
        /// Pops a buffer value, pushes an integer handle for the buffer.
        BufferLowerHandle { push: bool, ty: &'a Type } : [1] => [1],
        /// Pops a ptr/len, pushes a buffer wrapping that ptr/len of the memory
        /// from the origin module.
        BufferLiftPtrLen { push: bool, ty: &'a Type } : [3] => [1],
        /// Pops an i32, pushes a buffer wrapping that i32 handle.
        BufferLiftHandle { push: bool, ty: &'a Type } : [1] => [1],

        // records

        /// Pops a record value off the stack, decomposes the record to all of
        /// its fields, and then pushes the fields onto the stack.
        RecordLower {
            record: &'a Record,
            name: Option<&'a str>,
            ty: TypeId,
        } : [1] => [record.fields.len()],

        /// Pops all fields for a record off the stack and then composes them
        /// into a record.
        RecordLift {
            record: &'a Record,
            name: Option<&'a str>,
            ty: TypeId,
        } : [record.fields.len()] => [1],

        /// Converts a language-specific record-of-bools to a list of `i32`.
        FlagsLower {
            record: &'a Record,
            name: &'a str,
            ty: TypeId,
        } : [1] => [record.num_i32s()],
        FlagsLower64 {
            record: &'a Record,
            name: &'a str,
            ty: TypeId,
        } : [1] => [1],
        /// Converts a list of native wasm `i32` to a language-specific
        /// record-of-bools.
        FlagsLift {
            record: &'a Record,
            name: &'a str,
            ty: TypeId,
        } : [record.num_i32s()] => [1],
        FlagsLift64 {
            record: &'a Record,
            name: &'a str,
            ty: TypeId,
        } : [1] => [1],

        // variants

        /// This is a special instruction used for `VariantLower`
        /// instruction to determine the name of the payload, if present, to use
        /// within each block.
        ///
        /// Each sub-block will have this be the first instruction, and if it
        /// lowers a payload it will expect something bound to this name.
        VariantPayloadName : [0] => [1],

        /// TODO
        BufferPayloadName : [0] => [1],

        /// Pops a variant off the stack as well as `ty.cases.len()` blocks
        /// from the code generator. Uses each of those blocks and the value
        /// from the stack to produce `nresults` of items.
        VariantLower {
            variant: &'a Variant,
            name: Option<&'a str>,
            ty: TypeId,
            results: &'a [WasmType],
        } : [1] => [results.len()],

        /// Pops an `i32` off the stack as well as `ty.cases.len()` blocks
        /// from the code generator. Uses each of those blocks and the value
        /// from the stack to produce a final variant.
        VariantLift {
            variant: &'a Variant,
            name: Option<&'a str>,
            ty: TypeId,
        } : [1] => [1],

        // calling/control flow

        /// Represents a call to a raw WebAssembly API. The module/name are
        /// provided inline as well as the types if necessary.
        CallWasm {
            module: &'a str,
            name: &'a str,
            sig: &'a WasmSignature,
        } : [sig.params.len()] => [sig.results.len()],

        /// Same as `CallWasm`, except the dual where an interface is being
        /// called rather than a raw wasm function.
        CallInterface {
            module: &'a str,
            func: &'a Function,
        } : [func.params.len()] => [func.results.len()],

        /// Returns `amt` values on the stack. This is always the last
        /// instruction.
        Return { amt: usize, func: &'a Function } : [*amt] => [0],

        // ...

        /// An instruction from an extended instruction set that's specific to
        /// `*.witx` and the "Preview1" ABI.
        Witx {
            instr: &'a WitxInstruction<'a>,
        } : [instr.operands_len()] => [instr.results_len()],
    }
}

#[derive(Debug, PartialEq)]
pub enum Bitcast {
    // Upcasts
    F32ToF64,
    F32ToI32,
    F64ToI64,
    I32ToI64,
    F32ToI64,

    // Downcasts
    F64ToF32,
    I32ToF32,
    I64ToF64,
    I64ToI32,
    I64ToF32,

    None,
}

def_instruction! {
    #[derive(Debug)]
    pub enum WitxInstruction<'a> {
        /// Takes the value off the top of the stack and writes it into linear
        /// memory. Pushes the address in linear memory as an `i32`.
        AddrOf : [1] => [1],

        /// Converts a language-specific pointer value to a wasm `i32`.
        I32FromPointer : [1] => [1],
        /// Converts a language-specific pointer value to a wasm `i32`.
        I32FromConstPointer : [1] => [1],
        /// Converts a native wasm `i32` to a language-specific pointer.
        PointerFromI32 { ty: &'a Type }: [1] => [1],
        /// Converts a native wasm `i32` to a language-specific pointer.
        ConstPointerFromI32 { ty: &'a Type } : [1] => [1],

        /// This is a special instruction specifically for the original ABI of
        /// WASI.  The raw return `i32` of a function is re-pushed onto the
        /// stack for reuse.
        ReuseReturn : [0] => [1],
    }
}

/// Whether the glue code surrounding a call is lifting arguments and lowering
/// results or vice versa.
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum LiftLower {
    /// When the glue code lifts arguments and lowers results.
    ///
    /// ```text
    /// Wasm --lift-args--> SourceLanguage; call; SourceLanguage --lower-results--> Wasm
    /// ```
    LiftArgsLowerResults,
    /// When the glue code lowers arguments and lifts results.
    ///
    /// ```text
    /// SourceLanguage --lower-args--> Wasm; call; Wasm --lift-results--> SourceLanguage
    /// ```
    LowerArgsLiftResults,
}

/// Whether we are generating glue code to call an import or an export.
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum Direction {
    /// We are generating glue code to call an import.
    Import,
    /// We are generating glue code to call an export.
    Export,
}

/// Trait for language implementors to use to generate glue code between native
/// WebAssembly signatures and interface types signatures.
///
/// This is used as an implementation detail in interpreting the ABI between
/// interface types and wasm types. Eventually this will be driven by interface
/// types adapters themselves, but for now the ABI of a function dictates what
/// instructions are fed in.
///
/// Types implementing `Bindgen` are incrementally fed `Instruction` values to
/// generate code for. Instructions operate like a stack machine where each
/// instruction has a list of inputs and a list of outputs (provided by the
/// `emit` function).
pub trait Bindgen {
    /// The intermediate type for fragments of code for this type.
    ///
    /// For most languages `String` is a suitable intermediate type.
    type Operand: Clone;

    /// Emit code to implement the given instruction.
    ///
    /// Each operand is given in `operands` and can be popped off if ownership
    /// is required. It's guaranteed that `operands` has the appropriate length
    /// for the `inst` given, as specified with [`Instruction`].
    ///
    /// Each result variable should be pushed onto `results`. This function must
    /// push the appropriate number of results or binding generation will panic.
    fn emit(
        &mut self,
        iface: &Interface,
        inst: &Instruction<'_>,
        operands: &mut Vec<Self::Operand>,
        results: &mut Vec<Self::Operand>,
    );

    /// Allocates temporary space in linear memory for the type `ty`.
    ///
    /// This is called when calling some wasm functions where a return pointer
    /// is needed. Only used for the `Abi::Preview1` ABI.
    ///
    /// Returns an `Operand` which has type `i32` and is the base of the typed
    /// allocation in memory.
    fn allocate_typed_space(&mut self, iface: &Interface, ty: TypeId) -> Self::Operand;

    /// Allocates temporary space in linear memory for a fixed number of `i64`
    /// values.
    ///
    /// This is only called in the `Abi::Canonical` ABI for when a function
    /// would otherwise have multiple results.
    ///
    /// Returns an `Operand` which has type `i32` and points to the base of the
    /// fixed-size-array allocation.
    fn i64_return_pointer_area(&mut self, amt: usize) -> Self::Operand;

    /// Enters a new block of code to generate code for.
    ///
    /// This is currently exclusively used for constructing variants. When a
    /// variant is constructed a block here will be pushed for each case of a
    /// variant, generating the code necessary to translate a variant case.
    ///
    /// Blocks are completed with `finish_block` below. It's expected that `emit`
    /// will always push code (if necessary) into the "current block", which is
    /// updated by calling this method and `finish_block` below.
    fn push_block(&mut self);

    /// Indicates to the code generator that a block is completed, and the
    /// `operand` specified was the resulting value of the block.
    ///
    /// This method will be used to compute the value of each arm of lifting a
    /// variant. The `operand` will be `None` if the variant case didn't
    /// actually have any type associated with it. Otherwise it will be `Some`
    /// as the last value remaining on the stack representing the value
    /// associated with a variant's `case`.
    ///
    /// It's expected that this will resume code generation in the previous
    /// block before `push_block` was called. This must also save the results
    /// of the current block internally for instructions like `ResultLift` to
    /// use later.
    fn finish_block(&mut self, operand: &mut Vec<Self::Operand>);

    /// Returns size information that was previously calculated for all types.
    fn sizes(&self) -> &crate::sizealign::SizeAlign;

    /// Returns whether or not the specified element type is represented in a
    /// "canonical" form for lists. This dictates whether the `ListCanonLower`
    /// and `ListCanonLift` instructions are used or not.
    fn is_list_canonical(&self, iface: &Interface, element: &Type) -> bool;
}

impl Interface {
    /// Validates the parameters/results of a function are representable in its
    /// ABI.
    ///
    /// Returns an error string if they're not representable or returns `Ok` if
    /// they're indeed representable.
    pub fn validate_abi(&self, func: &Function) -> Result<(), String> {
        for (_, ty) in func.params.iter() {
            self.validate_abi_ty(func.abi, ty, true)?;
        }
        for (_, ty) in func.results.iter() {
            self.validate_abi_ty(func.abi, ty, false)?;
        }
        match func.abi {
            Abi::Preview1 => {
                // validated below...
            }
            Abi::Canonical => return Ok(()),
        }
        match func.results.len() {
            0 => Ok(()),
            1 => self.validate_preview1_return(&func.results[0].1),
            _ => Err("more than one result".to_string()),
        }
    }

    fn validate_preview1_return(&self, ty: &Type) -> Result<(), String> {
        let id = match ty {
            Type::Id(id) => *id,
            _ => return Ok(()),
        };
        match &self.types[id].kind {
            TypeDefKind::Type(t) => self.validate_preview1_return(t),
            TypeDefKind::Variant(v) => {
                let (ok, err) = match v.as_expected() {
                    Some(pair) => pair,
                    None => return Err("invalid return type".to_string()),
                };
                if let Some(ty) = ok {
                    let id = match ty {
                        Type::Id(id) => *id,
                        _ => return Err("only named types are allowed in results".to_string()),
                    };
                    match &self.types[id].kind {
                        TypeDefKind::Record(r) if r.is_tuple() => {
                            for field in r.fields.iter() {
                                self.validate_ty_named(&field.ty)?;
                            }
                        }
                        _ => {
                            self.validate_ty_named(ty)?;
                        }
                    }
                }

                if let Some(ty) = err {
                    let kind = self.validate_ty_named(ty)?;
                    if let TypeDefKind::Variant(v) = kind {
                        if v.is_enum() {
                            return Ok(());
                        }
                    }
                    return Err("invalid type in error payload of result".to_string());
                }
                Ok(())
            }
            TypeDefKind::Record(r) if r.is_flags() => Ok(()),
            TypeDefKind::Record(_)
            | TypeDefKind::List(_)
            | TypeDefKind::PushBuffer(_)
            | TypeDefKind::PullBuffer(_) => Err("invalid return type".to_string()),
            TypeDefKind::Pointer(_) | TypeDefKind::ConstPointer(_) => Ok(()),
        }
    }

    fn validate_ty_named(&self, ty: &Type) -> Result<&TypeDefKind, String> {
        let id = match ty {
            Type::Id(id) => *id,
            _ => return Err("only named types are allowed in results".to_string()),
        };
        let ty = &self.types[id];
        if ty.name.is_none() {
            return Err("only named types are allowed in results".to_string());
        }
        Ok(&ty.kind)
    }

    fn validate_abi_ty(&self, abi: Abi, ty: &Type, param: bool) -> Result<(), String> {
        let id = match ty {
            Type::Id(id) => *id,
            // Type::U8 { lang_c_char: true } => {
            //     if let Abi::Next = self {
            //         return Err("cannot use `(@witx char8)` in this ABI".to_string());
            //     }
            //     Ok(())
            // }
            // Type::U32 { lang_ptr_size: true } => {
            //     if let Abi::Next = self {
            //         return Err("cannot use `(@witx usize)` in this ABI".to_string());
            //     }
            //     Ok(())
            // }
            _ => return Ok(()),
        };
        match &self.types[id].kind {
            TypeDefKind::Type(t) => self.validate_abi_ty(abi, t, param),
            TypeDefKind::Record(r) => {
                for r in r.fields.iter() {
                    self.validate_abi_ty(abi, &r.ty, param)?;
                }
                Ok(())
            }
            TypeDefKind::Variant(v) => {
                for case in v.cases.iter() {
                    if let Some(ty) = &case.ty {
                        self.validate_abi_ty(abi, ty, param)?;
                    }
                }
                Ok(())
            }
            TypeDefKind::List(t) => self.validate_abi_ty(abi, t, param),
            TypeDefKind::Pointer(t) => {
                if let Abi::Canonical = abi {
                    return Err("cannot use `(@witx pointer)` in this ABI".to_string());
                }
                self.validate_abi_ty(abi, t, param)
            }
            TypeDefKind::ConstPointer(t) => {
                if let Abi::Canonical = abi {
                    return Err("cannot use `(@witx const_pointer)` in this ABI".to_string());
                }
                self.validate_abi_ty(abi, t, param)
            }
            TypeDefKind::PushBuffer(t) | TypeDefKind::PullBuffer(t) => {
                if !param {
                    return Err("cannot use buffers in the result position".to_string());
                }
                let param = match &self.types[id].kind {
                    TypeDefKind::PushBuffer(_) => false,
                    TypeDefKind::PullBuffer(_) => param,
                    _ => unreachable!(),
                };
                // If this is an output buffer then validate `t` as if it were a
                // result because the callee can't give us buffers back.
                self.validate_abi_ty(abi, t, param)
            }
        }
    }

    /// Get the WebAssembly type signature for this interface function
    ///
    /// The first entry returned is the list of parameters and the second entry
    /// is the list of results for the wasm function signature.
    pub fn wasm_signature(&self, dir: Direction, func: &Function) -> WasmSignature {
        let mut params = Vec::new();
        let mut results = Vec::new();
        for (_, param) in func.params.iter() {
            if let (Abi::Preview1, Type::Id(id)) = (func.abi, param) {
                match &self.types[*id].kind {
                    TypeDefKind::Variant(_) => {
                        params.push(WasmType::I32);
                        continue;
                    }
                    TypeDefKind::Record(r) if !r.is_flags() => {
                        params.push(WasmType::I32);
                        continue;
                    }
                    _ => {}
                }
            }
            self.push_wasm(func.abi, dir, param, &mut params);
        }

        for (_, result) in func.results.iter() {
            if let (Abi::Preview1, Type::Id(id)) = (func.abi, result) {
                match &self.types[*id].kind {
                    TypeDefKind::Variant(v) => {
                        results.push(v.tag.into());
                        if v.is_enum() {
                            continue;
                        }
                        // return pointer for payload, if any
                        if let Some(ty) = &v.cases[0].ty {
                            for _ in 0..self.preview1_num_types(ty) {
                                params.push(WasmType::I32);
                            }
                        }
                        continue;
                    }
                    _ => {}
                }
            }
            self.push_wasm(func.abi, dir, result, &mut results);
        }

        // Rust/C don't support multi-value well right now, so if a function
        // would have multiple results then instead truncate it. Imports take a
        // return pointer to write into and exports return a pointer they wrote
        // into.
        let mut retptr = None;
        if results.len() > 1 {
            retptr = Some(mem::take(&mut results));
            match dir {
                Direction::Import => {
                    params.push(WasmType::I32);
                }
                Direction::Export => {
                    results.push(WasmType::I32);
                }
            }
        }

        WasmSignature {
            params,
            results,
            retptr,
        }
    }

    fn preview1_num_types(&self, ty: &Type) -> usize {
        match ty {
            Type::Id(id) => match &self.types[*id].kind {
                TypeDefKind::Record(r) if r.is_tuple() => r.fields.len(),
                _ => 1,
            },
            _ => 1,
        }
    }

    fn push_wasm(&self, abi: Abi, dir: Direction, ty: &Type, result: &mut Vec<WasmType>) {
        match ty {
            Type::S8
            | Type::U8
            | Type::S16
            | Type::U16
            | Type::S32
            | Type::U32
            | Type::Char
            | Type::Handle(_)
            | Type::CChar
            | Type::Usize => result.push(WasmType::I32),

            Type::U64 | Type::S64 => result.push(WasmType::I64),
            Type::F32 => result.push(WasmType::F32),
            Type::F64 => result.push(WasmType::F64),

            Type::Id(id) => match &self.types[*id].kind {
                TypeDefKind::Type(t) => self.push_wasm(abi, dir, t, result),

                TypeDefKind::Record(r) if r.is_flags() => match self.flags_repr(r) {
                    Some(int) => result.push(int.into()),
                    None => {
                        for _ in 0..r.num_i32s() {
                            result.push(WasmType::I32);
                        }
                    }
                },

                TypeDefKind::Record(r) => {
                    for field in r.fields.iter() {
                        self.push_wasm(abi, dir, &field.ty, result);
                    }
                }

                TypeDefKind::List(_) => {
                    result.push(WasmType::I32);
                    result.push(WasmType::I32);
                }

                TypeDefKind::Pointer(_) | TypeDefKind::ConstPointer(_) => {
                    result.push(WasmType::I32);
                }

                TypeDefKind::PushBuffer(_) | TypeDefKind::PullBuffer(_) => {
                    result.push(WasmType::I32);
                    if dir == Direction::Import {
                        result.push(WasmType::I32);
                        result.push(WasmType::I32);
                    }
                }

                TypeDefKind::Variant(v) => {
                    result.push(v.tag.into());
                    let start = result.len();
                    let mut temp = Vec::new();

                    // Push each case's type onto a temporary vector, and then
                    // merge that vector into our final list starting at
                    // `start`. Note that this requires some degree of
                    // "unification" so we can handle things like `Result<i32,
                    // f32>` where that turns into `[i32 i32]` where the second
                    // `i32` might be the `f32` bitcasted.
                    for case in v.cases.iter() {
                        let ty = match &case.ty {
                            Some(ty) => ty,
                            None => continue,
                        };
                        self.push_wasm(abi, dir, ty, &mut temp);

                        for (i, ty) in temp.drain(..).enumerate() {
                            match result.get_mut(start + i) {
                                Some(prev) => *prev = unify(*prev, ty),
                                None => result.push(ty),
                            }
                        }
                    }
                }
            },
        }
    }

    pub fn flags_repr(&self, record: &Record) -> Option<Int> {
        match record.kind {
            RecordKind::Flags(Some(hint)) => Some(hint),
            RecordKind::Flags(None) if record.fields.len() <= 8 => Some(Int::U8),
            RecordKind::Flags(None) if record.fields.len() <= 16 => Some(Int::U16),
            RecordKind::Flags(None) if record.fields.len() <= 32 => Some(Int::U32),
            RecordKind::Flags(None) if record.fields.len() <= 64 => Some(Int::U64),
            RecordKind::Flags(None) => None,
            _ => panic!("not a flags record"),
        }
    }

    /// Generates an abstract sequence of instructions which represents this
    /// function being adapted as an imported function.
    ///
    /// The instructions here, when executed, will emulate a language with
    /// interface types calling the concrete wasm implementation. The parameters
    /// for the returned instruction sequence are the language's own
    /// interface-types parameters. One instruction in the instruction stream
    /// will be a `Call` which represents calling the actual raw wasm function
    /// signature.
    ///
    /// This function is useful, for example, if you're building a language
    /// generator for WASI bindings. This will document how to translate
    /// language-specific values into the wasm types to call a WASI function,
    /// and it will also automatically convert the results of the WASI function
    /// back to a language-specific value.
    pub fn call(
        &self,
        dir: Direction,
        lift_lower: LiftLower,
        func: &Function,
        bindgen: &mut impl Bindgen,
    ) {
        if Abi::Preview1 == func.abi {
            // The Preview1 ABI only works with WASI which is only intended
            // for use with these modes.
            if dir == Direction::Export {
                panic!("the preview1 ABI only supports import modes");
            }
        }
        Generator::new(self, func.abi, dir, lift_lower, bindgen).call(func);
    }
}

struct Generator<'a, B: Bindgen> {
    abi: Abi,
    dir: Direction,
    lift_lower: LiftLower,
    bindgen: &'a mut B,
    iface: &'a Interface,
    operands: Vec<B::Operand>,
    results: Vec<B::Operand>,
    stack: Vec<B::Operand>,
    return_pointers: Vec<B::Operand>,
}

impl<'a, B: Bindgen> Generator<'a, B> {
    fn new(
        iface: &'a Interface,
        abi: Abi,
        dir: Direction,
        lift_lower: LiftLower,
        bindgen: &'a mut B,
    ) -> Generator<'a, B> {
        Generator {
            iface,
            abi,
            dir,
            lift_lower,
            bindgen,
            operands: Vec::new(),
            results: Vec::new(),
            stack: Vec::new(),
            return_pointers: Vec::new(),
        }
    }

    fn call(&mut self, func: &Function) {
        let sig = self.iface.wasm_signature(self.dir, func);

        match self.lift_lower {
            LiftLower::LowerArgsLiftResults => {
                // Push all parameters for this function onto the stack, and
                // then batch-lower everything all at once.
                for nth in 0..func.params.len() {
                    self.emit(&Instruction::GetArg { nth });
                }
                self.lower_all(&func.params, None);

                // If necessary we may need to prepare a return pointer for this
                // ABI. The `Preview1` ABI has most return values returned
                // through pointers, and the `Canonical` ABI returns more-than-one
                // values through a return pointer.
                if self.dir == Direction::Import {
                    self.prep_return_pointer(&sig, &func.results);
                }

                // Now that all the wasm args are prepared we can call the
                // actual wasm function.
                assert_eq!(self.stack.len(), sig.params.len());
                self.emit(&Instruction::CallWasm {
                    module: &self.iface.name,
                    name: &func.name,
                    sig: &sig,
                });

                // In the `Canonical` ABI we model multiple return values by going
                // through memory. Remove that indirection here by loading
                // everything to simulate the function having many return values
                // in our stack discipline.
                if let Some(actual) = &sig.retptr {
                    if self.dir == Direction::Import {
                        assert_eq!(self.return_pointers.len(), 1);
                        self.stack.push(self.return_pointers.pop().unwrap());
                    }
                    self.load_retptr(actual);
                }

                // Batch-lift all result values now that all the function's return
                // values are on the stack.
                self.lift_all(&func.results);

                self.emit(&Instruction::Return {
                    func,
                    amt: func.results.len(),
                });
            }
            LiftLower::LiftArgsLowerResults => {
                // Use `GetArg` to push all relevant arguments onto the stack.
                // Note that we can't use the signature of this function
                // directly due to various conversions and return pointers, so
                // we need to somewhat manually calculate all the arguments
                // which are converted as interface types arguments below.
                let nargs = match self.abi {
                    Abi::Preview1 => {
                        func.params.len()
                            + func
                                .params
                                .iter()
                                .filter(|(_, t)| match t {
                                    Type::Id(id) => match &self.iface.types[*id].kind {
                                        TypeDefKind::List(_) => true,
                                        _ => false,
                                    },
                                    _ => false,
                                })
                                .count()
                    }
                    Abi::Canonical => {
                        sig.params.len()
                            - (self.dir == Direction::Import && sig.retptr.is_some()) as usize
                    }
                };
                for nth in 0..nargs {
                    self.emit(&Instruction::GetArg { nth });
                }

                // Once everything is on the stack we can lift all arguments
                // one-by-one into their interface-types equivalent.
                self.lift_all(&func.params);

                // ... and that allows us to call the interface types function
                self.emit(&Instruction::CallInterface {
                    module: &self.iface.name,
                    func,
                });

                // ... and at the end we lower everything back into return
                // values.
                self.lower_all(&func.results, Some(nargs));

                // Our ABI dictates that a list of returned types are returned
                // through memories, so after we've got all the values on the
                // stack perform all of the stores here.
                if let Some(tys) = &sig.retptr {
                    match self.dir {
                        Direction::Import => {
                            self.emit(&Instruction::GetArg {
                                nth: sig.params.len() - 1,
                            });
                        }
                        Direction::Export => {
                            let op = self.bindgen.i64_return_pointer_area(tys.len());
                            self.stack.push(op);
                        }
                    }
                    let retptr = self.store_retptr(tys);
                    if self.dir == Direction::Export {
                        self.stack.push(retptr);
                    }
                }

                self.emit(&Instruction::Return {
                    func,
                    amt: sig.results.len(),
                });
            }
        }

        assert!(
            self.stack.is_empty(),
            "stack has {} items remaining",
            self.stack.len()
        );
    }

    fn load_retptr(&mut self, types: &[WasmType]) {
        let rp = self.stack.pop().unwrap();
        for (i, ty) in types.iter().enumerate() {
            self.stack.push(rp.clone());
            let offset = (i * 8) as i32;
            match ty {
                WasmType::I32 => self.emit(&Instruction::I32Load { offset }),
                WasmType::I64 => self.emit(&Instruction::I64Load { offset }),
                WasmType::F32 => self.emit(&Instruction::F32Load { offset }),
                WasmType::F64 => self.emit(&Instruction::F64Load { offset }),
            }
        }
    }

    /// Assumes that the wasm values to create `tys` are all located on the
    /// stack.
    ///
    /// Inserts instructions necesesary to lift those types into their
    /// interface types equivalent.
    fn lift_all(&mut self, tys: &[(String, Type)]) {
        let mut temp = Vec::new();
        let operands = tys
            .iter()
            .rev()
            .map(|(_, ty)| {
                let ntys = match self.abi {
                    Abi::Preview1 => match ty {
                        Type::Id(id) => match &self.iface.types[*id].kind {
                            TypeDefKind::List(_) => 2,
                            _ => 1,
                        },
                        _ => 1,
                    },
                    Abi::Canonical => {
                        temp.truncate(0);
                        self.iface.push_wasm(self.abi, self.dir, ty, &mut temp);
                        temp.len()
                    }
                };
                self.stack
                    .drain(self.stack.len() - ntys..)
                    .collect::<Vec<_>>()
            })
            .collect::<Vec<_>>();
        for (operands, (_, ty)) in operands.into_iter().rev().zip(tys) {
            self.stack.extend(operands);
            self.lift(ty);
        }
    }

    /// Assumes that the value for `tys` is already on the stack, and then
    /// converts all of those values into their wasm types by lowering each
    /// argument in-order.
    fn lower_all<'b>(&mut self, tys: &[(String, Type)], mut nargs: Option<usize>) {
        let operands = self
            .stack
            .drain(self.stack.len() - tys.len()..)
            .collect::<Vec<_>>();
        for (operand, (_, ty)) in operands.into_iter().zip(tys) {
            self.stack.push(operand);
            self.lower(ty, nargs.as_mut());
        }
    }

    /// Assumes `types.len()` values are on the stack and stores them all into
    /// the return pointer of this function, specified in the last argument.
    ///
    /// This is only used with `Abi::Next`.
    fn store_retptr(&mut self, types: &[WasmType]) -> B::Operand {
        let retptr = self.stack.pop().unwrap();
        for (i, ty) in types.iter().enumerate().rev() {
            self.stack.push(retptr.clone());
            let offset = (i * 8) as i32;
            match ty {
                WasmType::I32 => self.emit(&Instruction::I32Store { offset }),
                WasmType::I64 => self.emit(&Instruction::I64Store { offset }),
                WasmType::F32 => self.emit(&Instruction::F32Store { offset }),
                WasmType::F64 => self.emit(&Instruction::F64Store { offset }),
            }
        }
        return retptr;
    }

    fn witx(&mut self, instr: &WitxInstruction<'_>) {
        self.emit(&Instruction::Witx { instr });
    }

    fn emit(&mut self, inst: &Instruction<'_>) {
        self.operands.clear();
        self.results.clear();

        let operands_len = inst.operands_len();
        assert!(
            self.stack.len() >= operands_len,
            "not enough operands on stack for {:?}",
            inst
        );
        self.operands
            .extend(self.stack.drain((self.stack.len() - operands_len)..));
        self.results.reserve(inst.results_len());

        self.bindgen
            .emit(self.iface, inst, &mut self.operands, &mut self.results);

        assert_eq!(
            self.results.len(),
            inst.results_len(),
            "{:?} expected {} results, got {}",
            inst,
            inst.results_len(),
            self.results.len()
        );
        self.stack.extend(self.results.drain(..));
    }

    fn push_block(&mut self) {
        self.bindgen.push_block();
    }

    fn finish_block(&mut self, size: usize) {
        self.operands.clear();
        assert!(
            size <= self.stack.len(),
            "not enough operands on stack for finishing block",
        );
        self.operands
            .extend(self.stack.drain((self.stack.len() - size)..));
        self.bindgen.finish_block(&mut self.operands);
    }

    fn lower(&mut self, ty: &Type, retptr: Option<&mut usize>) {
        use Instruction::*;
        use WitxInstruction::*;

        match *ty {
            Type::S8 => self.emit(&I32FromS8),
            Type::U8 => self.emit(&I32FromU8),
            Type::CChar => self.emit(&I32FromChar8),
            Type::S16 => self.emit(&I32FromS16),
            Type::U16 => self.emit(&I32FromU16),
            Type::S32 => self.emit(&I32FromS32),
            Type::U32 => self.emit(&I32FromU32),
            Type::Usize => self.emit(&I32FromUsize),
            Type::S64 => self.emit(&I64FromS64),
            Type::U64 => self.emit(&I64FromU64),
            Type::Char => self.emit(&I32FromChar),
            Type::F32 => self.emit(&F32FromIf32),
            Type::F64 => self.emit(&F64FromIf64),
            Type::Handle(ty) => {
                let borrowed = match self.lift_lower {
                    // This means that a return value is being lowered, which is
                    // never borrowed.
                    LiftLower::LiftArgsLowerResults => false,
                    // There's one of three possible situations we're in:
                    //
                    // * The handle is defined by the wasm module itself. This
                    //   is the only actual possible scenario today due to how
                    //   witx is defined. In this situation the handle is owned
                    //   by the host and "proof of ownership" is being offered
                    //   and there's no need to relinquish ownership.
                    //
                    // * The handle is defined by the host, and it's passing it
                    //   to a wasm module. This should use an owned conversion.
                    //   This isn't expressible in today's `*.witx` format.
                    //
                    // * The handle is defined by neither the host or the wasm
                    //   mdoule. This means that the host is passing a
                    //   capability from another wasm module into this one,
                    //   meaning it's doing so by reference since the host is
                    //   retaining access to its own
                    //
                    // Note, again, only the first bullet here is possible
                    // today, hence the hardcoded `true` value. We'll need to
                    // refactor `witx` to expose the other possibilities.
                    LiftLower::LowerArgsLiftResults => true,
                };
                if borrowed {
                    self.emit(&I32FromBorrowedHandle { ty });
                } else {
                    self.emit(&I32FromOwnedHandle { ty });
                }
            }
            Type::Id(id) => match &self.iface.types[id].kind {
                TypeDefKind::Type(t) => self.lower(t, retptr),
                TypeDefKind::Pointer(_) => self.witx(&I32FromPointer),
                TypeDefKind::ConstPointer(_) => self.witx(&I32FromConstPointer),
                TypeDefKind::List(element) => match self.abi {
                    Abi::Preview1 => self.emit(&ListCanonLower {
                        element,
                        realloc: None,
                    }),
                    Abi::Canonical => {
                        // Lowering parameters calling a wasm import means
                        // we don't need to pass ownership, but we pass
                        // ownership in all other cases.
                        let realloc = match (self.dir, self.lift_lower) {
                            (Direction::Import, LiftLower::LowerArgsLiftResults) => None,
                            _ => Some("canonical_abi_realloc"),
                        };
                        if self.is_char(element)
                            || self.bindgen.is_list_canonical(self.iface, element)
                        {
                            self.emit(&ListCanonLower { element, realloc });
                        } else {
                            self.push_block();
                            self.emit(&IterElem { element });
                            self.emit(&IterBasePointer);
                            let addr = self.stack.pop().unwrap();
                            self.write_to_memory(element, addr, 0);
                            self.finish_block(0);
                            self.emit(&ListLower { element, realloc });
                        }
                    }
                },
                TypeDefKind::PushBuffer(ty) | TypeDefKind::PullBuffer(ty) => {
                    let push = match &self.iface.types[id].kind {
                        TypeDefKind::PushBuffer(_) => true,
                        _ => false,
                    };
                    self.translate_buffer(push, ty);

                    // Buffers are only used in the parameter position, so if we
                    // are lowering them, then we had better be lowering args
                    // and lifting results.
                    assert!(self.lift_lower == LiftLower::LowerArgsLiftResults);

                    match self.dir {
                        Direction::Import => {
                            // When calling an imported function we're passing a raw view
                            // into memory, and the adapter will convert it into something
                            // else if necessary.
                            self.emit(&BufferLowerPtrLen { push, ty });
                        }
                        Direction::Export => {
                            // When calling an exported function we're passing a handle to
                            // the caller's memory, and this part of the adapter is
                            // responsible for converting it into something that's a handle.
                            self.emit(&BufferLowerHandle { push, ty });
                        }
                    }
                }
                TypeDefKind::Record(record) if record.is_flags() => {
                    match self.iface.flags_repr(record) {
                        Some(Int::U64) => self.emit(&FlagsLower64 {
                            record,
                            ty: id,
                            name: self.iface.types[id].name.as_ref().unwrap(),
                        }),
                        _ => self.emit(&FlagsLower {
                            record,
                            ty: id,
                            name: self.iface.types[id].name.as_ref().unwrap(),
                        }),
                    }
                }
                TypeDefKind::Record(record) => match self.abi {
                    Abi::Preview1 => self.witx(&AddrOf),

                    Abi::Canonical => {
                        self.emit(&RecordLower {
                            record,
                            ty: id,
                            name: self.iface.types[id].name.as_deref(),
                        });
                        let values = self
                            .stack
                            .drain(self.stack.len() - record.fields.len()..)
                            .collect::<Vec<_>>();
                        for (field, value) in record.fields.iter().zip(values) {
                            self.stack.push(value);
                            self.lower(&field.ty, None);
                        }
                    }
                },

                // Variants in the return position of an import must be a Result in
                // the preview1 ABI and they're a bit special about where all the
                // pieces are.
                TypeDefKind::Variant(v)
                    if self.abi == Abi::Preview1
                        && self.dir == Direction::Import
                        && self.lift_lower == LiftLower::LiftArgsLowerResults
                        && !v.is_enum() =>
                {
                    let retptr = retptr.unwrap();
                    let (ok, err) = v.as_expected().unwrap();
                    self.push_block();
                    self.emit(&VariantPayloadName);
                    let payload_name = self.stack.pop().unwrap();
                    if let Some(ok) = ok {
                        self.stack.push(payload_name.clone());
                        let store = |me: &mut Self, ty: &Type, n| {
                            me.emit(&GetArg { nth: *retptr + n });
                            let addr = me.stack.pop().unwrap();
                            me.write_to_memory(ty, addr, 0);
                        };
                        match *ok {
                            Type::Id(okid) => match &self.iface.types[okid].kind {
                                TypeDefKind::Record(record) if record.is_tuple() => {
                                    self.emit(&RecordLower {
                                        record,
                                        ty: id,
                                        name: self.iface.types[okid].name.as_deref(),
                                    });
                                    // Note that `rev()` is used here due to the order
                                    // that tuples are pushed onto the stack and how we
                                    // consume the last item first from the stack.
                                    for (i, field) in record.fields.iter().enumerate().rev() {
                                        store(self, &field.ty, i);
                                    }
                                }
                                _ => store(self, ok, 0),
                            },
                            _ => store(self, ok, 0),
                        }
                    };
                    self.emit(&I32Const { val: 0 });
                    self.finish_block(1);

                    self.push_block();
                    self.emit(&VariantPayloadName);
                    let payload_name = self.stack.pop().unwrap();
                    if let Some(ty) = err {
                        self.stack.push(payload_name.clone());
                        self.lower(ty, None);
                    }
                    self.finish_block(1);

                    self.emit(&VariantLower {
                        variant: v,
                        ty: id,
                        name: self.iface.types[id].name.as_deref(),
                        results: &[WasmType::I32],
                    });
                }

                // Variant arguments in the Preview1 ABI are all passed by pointer
                TypeDefKind::Variant(v)
                    if self.abi == Abi::Preview1
                        && self.dir == Direction::Import
                        && self.lift_lower == LiftLower::LowerArgsLiftResults
                        && !v.is_enum() =>
                {
                    self.witx(&AddrOf)
                }

                TypeDefKind::Variant(v) => {
                    let mut results = Vec::new();
                    let mut temp = Vec::new();
                    let mut casts = Vec::new();
                    self.iface.push_wasm(self.abi, self.dir, ty, &mut results);
                    for (i, case) in v.cases.iter().enumerate() {
                        self.push_block();
                        self.emit(&VariantPayloadName);
                        let payload_name = self.stack.pop().unwrap();
                        self.emit(&I32Const { val: i as i32 });
                        let mut pushed = 1;
                        if let Some(ty) = &case.ty {
                            // Using the payload of this block we lower the type to
                            // raw wasm values.
                            self.stack.push(payload_name.clone());
                            self.lower(ty, None);

                            // Determine the types of all the wasm values we just
                            // pushed, and record how many. If we pushed too few
                            // then we'll need to push some zeros after this.
                            temp.truncate(0);
                            self.iface.push_wasm(self.abi, self.dir, ty, &mut temp);
                            pushed += temp.len();

                            // For all the types pushed we may need to insert some
                            // bitcasts. This will go through and cast everything
                            // to the right type to ensure all blocks produce the
                            // same set of results.
                            casts.truncate(0);
                            for (actual, expected) in temp.iter().zip(&results[1..]) {
                                casts.push(cast(*actual, *expected));
                            }
                            if casts.iter().any(|c| *c != Bitcast::None) {
                                self.emit(&Bitcasts { casts: &casts });
                            }
                        }

                        // If we haven't pushed enough items in this block to match
                        // what other variants are pushing then we need to push
                        // some zeros.
                        if pushed < results.len() {
                            self.emit(&ConstZero {
                                tys: &results[pushed..],
                            });
                        }
                        self.finish_block(results.len());
                    }
                    self.emit(&VariantLower {
                        variant: v,
                        ty: id,
                        results: &results,
                        name: self.iface.types[id].name.as_deref(),
                    });
                }
            },
        }
    }

    fn prep_return_pointer(&mut self, sig: &WasmSignature, results: &[(String, Type)]) {
        match self.abi {
            Abi::Preview1 => {
                assert!(results.len() <= 1);
                let ty = match results.get(0) {
                    Some((_, ty)) => ty,
                    None => return,
                };
                // Return pointers are only needed for `Result<T, _>`...
                let variant = match ty {
                    Type::Id(id) => match &self.iface.types[*id].kind {
                        TypeDefKind::Variant(v) => v,
                        _ => return,
                    },
                    _ => return,
                };
                // ... and only if `T` is actually present in `Result<T, _>`
                let ok = match &variant.cases[0].ty {
                    Some(Type::Id(id)) => *id,
                    _ => return,
                };

                // Tuples have each individual item in a separate return pointer while
                // all other types go through a singular return pointer.
                let iface = self.iface;
                let mut prep = |ty: TypeId| {
                    let ptr = self.bindgen.allocate_typed_space(iface, ty);
                    self.return_pointers.push(ptr.clone());
                    self.stack.push(ptr);
                };
                match &iface.types[ok].kind {
                    TypeDefKind::Record(r) if r.is_tuple() => {
                        for field in r.fields.iter() {
                            match field.ty {
                                Type::Id(id) => prep(id),
                                _ => unreachable!(),
                            }
                        }
                    }
                    _ => prep(ok),
                }
            }
            // If a return pointer was automatically injected into this function
            // then we need to allocate a proper amount of space for it and then
            // add it to the stack to get passed to the callee.
            Abi::Canonical => {
                if let Some(results) = &sig.retptr {
                    let ptr = self.bindgen.i64_return_pointer_area(results.len());
                    self.return_pointers.push(ptr.clone());
                    self.stack.push(ptr.clone());
                }
            }
        }
    }

    /// Note that in general everything in this function is the opposite of the
    /// `lower` function above. This is intentional and should be kept this way!
    fn lift(&mut self, ty: &Type) {
        use Instruction::*;
        use WitxInstruction::*;

        match *ty {
            Type::S8 => self.emit(&S8FromI32),
            Type::CChar => self.emit(&Char8FromI32),
            Type::U8 => self.emit(&U8FromI32),
            Type::S16 => self.emit(&S16FromI32),
            Type::U16 => self.emit(&U16FromI32),
            Type::S32 => self.emit(&S32FromI32),
            Type::Usize => self.emit(&UsizeFromI32),
            Type::U32 => self.emit(&U32FromI32),
            Type::S64 => self.emit(&S64FromI64),
            Type::U64 => self.emit(&U64FromI64),
            Type::Char => self.emit(&CharFromI32),
            Type::F32 => self.emit(&If32FromF32),
            Type::F64 => self.emit(&If64FromF64),
            Type::Handle(ty) => {
                // For more information on these values see the comments in
                // `lower` above.
                let borrowed = match self.lift_lower {
                    LiftLower::LiftArgsLowerResults => true,
                    LiftLower::LowerArgsLiftResults => false,
                };
                if borrowed {
                    self.emit(&HandleBorrowedFromI32 { ty });
                } else {
                    self.emit(&HandleOwnedFromI32 { ty });
                }
            }
            Type::Id(id) => match &self.iface.types[id].kind {
                TypeDefKind::Type(t) => self.lift(t),
                TypeDefKind::Pointer(ty) => self.witx(&PointerFromI32 { ty }),
                TypeDefKind::ConstPointer(ty) => self.witx(&ConstPointerFromI32 { ty }),
                TypeDefKind::List(element) => match self.abi {
                    Abi::Preview1 => self.emit(&ListCanonLift {
                        element,
                        free: None,
                        ty: id,
                    }),
                    Abi::Canonical => {
                        // Lifting the arguments of a defined import means that, if
                        // possible, the caller still retains ownership and we don't
                        // free anything.
                        let free = match (self.dir, self.lift_lower) {
                            (Direction::Import, LiftLower::LiftArgsLowerResults) => None,
                            _ => Some("canonical_abi_free"),
                        };
                        if self.is_char(element)
                            || self.bindgen.is_list_canonical(self.iface, element)
                        {
                            self.emit(&ListCanonLift {
                                element,
                                free,
                                ty: id,
                            });
                        } else {
                            self.push_block();
                            self.emit(&IterBasePointer);
                            let addr = self.stack.pop().unwrap();
                            self.read_from_memory(element, addr, 0);
                            self.finish_block(1);
                            self.emit(&ListLift {
                                element,
                                free,
                                ty: id,
                            });
                        }
                    }
                },
                TypeDefKind::PushBuffer(ty) | TypeDefKind::PullBuffer(ty) => {
                    let push = match &self.iface.types[id].kind {
                        TypeDefKind::PushBuffer(_) => true,
                        _ => false,
                    };
                    self.translate_buffer(push, ty);
                    // Buffers are only used in the parameter position, which
                    // means lifting a buffer should only happen when we are
                    // lifting arguments and lowering results.
                    assert!(self.lift_lower == LiftLower::LiftArgsLowerResults);

                    match self.dir {
                        Direction::Import => {
                            // When calling a defined imported function then we're coming
                            // from a pointer/length, and the embedding context will figure
                            // out what to do with that pointer/length.
                            self.emit(&BufferLiftPtrLen { push, ty })
                        }
                        Direction::Export => {
                            // When calling an exported function we're given a handle to the
                            // buffer, which is then interpreted in the calling context.
                            self.emit(&BufferLiftHandle { push, ty })
                        }
                    }
                }
                TypeDefKind::Record(record) if record.is_flags() => {
                    match self.iface.flags_repr(record) {
                        Some(Int::U64) => self.emit(&FlagsLift64 {
                            record,
                            ty: id,
                            name: self.iface.types[id].name.as_ref().unwrap(),
                        }),
                        _ => self.emit(&FlagsLift {
                            record,
                            ty: id,
                            name: self.iface.types[id].name.as_ref().unwrap(),
                        }),
                    }
                }
                TypeDefKind::Record(record) => match self.abi {
                    Abi::Preview1 => {
                        let addr = self.stack.pop().unwrap();
                        self.read_from_memory(ty, addr, 0);
                    }
                    Abi::Canonical => {
                        let mut temp = Vec::new();
                        self.iface.push_wasm(self.abi, self.dir, ty, &mut temp);
                        let mut args = self
                            .stack
                            .drain(self.stack.len() - temp.len()..)
                            .collect::<Vec<_>>();
                        for field in record.fields.iter() {
                            temp.truncate(0);
                            self.iface
                                .push_wasm(self.abi, self.dir, &field.ty, &mut temp);
                            self.stack.extend(args.drain(..temp.len()));
                            self.lift(&field.ty);
                        }
                        self.emit(&RecordLift {
                            record,
                            ty: id,
                            name: self.iface.types[id].name.as_deref(),
                        });
                    }
                },

                // Variants in the return position of an import must be a Result in
                // the preview1 ABI and they're a bit special about where all the
                // pieces are.
                TypeDefKind::Variant(v)
                    if self.abi == Abi::Preview1
                        && self.dir == Direction::Import
                        && self.lift_lower == LiftLower::LowerArgsLiftResults
                        && !v.is_enum() =>
                {
                    let (ok, err) = v.as_expected().unwrap();
                    self.push_block();
                    if let Some(ok) = ok {
                        let mut n = 0;
                        let mut load = |me: &mut Self, ty: &Type| {
                            me.read_from_memory(ty, me.return_pointers[n].clone(), 0);
                            n += 1;
                        };
                        match *ok {
                            Type::Id(okid) => match &self.iface.types[okid].kind {
                                TypeDefKind::Record(record) if record.is_tuple() => {
                                    for field in record.fields.iter() {
                                        load(self, &field.ty);
                                    }
                                    self.emit(&RecordLift {
                                        record,
                                        ty: okid,
                                        name: self.iface.types[okid].name.as_deref(),
                                    });
                                }
                                _ => load(self, ok),
                            },
                            _ => load(self, ok),
                        }
                    }
                    self.finish_block(ok.is_some() as usize);

                    self.push_block();
                    if let Some(ty) = err {
                        self.witx(&ReuseReturn);
                        self.lift(ty);
                    }
                    self.finish_block(err.is_some() as usize);

                    self.emit(&VariantLift {
                        variant: v,
                        ty: id,
                        name: self.iface.types[id].name.as_deref(),
                    });
                }

                // Variant arguments in the Preview1 ABI are all passed by pointer,
                // so we read them here.
                TypeDefKind::Variant(v)
                    if self.abi == Abi::Preview1
                        && self.dir == Direction::Import
                        && self.lift_lower == LiftLower::LiftArgsLowerResults
                        && !v.is_enum() =>
                {
                    let addr = self.stack.pop().unwrap();
                    self.read_from_memory(ty, addr, 0)
                }

                TypeDefKind::Variant(v) => {
                    let mut params = Vec::new();
                    let mut temp = Vec::new();
                    let mut casts = Vec::new();
                    self.iface.push_wasm(self.abi, self.dir, ty, &mut params);
                    let block_inputs = self
                        .stack
                        .drain(self.stack.len() + 1 - params.len()..)
                        .collect::<Vec<_>>();
                    for case in v.cases.iter() {
                        self.push_block();
                        if let Some(ty) = &case.ty {
                            // Push only the values we need for this variant onto
                            // the stack.
                            temp.truncate(0);
                            self.iface.push_wasm(self.abi, self.dir, ty, &mut temp);
                            self.stack
                                .extend(block_inputs[..temp.len()].iter().cloned());

                            // Cast all the types we have on the stack to the actual
                            // types needed for this variant, if necessary.
                            casts.truncate(0);
                            for (actual, expected) in temp.iter().zip(&params[1..]) {
                                casts.push(cast(*expected, *actual));
                            }
                            if casts.iter().any(|c| *c != Bitcast::None) {
                                self.emit(&Bitcasts { casts: &casts });
                            }

                            // Then recursively lift this variant's payload.
                            self.lift(ty);
                        }
                        self.finish_block(case.ty.is_some() as usize);
                    }
                    self.emit(&VariantLift {
                        variant: v,
                        ty: id,
                        name: self.iface.types[id].name.as_deref(),
                    });
                }
            },
        }
    }

    fn write_to_memory(&mut self, ty: &Type, addr: B::Operand, offset: i32) {
        use Instruction::*;

        match *ty {
            // Builtin types need different flavors of storage instructions
            // depending on the size of the value written.
            Type::U8 | Type::S8 | Type::CChar => {
                self.lower_and_emit(ty, addr, &I32Store8 { offset })
            }
            Type::U16 | Type::S16 => self.lower_and_emit(ty, addr, &I32Store16 { offset }),
            Type::U32 | Type::S32 | Type::Usize | Type::Handle(_) | Type::Char => {
                self.lower_and_emit(ty, addr, &I32Store { offset })
            }
            Type::U64 | Type::S64 => self.lower_and_emit(ty, addr, &I64Store { offset }),
            Type::F32 => self.lower_and_emit(ty, addr, &F32Store { offset }),
            Type::F64 => self.lower_and_emit(ty, addr, &F64Store { offset }),

            Type::Id(id) => match &self.iface.types[id].kind {
                TypeDefKind::Type(t) => self.write_to_memory(t, addr, offset),
                TypeDefKind::Pointer(_) | TypeDefKind::ConstPointer(_) => {
                    self.lower_and_emit(ty, addr, &I32Store { offset });
                }

                // After lowering the list there's two i32 values on the stack
                // which we write into memory, writing the pointer into the low address
                // and the length into the high address.
                TypeDefKind::List(_) => {
                    self.lower(ty, None);
                    self.stack.push(addr.clone());
                    self.emit(&I32Store { offset: offset + 4 });
                    self.stack.push(addr);
                    self.emit(&I32Store { offset });
                }

                // Lower the buffer to its raw values, and then write the values
                // into memory, which may be more than one value depending on
                // our import/export direction.
                TypeDefKind::PushBuffer(_) | TypeDefKind::PullBuffer(_) => {
                    self.lower(ty, None);
                    if self.dir == Direction::Import {
                        self.stack.push(addr.clone());
                        self.emit(&I32Store { offset: offset + 8 });
                        self.stack.push(addr.clone());
                        self.emit(&I32Store { offset: offset + 4 });
                    }
                    self.stack.push(addr);
                    self.emit(&I32Store { offset });
                }

                TypeDefKind::Record(r) if r.is_flags() => {
                    self.lower(ty, None);
                    match self.iface.flags_repr(r) {
                        Some(repr) => {
                            self.stack.push(addr);
                            self.store_intrepr(offset, repr);
                        }
                        None => {
                            for i in 0..r.num_i32s() {
                                self.stack.push(addr.clone());
                                self.emit(&I32Store {
                                    offset: offset + (i as i32) * 4,
                                });
                            }
                        }
                    }
                }

                // Decompose the record into its components and then write all
                // the components into memory one-by-one.
                TypeDefKind::Record(record) => {
                    self.emit(&RecordLower {
                        record,
                        ty: id,
                        name: self.iface.types[id].name.as_deref(),
                    });
                    let fields = self
                        .stack
                        .drain(self.stack.len() - record.fields.len()..)
                        .collect::<Vec<_>>();
                    for ((field_offset, op), field) in self
                        .bindgen
                        .sizes()
                        .field_offsets(record)
                        .into_iter()
                        .zip(fields)
                        .zip(&record.fields)
                    {
                        self.stack.push(op);
                        self.write_to_memory(
                            &field.ty,
                            addr.clone(),
                            offset + (field_offset as i32),
                        );
                    }
                }

                // Each case will get its own block, and the first item in each
                // case is writing the discriminant. After that if we have a
                // payload we write the payload after the discriminant, aligned up
                // to the type's alignment.
                TypeDefKind::Variant(v) => {
                    let payload_offset = offset + (self.bindgen.sizes().payload_offset(v) as i32);
                    for (i, case) in v.cases.iter().enumerate() {
                        self.push_block();
                        self.emit(&VariantPayloadName);
                        let payload_name = self.stack.pop().unwrap();
                        self.emit(&I32Const { val: i as i32 });
                        self.stack.push(addr.clone());
                        self.store_intrepr(offset, v.tag);
                        if let Some(ty) = &case.ty {
                            self.stack.push(payload_name.clone());
                            self.write_to_memory(ty, addr.clone(), payload_offset);
                        }
                        self.finish_block(0);
                    }
                    self.emit(&VariantLower {
                        variant: v,
                        ty: id,
                        results: &[],
                        name: self.iface.types[id].name.as_deref(),
                    });
                }
            },
        }
    }

    fn lower_and_emit(&mut self, ty: &Type, addr: B::Operand, instr: &Instruction) {
        self.lower(ty, None);
        self.stack.push(addr);
        self.emit(instr);
    }

    fn read_from_memory(&mut self, ty: &Type, addr: B::Operand, offset: i32) {
        use Instruction::*;

        match *ty {
            Type::U8 | Type::CChar => self.emit_and_lift(ty, addr, &I32Load8U { offset }),
            Type::S8 => self.emit_and_lift(ty, addr, &I32Load8S { offset }),
            Type::U16 => self.emit_and_lift(ty, addr, &I32Load16U { offset }),
            Type::S16 => self.emit_and_lift(ty, addr, &I32Load16S { offset }),
            Type::U32 | Type::S32 | Type::Char | Type::Usize | Type::Handle(_) => {
                self.emit_and_lift(ty, addr, &I32Load { offset })
            }
            Type::U64 | Type::S64 => self.emit_and_lift(ty, addr, &I64Load { offset }),
            Type::F32 => self.emit_and_lift(ty, addr, &F32Load { offset }),
            Type::F64 => self.emit_and_lift(ty, addr, &F64Load { offset }),

            Type::Id(id) => match &self.iface.types[id].kind {
                TypeDefKind::Type(t) => self.read_from_memory(t, addr, offset),
                TypeDefKind::Pointer(_) | TypeDefKind::ConstPointer(_) => {
                    self.emit_and_lift(ty, addr, &I32Load { offset })
                }

                // Read the pointer/len and then perform the standard lifting
                // proceses.
                TypeDefKind::List(_) => {
                    self.stack.push(addr.clone());
                    self.emit(&I32Load { offset });
                    self.stack.push(addr);
                    self.emit(&I32Load { offset: offset + 4 });
                    self.lift(ty);
                }

                // Read the requisite number of values from memory and then lift as
                // appropriate.
                TypeDefKind::PushBuffer(_) | TypeDefKind::PullBuffer(_) => {
                    self.stack.push(addr.clone());
                    self.emit(&I32Load { offset });
                    if self.dir == Direction::Import
                        && self.lift_lower == LiftLower::LiftArgsLowerResults
                    {
                        self.stack.push(addr.clone());
                        self.emit(&I32Load { offset: offset + 4 });
                        self.stack.push(addr);
                        self.emit(&I32Load { offset: offset + 8 });
                    }
                    self.lift(ty);
                }

                TypeDefKind::Record(r) if r.is_flags() => {
                    match self.iface.flags_repr(r) {
                        Some(repr) => {
                            self.stack.push(addr);
                            self.load_intrepr(offset, repr);
                        }
                        None => {
                            for i in 0..r.num_i32s() {
                                self.stack.push(addr.clone());
                                self.emit(&I32Load {
                                    offset: offset + (i as i32) * 4,
                                });
                            }
                        }
                    }
                    self.lift(ty);
                }

                // Read and lift each field individually, adjusting the offset
                // as we go along, then aggregate all the fields into the
                // record.
                TypeDefKind::Record(record) => {
                    for (field_offset, field) in self
                        .bindgen
                        .sizes()
                        .field_offsets(record)
                        .into_iter()
                        .zip(&record.fields)
                    {
                        self.read_from_memory(
                            &field.ty,
                            addr.clone(),
                            offset + (field_offset as i32),
                        );
                    }
                    self.emit(&RecordLift {
                        record,
                        ty: id,
                        name: self.iface.types[id].name.as_deref(),
                    });
                }

                // Each case will get its own block, and we'll dispatch to the
                // right block based on the `i32.load` we initially perform. Each
                // individual block is pretty simple and just reads the payload type
                // from the corresponding offset if one is available.
                TypeDefKind::Variant(variant) => {
                    self.stack.push(addr.clone());
                    self.load_intrepr(offset, variant.tag);
                    let payload_offset =
                        offset + (self.bindgen.sizes().payload_offset(variant) as i32);
                    for case in variant.cases.iter() {
                        self.push_block();
                        if let Some(ty) = &case.ty {
                            self.read_from_memory(ty, addr.clone(), payload_offset);
                        }
                        self.finish_block(case.ty.is_some() as usize);
                    }
                    self.emit(&VariantLift {
                        variant,
                        ty: id,
                        name: self.iface.types[id].name.as_deref(),
                    });
                }
            },
        }
    }

    fn emit_and_lift(&mut self, ty: &Type, addr: B::Operand, instr: &Instruction) {
        self.stack.push(addr);
        self.emit(instr);
        self.lift(ty);
    }

    fn load_intrepr(&mut self, offset: i32, repr: Int) {
        self.emit(&match repr {
            Int::U64 => Instruction::I64Load { offset },
            Int::U32 => Instruction::I32Load { offset },
            Int::U16 => Instruction::I32Load16U { offset },
            Int::U8 => Instruction::I32Load8U { offset },
        });
    }

    fn store_intrepr(&mut self, offset: i32, repr: Int) {
        self.emit(&match repr {
            Int::U64 => Instruction::I64Store { offset },
            Int::U32 => Instruction::I32Store { offset },
            Int::U16 => Instruction::I32Store16 { offset },
            Int::U8 => Instruction::I32Store8 { offset },
        });
    }

    fn translate_buffer(&mut self, push: bool, ty: &Type) {
        let do_write = match self.lift_lower {
            // For declared items, input/output is defined in the context of
            // what the callee will do. The callee will read input buffers,
            // meaning we write to them, and write to output buffers, meaning
            // we'll read from them.
            LiftLower::LowerArgsLiftResults => !push,

            // Defined item mirror declared imports because buffers are
            // defined from the caller's perspective, so we don't invert the
            // `out` setting like above.
            LiftLower::LiftArgsLowerResults => push,
        };
        self.emit(&Instruction::IterBasePointer);
        let addr = self.stack.pop().unwrap();
        if do_write {
            self.push_block();
            self.emit(&Instruction::BufferPayloadName);
            self.write_to_memory(ty, addr, 0);
            self.finish_block(0);
        } else {
            self.push_block();
            self.read_from_memory(ty, addr, 0);
            self.finish_block(1);
        }
    }

    fn is_char(&self, ty: &Type) -> bool {
        match ty {
            Type::Char => true,
            Type::Id(id) => match &self.iface.types[*id].kind {
                TypeDefKind::Type(t) => self.is_char(t),
                _ => false,
            },
            _ => false,
        }
    }
}

fn cast(from: WasmType, to: WasmType) -> Bitcast {
    use WasmType::*;

    match (from, to) {
        (I32, I32) | (I64, I64) | (F32, F32) | (F64, F64) => Bitcast::None,

        (I32, I64) => Bitcast::I32ToI64,
        (F32, F64) => Bitcast::F32ToF64,
        (F32, I32) => Bitcast::F32ToI32,
        (F64, I64) => Bitcast::F64ToI64,

        (I64, I32) => Bitcast::I64ToI32,
        (F64, F32) => Bitcast::F64ToF32,
        (I32, F32) => Bitcast::I32ToF32,
        (I64, F64) => Bitcast::I64ToF64,

        (F32, I64) => Bitcast::F32ToI64,
        (I64, F32) => Bitcast::I64ToF32,
        (F64, I32) | (I32, F64) => unreachable!(),
    }
}