hyperlight_component_util/

elaborate.rs

/*
Copyright 2025 The Hyperlight Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
 */

//! Component type elaboration
//!
//! This is a pretty direct port of the relevant sections of the OCaml
//! reference interpreter, except that the approach to substitutions has
//! been changed significantly. (Although the core capture-avoiding
//! substitution routines are of course the same, the ways in which
//! substitutions are represented/constructed are quite different; see
//! substitute.rs for more details of the approach here).

use wasmparser::{
    ComponentAlias, ComponentDefinedType, ComponentFuncType, ComponentOuterAliasKind,
    ComponentType, ComponentTypeDeclaration, ComponentTypeRef, ComponentValType,
    CompositeInnerType, CoreType, InstanceTypeDeclaration, ModuleTypeDeclaration, OuterAliasKind,
    PrimitiveValType, TypeBounds, TypeRef,
};

use crate::etypes::{
    BoundedTyvar, Component, CoreDefined, CoreExportDecl, CoreExternDesc, CoreModule,
    CoreOrComponentExternDesc, Ctx, Defined, ExternDecl, ExternDesc, FloatWidth, Func, Handleable,
    Instance, IntWidth, Name, Param, QualifiedInstance, RecordField, Resource, ResourceId,
    TypeBound, Tyvar, Value, VariantCase,
};
use crate::substitute::{self, Substitution, Unvoidable};
use crate::tv::ResolvedTyvar;
use crate::wf;

mod basic_conversions {
    //! Basic utility conversions between various spec and wasmparser
    //! representations of extern kind/sorts

    use wasmparser::{ComponentExternalKind, ExternalKind};

    use crate::etypes::{CoreExternDesc, ExternDesc};
    use crate::structure::{CoreSort, Sort};

    pub(super) fn sort_matches_core_ed(sort: Sort, ed: &CoreExternDesc) {
        match (sort, ed) {
            (Sort::Core(CoreSort::Func), CoreExternDesc::Func(_)) => (),
            (Sort::Core(CoreSort::Table), CoreExternDesc::Table(_)) => (),
            (Sort::Core(CoreSort::Memory), CoreExternDesc::Memory(_)) => (),
            (Sort::Core(CoreSort::Global), CoreExternDesc::Global(_)) => (),
            _ => panic!("sort does not match core extern descriptor"),
        }
    }

    pub(super) fn external_kind(k: ExternalKind) -> Sort {
        match k {
            ExternalKind::Func => Sort::Core(CoreSort::Func),
            ExternalKind::Table => Sort::Core(CoreSort::Table),
            ExternalKind::Memory => Sort::Core(CoreSort::Memory),
            ExternalKind::Global => Sort::Core(CoreSort::Global),
            ExternalKind::Tag => panic!("core type tags are not supported"),
        }
    }

    pub(super) fn sort_matches_ed<'a>(sort: Sort, ed: &ExternDesc<'a>) {
        match (sort, ed) {
            (Sort::Core(CoreSort::Module), ExternDesc::CoreModule(_)) => (),
            (Sort::Func, ExternDesc::Func(_)) => (),
            (Sort::Type, ExternDesc::Type(_)) => (),
            (Sort::Instance, ExternDesc::Instance(_)) => (),
            (Sort::Component, ExternDesc::Component(_)) => (),
            _ => panic!("sort does not match extern descriptor"),
        }
    }

    pub(super) fn component_external_kind(k: ComponentExternalKind) -> Sort {
        match k {
            ComponentExternalKind::Module => Sort::Core(CoreSort::Module),
            ComponentExternalKind::Func => Sort::Func,
            ComponentExternalKind::Value => Sort::Value,
            ComponentExternalKind::Type => Sort::Type,
            ComponentExternalKind::Instance => Sort::Instance,
            ComponentExternalKind::Component => Sort::Component,
        }
    }
}
use basic_conversions::*;

#[derive(Debug)]
#[allow(dead_code)]
/// Elaboration-specific errors
pub enum Error<'a> {
    /// Innerizing an outer alias failed; this usually means that the
    /// outer alias refers to a resource type or something like that.
    InvalidOuterAlias(substitute::InnerizeError),
    /// Innerizing an outer alias resulted in an ill-formed type; this
    /// often also means that the outer alias refers to a resource
    /// type or similar.
    IllFormedOuterAlias(wf::Error<'a>),
    /// The component type declarator should never have a resource
    /// type in it, even though this is allowed by the grammar, since
    /// there is no export (or instantiation) to generatively give it
    /// identity.
    ResourceInDeclarator,
    /// A the typeidx inside an own/borrow handle should always point
    /// to a resource type (either a bare resource, or, more usually,
    /// an imported/exported type variable that is bounded by `(sub
    /// resource)`.
    HandleToNonResource,
    /// Complex valtypes are allowed to use indirect type indices to
    /// refer to another type, but the type index space is also used
    /// for instance types, bare resource types, etc.  A malformed
    /// complex value type which refers to a non-value type will
    /// result in this error.
    ValTypeRefToNonVal(Defined<'a>),
    /// The finalisation/closing of a component or instance type
    /// failed. This usually means that an exported type is referring
    /// to a non-exported type variable or something along those
    /// lines, which makes it impossible for the exported type to be
    /// lifted out of the context.
    ClosingError(substitute::ClosingError),
    /// A finished component or instance type was ill-formed
    IllFormed(wf::Error<'a>),
}
impl<'a> From<substitute::ClosingError> for Error<'a> {
    fn from(e: substitute::ClosingError) -> Error<'a> {
        Error::ClosingError(e)
    }
}

/// # Elaboration
///
/// Most of this is a very direct translation of the specification
/// (section 3.4 Type Elaboration).
impl<'p, 'a> Ctx<'p, 'a> {
    pub fn elab_component<'c>(
        &'c mut self,
        decls: &[ComponentTypeDeclaration<'a>],
    ) -> Result<Component<'a>, Error<'a>> {
        let mut ctx = Ctx::new(Some(self), false);
        let mut imports = Vec::new();
        let mut exports = Vec::new();
        for decl in decls {
            let (import, export) = ctx.elab_component_decl(decl)?;
            if let Some(import) = import {
                imports.push(import);
            }
            if let Some(export) = export {
                exports.push(export);
            }
        }
        ctx.finish_component(&imports, &exports)
    }

    fn elab_core_module_decl<'c>(
        &'c mut self,
        decl: &ModuleTypeDeclaration<'a>,
    ) -> (Option<wasmparser::Import<'a>>, Option<CoreExportDecl<'a>>) {
        match decl {
            ModuleTypeDeclaration::Import(i) => (Some(*i), None),
            ModuleTypeDeclaration::Type(rg) => {
                let ct = self.elab_core_type_rec(rg);
                self.core.types.push(ct);
                (None, None)
            }
            ModuleTypeDeclaration::OuterAlias {
                kind: OuterAliasKind::Type,
                count,
                index,
            } => {
                let ct = self.parents().nth(*count as usize).unwrap().core.types[*index as usize]
                    .clone();
                self.core.types.push(ct);
                (None, None)
            }
            ModuleTypeDeclaration::Export { name, ty } => (
                None,
                Some(CoreExportDecl {
                    name: Name { name },
                    desc: match ty {
                        TypeRef::Func(n) => match &self.core.types[*n as usize] {
                            CoreDefined::Func(ft) => CoreExternDesc::Func(ft.clone()),
                            _ => panic!(
                                "internal invariant violation: WasmParser function TypeRef refers to non-function"
                            ),
                        },
                        TypeRef::Table(tt) => CoreExternDesc::Table(*tt),
                        TypeRef::Memory(mt) => CoreExternDesc::Memory(*mt),
                        TypeRef::Global(gt) => CoreExternDesc::Global(*gt),
                        TypeRef::Tag(_) => panic!("core type tags are not supported"),
                    },
                }),
            ),
        }
    }

    fn elab_core_module<'c>(&'c mut self, decls: &[ModuleTypeDeclaration<'a>]) -> CoreModule<'a> {
        let mut ctx = Ctx::new(Some(self), false);
        let mut imports = Vec::new();
        let mut exports = Vec::new();
        for decl in decls {
            let (import, export) = ctx.elab_core_module_decl(decl);
            if let Some(import) = import {
                imports.push(import)
            }
            if let Some(export) = export {
                exports.push(export)
            }
        }
        CoreModule {
            _imports: imports,
            _exports: exports,
        }
    }

    fn elab_core_type_rec<'c>(&'c mut self, rg: &wasmparser::RecGroup) -> CoreDefined<'a> {
        match &rg.types().nth(0).unwrap().composite_type.inner {
            CompositeInnerType::Func(ft) => CoreDefined::Func(ft.clone()),
            _ => panic!("GC core types are not presently supported"),
        }
    }

    fn elab_core_type<'c>(&'c mut self, ct: &wasmparser::CoreType<'a>) -> CoreDefined<'a> {
        match ct {
            CoreType::Rec(rg) => self.elab_core_type_rec(rg),
            CoreType::Module(ds) => CoreDefined::Module(self.elab_core_module(ds)),
        }
    }

    /// This tries to handle pretty much everything involved in alias
    /// resolution and well-formedness checking. Since both core and
    /// component aliases are largely similar, it can handle both and
    /// has to return a union of core/component extern descriptors
    /// that does not exist in the spec.
    fn resolve_alias<'c>(
        &'c mut self,
        alias: &ComponentAlias<'a>,
    ) -> Result<CoreOrComponentExternDesc<'a>, Error<'a>> {
        match alias {
            ComponentAlias::InstanceExport {
                kind,
                instance_index,
                name,
            } => {
                let it = &self.instances[*instance_index as usize];
                let ed = &it
                    .exports
                    .iter()
                    .find(|e| e.kebab_name == *name)
                    .unwrap()
                    .desc;
                let sort = component_external_kind(*kind);
                sort_matches_ed(sort, ed);
                Ok(CoreOrComponentExternDesc::Component(ed.clone()))
            }
            ComponentAlias::CoreInstanceExport {
                kind,
                instance_index,
                name,
            } => {
                let it = &self.core.instances[*instance_index as usize];
                let ed = &it
                    .exports
                    .iter()
                    .find(|e| e.name.name == *name)
                    .unwrap()
                    .desc;
                let sort = external_kind(*kind);
                sort_matches_core_ed(sort, ed);
                Ok(CoreOrComponentExternDesc::Core(ed.clone()))
            }
            ComponentAlias::Outer { kind, count, index } => {
                if *kind != ComponentOuterAliasKind::Type {
                    panic!("In types, only outer type aliases are allowed");
                }
                // Walk through each of the contexts between us and
                // the targeted type, so that we can innerize each one
                let mut ctxs = self.parents().take(*count as usize + 1).collect::<Vec<_>>();
                ctxs.reverse();
                let mut target_type = ctxs[0].types[*index as usize].clone();
                let mut ob_crossed = false;
                for ctxs_ in ctxs.windows(2) {
                    ob_crossed |= ctxs_[1].outer_boundary;
                    let sub = substitute::Innerize::new(ctxs_[0], ctxs_[1].outer_boundary);
                    target_type = sub
                        .defined(&target_type)
                        .map_err(Error::InvalidOuterAlias)?;
                }
                if ob_crossed {
                    self.wf_defined(wf::DefinedTypePosition::export(), &target_type)
                        .map_err(Error::IllFormedOuterAlias)?;
                }
                Ok(CoreOrComponentExternDesc::Component(ExternDesc::Type(
                    target_type,
                )))
            }
        }
    }

    /// Add a core extern descriptor to the context: whatever it
    /// describes is added to the relevant index space
    fn add_core_ed<'c>(&'c mut self, ed: CoreExternDesc) {
        match ed {
            CoreExternDesc::Func(ft) => self.core.funcs.push(ft),
            CoreExternDesc::Table(tt) => self.core.tables.push(tt),
            CoreExternDesc::Memory(mt) => self.core.mems.push(mt),
            CoreExternDesc::Global(gt) => self.core.globals.push(gt),
        }
    }

    /// Add an extern descriptor to the context: whatever it describes
    /// is added to the relevant index space. Note that this does not
    /// handle stripping the type variables off of an instance type
    /// (since `ExternDesc::Instance` doesn't have them); that should
    /// have been done earlier. See for example the export instance
    /// declarator case below, which converts the bound variables on
    /// the instance type to context evars, and fixes them up in the
    /// instance type, before calling add_ed.
    fn add_ed<'c>(&'c mut self, ed: &ExternDesc<'a>) {
        match ed {
            ExternDesc::CoreModule(cmd) => self.core.modules.push(cmd.clone()),
            ExternDesc::Func(ft) => self.funcs.push(ft.clone()),
            ExternDesc::Type(dt) => self.types.push(dt.clone()),
            ExternDesc::Instance(it) => self.instances.push(it.clone()),
            ExternDesc::Component(ct) => self.components.push(ct.clone()),
        }
    }

    fn add_core_or_component_ed<'c>(&'c mut self, ed: CoreOrComponentExternDesc<'a>) {
        match ed {
            CoreOrComponentExternDesc::Core(ced) => self.add_core_ed(ced),
            CoreOrComponentExternDesc::Component(ed) => self.add_ed(&ed),
        }
    }

    fn elab_value<'c>(&'c mut self, ctr: &ComponentValType) -> Result<Value<'a>, Error<'a>> {
        match ctr {
            ComponentValType::Type(n) => match &self.types[*n as usize] {
                Defined::Value(vt) => Ok(vt.clone()),
                dt @ Defined::Handleable(Handleable::Var(tv)) => match self.resolve_tyvar(tv) {
                    ResolvedTyvar::Definite(Defined::Value(vt)) => {
                        Ok(Value::Var(Some(tv.clone()), Box::new(vt)))
                    }
                    _ => Err(Error::ValTypeRefToNonVal(dt.clone())),
                },
                dt => Err(Error::ValTypeRefToNonVal(dt.clone())),
            },
            ComponentValType::Primitive(pt) => Ok(match pt {
                PrimitiveValType::Bool => Value::Bool,
                PrimitiveValType::S8 => Value::S(IntWidth::I8),
                PrimitiveValType::U8 => Value::U(IntWidth::I8),
                PrimitiveValType::S16 => Value::S(IntWidth::I16),
                PrimitiveValType::U16 => Value::U(IntWidth::I16),
                PrimitiveValType::S32 => Value::S(IntWidth::I32),
                PrimitiveValType::U32 => Value::U(IntWidth::I32),
                PrimitiveValType::S64 => Value::S(IntWidth::I64),
                PrimitiveValType::U64 => Value::U(IntWidth::I64),
                PrimitiveValType::F32 => Value::F(FloatWidth::F32),
                PrimitiveValType::F64 => Value::F(FloatWidth::F64),
                PrimitiveValType::Char => Value::Char,
                PrimitiveValType::String => Value::String,
                PrimitiveValType::ErrorContext => panic!("async not yet supported"),
            }),
        }
    }

    fn elab_defined_value<'c>(
        &'c mut self,
        vt: &ComponentDefinedType<'a>,
    ) -> Result<Value<'a>, Error<'a>> {
        match vt {
            ComponentDefinedType::Primitive(pvt) => {
                self.elab_value(&ComponentValType::Primitive(*pvt))
            }
            ComponentDefinedType::Record(rfs) => {
                let rfs = rfs
                    .iter()
                    .map(|(name, ty)| {
                        Ok::<_, Error<'a>>(RecordField {
                            name: Name { name },
                            ty: self.elab_value(ty)?,
                        })
                    })
                    .collect::<Result<Vec<_>, Error<'a>>>()?;
                Ok(Value::Record(rfs))
            }
            ComponentDefinedType::Variant(vcs) => {
                let vcs = vcs
                    .iter()
                    .map(|vc| {
                        Ok(VariantCase {
                            name: Name { name: vc.name },
                            ty: vc.ty.as_ref().map(|ty| self.elab_value(ty)).transpose()?,
                            refines: vc.refines,
                        })
                    })
                    .collect::<Result<Vec<_>, Error<'a>>>()?;
                Ok(Value::Variant(vcs))
            }
            ComponentDefinedType::List(vt) => Ok(Value::List(Box::new(self.elab_value(vt)?))),
            ComponentDefinedType::Tuple(vts) => Ok(Value::Tuple(
                vts.iter()
                    .map(|vt| self.elab_value(vt))
                    .collect::<Result<Vec<_>, Error<'a>>>()?,
            )),
            ComponentDefinedType::Flags(ns) => {
                Ok(Value::Flags(ns.iter().map(|n| Name { name: n }).collect()))
            }
            ComponentDefinedType::Enum(ns) => {
                Ok(Value::Enum(ns.iter().map(|n| Name { name: n }).collect()))
            }
            ComponentDefinedType::Option(vt) => Ok(Value::Option(Box::new(self.elab_value(vt)?))),
            ComponentDefinedType::Result { ok, err } => Ok(Value::Result(
                Box::new(ok.map(|ok| self.elab_value(&ok)).transpose()?),
                Box::new(err.map(|err| self.elab_value(&err)).transpose()?),
            )),
            ComponentDefinedType::Own(n) => match &self.types[*n as usize] {
                Defined::Handleable(h) => Ok(Value::Own(h.clone())),
                _ => Err(Error::HandleToNonResource),
            },
            ComponentDefinedType::Borrow(n) => match &self.types[*n as usize] {
                Defined::Handleable(h) => Ok(Value::Borrow(h.clone())),
                _ => Err(Error::HandleToNonResource),
            },
            ComponentDefinedType::FixedSizeList(vt, size) => {
                Ok(Value::FixList(Box::new(self.elab_value(vt)?), *size))
            }
            ComponentDefinedType::Future(_) | ComponentDefinedType::Stream(_) => {
                panic!("async not yet supported")
            }
        }
    }

    fn elab_func<'c>(&'c mut self, ft: &ComponentFuncType<'a>) -> Result<Func<'a>, Error<'a>> {
        Ok(Func {
            params: ft
                .params
                .iter()
                .map(|(n, vt)| {
                    Ok(Param {
                        name: Name { name: n },
                        ty: self.elab_value(vt)?,
                    })
                })
                .collect::<Result<Vec<_>, Error<'a>>>()?,
            result: ft
                .result
                .as_ref()
                .map(|vt| self.elab_value(vt))
                .transpose()?,
        })
    }

    /// Elaborate an extern descriptor. This returns any evars that
    /// are implied by the descriptor separately, to simplify
    /// converting them to context e/u vars, which is usually what you
    /// want to do.
    fn elab_extern_desc<'c>(
        &'c mut self,
        ed: &ComponentTypeRef,
    ) -> Result<(Vec<BoundedTyvar<'a>>, ExternDesc<'a>), Error<'a>> {
        match ed {
            ComponentTypeRef::Module(i) => match &self.core.types[*i as usize] {
                CoreDefined::Module(mt) => Ok((vec![], ExternDesc::CoreModule(mt.clone()))),
                _ => {
                    panic!("internal invariant violation: bad sort for ComponentTypeRef to Module")
                }
            },
            ComponentTypeRef::Func(i) => match &self.types[*i as usize] {
                Defined::Func(ft) => Ok((vec![], ExternDesc::Func(ft.clone()))),
                _ => panic!("internal invariant violation: bad sort for ComponentTypeRef to Func"),
            },
            ComponentTypeRef::Value(_) => panic!("First-class values are not yet supported"),
            ComponentTypeRef::Type(tb) => {
                let bound = match tb {
                    TypeBounds::Eq(i) => TypeBound::Eq(self.types[*i as usize].clone()),
                    TypeBounds::SubResource => TypeBound::SubResource,
                };
                let dt = Defined::Handleable(Handleable::Var(Tyvar::Bound(0)));
                Ok((vec![BoundedTyvar::new(bound)], ExternDesc::Type(dt)))
            }
            ComponentTypeRef::Instance(i) => match &self.types[*i as usize] {
                Defined::Instance(qit) => Ok((
                    qit.evars.clone(),
                    ExternDesc::Instance(qit.unqualified.clone()),
                )),
                _ => panic!(
                    "internal invariant violation: bad sort for ComponentTypeRef to Instance"
                ),
            },
            ComponentTypeRef::Component(i) => match &self.types[*i as usize] {
                Defined::Component(ct) => Ok((vec![], ExternDesc::Component(ct.clone()))),
                _ => panic!(
                    "internal invariant violation: bad sort for ComponentTypeRef to Component"
                ),
            },
        }
    }

    fn elab_instance_decl<'c>(
        &'c mut self,
        decl: &InstanceTypeDeclaration<'a>,
    ) -> Result<Option<ExternDecl<'a>>, Error<'a>> {
        match decl {
            InstanceTypeDeclaration::CoreType(ct) => {
                let ct = self.elab_core_type(ct);
                self.core.types.push(ct);
                Ok(None)
            }
            InstanceTypeDeclaration::Type(t) => {
                let t = self.elab_defined(t)?;
                if let Defined::Handleable(_) = t {
                    return Err(Error::ResourceInDeclarator);
                }
                self.types.push(t);
                Ok(None)
            }
            InstanceTypeDeclaration::Alias(a) => {
                let ed = self.resolve_alias(a)?;
                self.add_core_or_component_ed(ed);
                Ok(None)
            }
            InstanceTypeDeclaration::Export { name, ty } => {
                let (vs, ed) = self.elab_extern_desc(ty)?;
                let sub = self.bound_to_evars(Some(name.0), &vs);
                let ed = sub.extern_desc(&ed).not_void();
                self.add_ed(&ed);
                Ok(Some(ExternDecl {
                    kebab_name: name.0,
                    desc: ed,
                }))
            }
        }
    }

    fn elab_instance<'c>(
        &'c mut self,
        decls: &[InstanceTypeDeclaration<'a>],
    ) -> Result<QualifiedInstance<'a>, Error<'a>> {
        let mut ctx = Ctx::new(Some(self), false);
        let mut exports = Vec::new();
        for decl in decls {
            let export = ctx.elab_instance_decl(decl)?;
            if let Some(export) = export {
                exports.push(export);
            }
        }
        ctx.finish_instance(&exports)
    }

    /// Convert instance variables in the context into bound variables
    /// in the type. This is pulled out separately from raising the
    /// resulting type so that it can be shared between
    /// [`Ctx::finish_instance`] and [`Ctx::finish_component`], which
    /// have different requirements in that respect.
    fn finish_instance_evars(
        self,
        exports: &[ExternDecl<'a>],
    ) -> Result<QualifiedInstance<'a>, Error<'a>> {
        let mut evars = Vec::new();
        let mut sub = substitute::Closing::new(false);
        for (bound, _) in self.evars {
            let bound = sub.bounded_tyvar(&bound)?;
            evars.push(bound);
            sub.next_e();
        }
        let unqualified = sub.instance(&Instance {
            exports: exports.to_vec(),
        })?;
        Ok(QualifiedInstance { evars, unqualified })
    }

    /// The equivalent of the \oplus in the spec.  This has to deal
    /// with more bookkeeping because of our variable representation:
    /// the free variables in the exports need to be converted to
    /// bound variables, and any free variables referring to upper
    /// contexts need to have their parent/outer index reduced by one
    /// to deal with this context ending.
    fn finish_instance(
        self,
        exports: &[ExternDecl<'a>],
    ) -> Result<QualifiedInstance<'a>, Error<'a>> {
        // When we do the well-formedness check in a minute, we need
        // to use the parent ctx, because the closing substitution has
        // already been applied.
        let fallback_parent = Ctx::new(None, false);
        let parent_ctx = self.parent.unwrap_or(&fallback_parent);

        let qi = self.finish_instance_evars(exports)?;
        let raise_u_sub = substitute::Closing::new(true);
        let it = raise_u_sub.qualified_instance(&qi)?;
        parent_ctx
            .wf_qualified_instance(wf::DefinedTypePosition::internal(), &it)
            .map_err(Error::IllFormed)?;
        Ok(it)
    }

    fn elab_component_decl<'c>(
        &'c mut self,
        decl: &ComponentTypeDeclaration<'a>,
    ) -> Result<(Option<ExternDecl<'a>>, Option<ExternDecl<'a>>), Error<'a>> {
        match decl {
            ComponentTypeDeclaration::CoreType(ct) => {
                let ct = self.elab_core_type(ct);
                self.core.types.push(ct);
                Ok((None, None))
            }
            ComponentTypeDeclaration::Type(t) => {
                let t = self.elab_defined(t)?;
                if let Defined::Handleable(_) = t {
                    return Err(Error::ResourceInDeclarator);
                }
                self.types.push(t);
                Ok((None, None))
            }
            ComponentTypeDeclaration::Alias(a) => {
                let ed = self.resolve_alias(a)?;
                self.add_core_or_component_ed(ed);
                Ok((None, None))
            }
            ComponentTypeDeclaration::Export { name, ty, .. } => {
                let (vs, ed) = self.elab_extern_desc(ty)?;
                let sub = self.bound_to_evars(Some(name.0), &vs);
                let ed = sub.extern_desc(&ed).not_void();
                self.add_ed(&ed);
                Ok((
                    None,
                    Some(ExternDecl {
                        kebab_name: name.0,
                        desc: ed,
                    }),
                ))
            }
            ComponentTypeDeclaration::Import(i) => {
                let (vs, ed) = self.elab_extern_desc(&i.ty)?;
                let sub = self.bound_to_uvars(Some(i.name.0), &vs, true);
                let ed = sub.extern_desc(&ed).not_void();
                self.add_ed(&ed);
                Ok((
                    Some(ExternDecl {
                        kebab_name: i.name.0,
                        desc: ed,
                    }),
                    None,
                ))
            }
        }
    }

    /// Similar to [`Ctx::finish_instance`], but for components; this
    /// has to cover uvars as well as evars.
    fn finish_component(
        self,
        imports: &[ExternDecl<'a>],
        exports: &[ExternDecl<'a>],
    ) -> Result<Component<'a>, Error<'a>> {
        // When we do the well-formedness check in a minute, we need
        // to use the parent ctx, because the closing substitution has
        // already been applied.
        let fallback_parent = Ctx::new(None, false);
        let parent_ctx = self.parent.unwrap_or(&fallback_parent);

        let mut uvars = Vec::new();
        let mut sub = substitute::Closing::new(true);
        for (bound, imported) in &self.uvars {
            let bound = sub.bounded_tyvar(bound)?;
            uvars.push(bound);
            sub.next_u(*imported);
        }
        let imports = imports
            .iter()
            .map(|ed| sub.extern_decl(ed).map_err(Into::into))
            .collect::<Result<Vec<ExternDecl<'a>>, Error<'a>>>()?;
        let instance = sub.qualified_instance(&self.finish_instance_evars(exports)?)?;
        let ct = Component {
            uvars,
            imports,
            instance,
        };
        parent_ctx
            .wf_component(wf::DefinedTypePosition::internal(), &ct)
            .map_err(Error::IllFormed)?;
        Ok(ct)
    }

    fn elab_defined<'c>(&'c mut self, dt: &ComponentType<'a>) -> Result<Defined<'a>, Error<'a>> {
        match dt {
            ComponentType::Defined(vt) => Ok(Defined::Value(self.elab_defined_value(vt)?)),
            ComponentType::Func(ft) => Ok(Defined::Func(self.elab_func(ft)?)),
            ComponentType::Component(cds) => Ok(Defined::Component(self.elab_component(cds)?)),
            ComponentType::Instance(ids) => Ok(Defined::Instance(self.elab_instance(ids)?)),
            ComponentType::Resource { dtor, .. } => {
                let rid = ResourceId {
                    id: self.rtypes.len() as u32,
                };
                self.rtypes.push(Resource { _dtor: *dtor });
                Ok(Defined::Handleable(Handleable::Resource(rid)))
            }
        }
    }
}