facet_deserialize/

lib.rs

#![cfg_attr(not(feature = "std"), no_std)]
#![warn(missing_docs)]
#![warn(clippy::std_instead_of_core)]
#![warn(clippy::std_instead_of_alloc)]
#![deny(unsafe_code)]
#![doc = include_str!("../README.md")]

extern crate alloc;

use alloc::string::ToString;
use alloc::{vec, vec::Vec};

mod error;
use alloc::borrow::Cow;

pub use error::*;

mod span;
use facet_core::{Characteristic, Def, Facet, FieldFlags, ScalarAffinity};
use owo_colors::OwoColorize;
pub use span::*;

use facet_reflect::{HeapValue, ReflectError, Wip};
use log::trace;

#[derive(PartialEq, Debug, Clone)]
/// A scalar value used during deserialization.
/// `u64` and `i64` are separated because `i64` doesn't fit in `u64`,
/// but having `u64` is a fast path for 64-bit architectures — no need to
/// go through `u128` / `i128` for everything
pub enum Scalar<'input> {
    /// Owned or borrowed string data.
    String(Cow<'input, str>),
    /// Unsigned 64-bit integer scalar.
    U64(u64),
    /// Signed 64-bit integer scalar.
    I64(i64),
    /// 64-bit floating-point scalar.
    F64(f64),
    /// Boolean scalar.
    Bool(bool),
    /// Null scalar (e.g. for formats supporting explicit null).
    Null,
}

#[derive(PartialEq, Debug, Clone)]
/// Expected next input token or structure during deserialization.
pub enum Expectation {
    /// Accept a value.
    Value,
    /// Expect an object key or the end of an object.
    ObjectKeyOrObjectClose,
    /// Expect a value inside an object.
    ObjectVal,
    /// Expect a list item or the end of a list.
    ListItemOrListClose,
}

#[derive(PartialEq, Debug, Clone)]
/// Outcome of parsing the next input element.
pub enum Outcome<'input> {
    /// Parsed a scalar value.
    Scalar(Scalar<'input>),
    /// Starting a list/array.
    ListStarted,
    /// Ending a list/array.
    ListEnded,
    /// Starting an object/map.
    ObjectStarted,
    /// Ending an object/map.
    ObjectEnded,
}

impl<'input> From<Scalar<'input>> for Outcome<'input> {
    fn from(scalar: Scalar<'input>) -> Self {
        Outcome::Scalar(scalar)
    }
}

use core::fmt;

/// Display implementation for `Outcome`, focusing on user-friendly descriptions.
impl fmt::Display for Outcome<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Outcome::Scalar(scalar) => write!(f, "scalar {}", scalar),
            Outcome::ListStarted => write!(f, "list start"),
            Outcome::ListEnded => write!(f, "list end"),
            Outcome::ObjectStarted => write!(f, "object start"),
            Outcome::ObjectEnded => write!(f, "object end"),
        }
    }
}

/// Display implementation for `Scalar`, for use in displaying `Outcome`.
impl fmt::Display for Scalar<'_> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Scalar::String(s) => write!(f, "string \"{}\"", s),
            Scalar::U64(val) => write!(f, "u64 {}", val),
            Scalar::I64(val) => write!(f, "i64 {}", val),
            Scalar::F64(val) => write!(f, "f64 {}", val),
            Scalar::Bool(val) => write!(f, "bool {}", val),
            Scalar::Null => write!(f, "null"),
        }
    }
}

impl Outcome<'_> {
    fn into_owned(self) -> Outcome<'static> {
        match self {
            Outcome::Scalar(scalar) => {
                let owned_scalar = match scalar {
                    Scalar::String(cow) => Scalar::String(Cow::Owned(cow.into_owned())),
                    Scalar::U64(val) => Scalar::U64(val),
                    Scalar::I64(val) => Scalar::I64(val),
                    Scalar::F64(val) => Scalar::F64(val),
                    Scalar::Bool(val) => Scalar::Bool(val),
                    Scalar::Null => Scalar::Null,
                };
                Outcome::Scalar(owned_scalar)
            }
            Outcome::ListStarted => Outcome::ListStarted,
            Outcome::ListEnded => Outcome::ListEnded,
            Outcome::ObjectStarted => Outcome::ObjectStarted,
            Outcome::ObjectEnded => Outcome::ObjectEnded,
        }
    }
}

/// Carries the current parsing state and the in-progress value during deserialization.
/// This bundles the mutable context that must be threaded through parsing steps.
pub struct NextData<'input: 'facet, 'facet> {
    /// The offset we're supposed to start parsing from
    start: usize,

    /// Controls the parsing flow and stack state.
    runner: StackRunner<'input>,

    /// Holds the intermediate representation of the value being built.
    pub wip: Wip<'facet>,
}

impl<'input: 'facet, 'facet> NextData<'input, 'facet> {
    /// Returns the input (from the start! not from the current position)
    pub fn input(&self) -> &'input [u8] {
        self.runner.input
    }

    /// Returns the parsing start offset.
    pub fn start(&self) -> usize {
        self.start
    }
}

/// The result of advancing the parser: updated state and parse outcome or error.
pub type NextResult<'input, 'facet, T, E> = (NextData<'input, 'facet>, Result<T, E>);

/// Trait defining a deserialization format.
/// Provides the next parsing step based on current state and expected input.
pub trait Format {
    /// Advance the parser with current state and expectation, producing the next outcome or error.
    fn next<'input, 'facet>(
        &mut self,
        nd: NextData<'input, 'facet>,
        expectation: Expectation,
    ) -> NextResult<'input, 'facet, Spanned<Outcome<'input>>, Spanned<DeserErrorKind>>;

    /// Skip the next value; used to ignore an input.
    fn skip<'input, 'facet>(
        &mut self,
        nd: NextData<'input, 'facet>,
    ) -> NextResult<'input, 'facet, Span, Spanned<DeserErrorKind>>;
}

/// Instructions guiding the parsing flow, indicating the next expected action or token.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Instruction {
    /// Expect a value, specifying the context or reason.
    Value(ValueReason),
    /// Skip the next value; used to ignore an input.
    SkipValue,
    /// Indicate completion of a structure or value; triggers popping from stack.
    Pop(PopReason),
    /// Expect an object key or the end of an object.
    ObjectKeyOrObjectClose,
    /// Expect a list item or the end of a list.
    ListItemOrListClose,
}

/// Reasons for expecting a value, reflecting the current parse context.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ValueReason {
    /// Parsing at the root level.
    TopLevel,
    /// Parsing a value inside an object.
    ObjectVal,
}

/// Reasons for popping a state from the stack, indicating why a scope is ended.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum PopReason {
    /// Ending the top-level parsing scope.
    TopLevel,
    /// Ending a value within an object.
    ObjectVal,
    /// Ending value within a list
    ListVal,
    /// Ending a `Some()` in an option
    Some,
}

/// Deserialize a value of type `T` from raw input bytes using format `F`.
///
/// This function sets up the initial working state and drives the deserialization process,
/// ensuring that the resulting value is fully materialized and valid.
pub fn deserialize<'input, 'facet, T, F>(
    input: &'input [u8],
    format: F,
) -> Result<T, DeserError<'input>>
where
    T: Facet<'facet>,
    F: Format,
    'input: 'facet,
{
    let wip = Wip::alloc_shape(T::SHAPE).map_err(|e| DeserError {
        input: input.into(),
        span: Span { start: 0, len: 0 },
        kind: DeserErrorKind::ReflectError(e),
    })?;
    deserialize_wip(wip, input, format)?
        .materialize()
        .map_err(|e| DeserError::new_reflect(e, input, Span { start: 0, len: 0 }))
}

/// Deserializes a working-in-progress value into a fully materialized heap value.
/// This function drives the parsing loop until the entire input is consumed and the value is complete.
pub fn deserialize_wip<'input, 'facet, F>(
    mut wip: Wip<'facet>,
    input: &'input [u8],
    mut format: F,
) -> Result<HeapValue<'facet>, DeserError<'input>>
where
    F: Format,
    'input: 'facet,
{
    // This struct is just a bundle of the state that we need to pass around all the time.
    let mut runner = StackRunner {
        original_input: input,
        input,
        stack: vec![
            Instruction::Pop(PopReason::TopLevel),
            Instruction::Value(ValueReason::TopLevel),
        ],
        last_span: Span::new(0, 0),
    };

    macro_rules! next {
        ($runner:ident, $wip:ident, $expectation:expr, $method:ident) => {{
            let nd = NextData {
                start: $runner.last_span.end(), // or supply the appropriate start value if available
                runner: $runner,
                wip: $wip,
            };
            let (nd, res) = format.next(nd, $expectation);
            $runner = nd.runner;
            $wip = nd.wip;
            let outcome = res.map_err(|span_kind| {
                $runner.last_span = span_kind.span;
                $runner.err(span_kind.node)
            })?;
            $runner.last_span = outcome.span;
            trace!("Got outcome {}", outcome.blue());
            $wip = $runner.$method($wip, outcome)?;
        }};
    }

    loop {
        let frame_count = wip.frames_count();
        debug_assert!(
            frame_count
                >= runner
                    .stack
                    .iter()
                    .filter(|f| matches!(f, Instruction::Pop(_)))
                    .count()
        );

        let insn = match runner.stack.pop() {
            Some(insn) => insn,
            None => unreachable!("Instruction stack is empty"),
        };

        trace!("[{frame_count}] Instruction {:?}", insn.yellow());

        match insn {
            Instruction::Pop(reason) => {
                wip = runner.pop(wip, reason)?;

                if reason == PopReason::TopLevel {
                    return wip.build().map_err(|e| runner.reflect_err(e));
                } else {
                    wip = wip.pop().map_err(|e| runner.reflect_err(e))?;
                }
            }
            Instruction::Value(_why) => {
                let expectation = match _why {
                    ValueReason::TopLevel => Expectation::Value,
                    ValueReason::ObjectVal => Expectation::ObjectVal,
                };
                next!(runner, wip, expectation, value);
            }
            Instruction::ObjectKeyOrObjectClose => {
                next!(
                    runner,
                    wip,
                    Expectation::ObjectKeyOrObjectClose,
                    object_key_or_object_close
                );
            }
            Instruction::ListItemOrListClose => {
                next!(
                    runner,
                    wip,
                    Expectation::ListItemOrListClose,
                    list_item_or_list_close
                );
            }
            Instruction::SkipValue => {
                // Call F::skip to skip over the next value in the input
                let nd = NextData {
                    start: runner.last_span.end(),
                    runner,
                    wip,
                };
                let (nd, res) = format.skip(nd);
                runner = nd.runner;
                wip = nd.wip;
                // Only propagate error, don't modify wip, since skip just advances input
                let span = res.map_err(|span_kind| {
                    runner.last_span = span_kind.span;
                    runner.err(span_kind.node)
                })?;
                // do the actual skip
                runner.last_span = span;
            }
        }
    }
}

#[doc(hidden)]
/// Maintains the parsing state and context necessary to drive deserialization.
///
/// This struct tracks what the parser expects next, manages input position,
/// and remembers the span of the last processed token to provide accurate error reporting.
pub struct StackRunner<'input> {
    /// A version of the input that doesn't advance as we parse.
    original_input: &'input [u8],
    /// The raw input data being deserialized.
    pub input: &'input [u8],

    /// Stack of parsing instructions guiding the control flow.
    pub stack: Vec<Instruction>,
    /// Span of the last processed token, for accurate error reporting.
    pub last_span: Span,
}

impl<'input> StackRunner<'input> {
    /// Convenience function to create a DeserError using the original input and last_span.
    fn err(&self, kind: DeserErrorKind) -> DeserError<'input> {
        DeserError::new(kind, self.original_input, self.last_span)
    }

    /// Convenience function to create a DeserError from a ReflectError,
    /// using the original input and last_span for context.
    fn reflect_err(&self, err: ReflectError) -> DeserError<'input> {
        DeserError::new_reflect(err, self.original_input, self.last_span)
    }

    fn pop<'facet>(
        &mut self,
        mut wip: Wip<'facet>,
        reason: PopReason,
    ) -> Result<Wip<'facet>, DeserError<'input>> {
        trace!("Popping because {:?}", reason.yellow());

        let container_shape = wip.shape();
        match container_shape.def {
            Def::Struct(sd) => {
                let mut has_unset = false;

                trace!("Let's check all fields are initialized");
                for (index, field) in sd.fields.iter().enumerate() {
                    let is_set = wip.is_field_set(index).map_err(|err| {
                        trace!("Error checking field set status: {:?}", err);
                        self.reflect_err(err)
                    })?;
                    if !is_set {
                        if field.flags.contains(FieldFlags::DEFAULT) {
                            wip = wip.field(index).map_err(|e| self.reflect_err(e))?;
                            if let Some(default_in_place_fn) = field.vtable.default_fn {
                                wip = wip
                                    .put_from_fn(default_in_place_fn)
                                    .map_err(|e| self.reflect_err(e))?;
                                trace!(
                                    "Field #{} {:?} was set to default value (via custom fn)",
                                    index.yellow(),
                                    field.blue()
                                );
                            } else {
                                if !field.shape().is(Characteristic::Default) {
                                    return Err(self.reflect_err(
                                        ReflectError::DefaultAttrButNoDefaultImpl {
                                            shape: field.shape(),
                                        },
                                    ));
                                }
                                wip = wip.put_default().map_err(|e| self.reflect_err(e))?;
                                trace!(
                                    "Field #{} {:?} was set to default value (via default impl)",
                                    index.yellow(),
                                    field.blue()
                                );
                            }
                            wip = wip.pop().map_err(|e| self.reflect_err(e))?;
                        } else {
                            trace!(
                                "Field #{} {:?} is not initialized",
                                index.yellow(),
                                field.blue()
                            );
                            has_unset = true;
                        }
                    }
                }

                if has_unset && container_shape.has_default_attr() {
                    // let's allocate and build a default value
                    let default_val = Wip::alloc_shape(container_shape)
                        .map_err(|e| self.reflect_err(e))?
                        .put_default()
                        .map_err(|e| self.reflect_err(e))?
                        .build()
                        .map_err(|e| self.reflect_err(e))?;
                    let peek = default_val.peek().into_struct().unwrap();

                    for (index, field) in sd.fields.iter().enumerate() {
                        let is_set = wip.is_field_set(index).map_err(|err| {
                            trace!("Error checking field set status: {:?}", err);
                            self.reflect_err(err)
                        })?;
                        if !is_set {
                            let address_of_field_from_default = peek.field(index).unwrap().data();
                            wip = wip.field(index).map_err(|e| self.reflect_err(e))?;
                            wip = wip
                                .put_shape(address_of_field_from_default, field.shape())
                                .map_err(|e| self.reflect_err(e))?;
                            wip = wip.pop().map_err(|e| self.reflect_err(e))?;
                        }
                    }
                }
            }
            Def::Enum(_) => {
                trace!(
                    "TODO: make sure enums are initialized (support container-level and field-level default, etc.)"
                );
            }
            _ => {
                trace!(
                    "Thing being popped is not a container I guess (it's a {})",
                    wip.shape()
                );
            }
        }
        Ok(wip)
    }

    /// Internal common handler for GotScalar outcome, to deduplicate code.
    fn handle_scalar<'facet>(
        &self,
        wip: Wip<'facet>,
        scalar: Scalar<'input>,
    ) -> Result<Wip<'facet>, DeserError<'input>> {
        match scalar {
            Scalar::String(cow) => {
                match wip.innermost_shape().def {
                    Def::Enum(_) => {
                        if wip.selected_variant().is_some() {
                            // If we already have a variant selected, just put the string
                            wip.put(cow.to_string()).map_err(|e| self.reflect_err(e))
                        } else {
                            // Try to select the variant
                            match wip.find_variant(&cow) {
                                Some((variant_index, _)) => {
                                    wip.variant(variant_index).map_err(|e| self.reflect_err(e))
                                }
                                None => Err(self.err(DeserErrorKind::NoSuchVariant {
                                    name: cow.to_string(),
                                    enum_shape: wip.innermost_shape(),
                                })),
                            }
                        }
                    }
                    _ => wip.put(cow.to_string()).map_err(|e| self.reflect_err(e)),
                }
            }
            Scalar::U64(value) => wip.put(value).map_err(|e| self.reflect_err(e)),
            Scalar::I64(value) => wip.put(value).map_err(|e| self.reflect_err(e)),
            Scalar::F64(value) => wip.put(value).map_err(|e| self.reflect_err(e)),
            Scalar::Bool(value) => wip.put(value).map_err(|e| self.reflect_err(e)),
            Scalar::Null => wip.put_default().map_err(|e| self.reflect_err(e)),
        }
    }

    /// Handle value parsing
    fn value<'facet>(
        &mut self,
        mut wip: Wip<'facet>,
        outcome: Spanned<Outcome<'input>>,
    ) -> Result<Wip<'facet>, DeserError<'input>> {
        trace!(
            "Handling value at wip shape {} (wip innermost shape {})",
            wip.shape().blue(),
            wip.innermost_shape().yellow()
        );

        match outcome.node {
            Outcome::Scalar(Scalar::Null) => {
                return wip.put_default().map_err(|e| self.reflect_err(e));
            }
            _ => {
                if matches!(wip.shape().def, Def::Option(_)) {
                    trace!("Starting Some(_) option for {}", wip.shape().blue());
                    wip = wip.push_some().map_err(|e| self.reflect_err(e))?;
                    self.stack.push(Instruction::Pop(PopReason::Some));
                }
            }
        }

        match outcome.node {
            Outcome::Scalar(s) => {
                wip = self.handle_scalar(wip, s)?;
            }
            Outcome::ListStarted => {
                let shape = wip.innermost_shape();
                match shape.def {
                    Def::Array(_) => {
                        trace!("Array starting for array ({})!", shape.blue());
                    }
                    Def::Slice(_) => {
                        trace!("Array starting for slice ({})!", shape.blue());
                    }
                    Def::List(_) => {
                        trace!("Array starting for list ({})!", shape.blue());
                        wip = wip.put_default().map_err(|e| self.reflect_err(e))?;
                    }
                    Def::Enum(_) => {
                        trace!("Array starting for enum ({})!", shape.blue());
                    }
                    Def::Struct(_) => {
                        trace!("Array starting for tuple ({})!", shape.blue());
                        wip = wip.put_default().map_err(|e| self.reflect_err(e))?;
                    }
                    Def::Scalar(sd) => {
                        if matches!(sd.affinity, ScalarAffinity::Empty(_)) {
                            trace!("Empty tuple/scalar, nice");
                            wip = wip.put_default().map_err(|e| self.reflect_err(e))?;
                        } else {
                            return Err(self.err(DeserErrorKind::UnsupportedType {
                                got: shape,
                                wanted: "array, list, tuple, or slice",
                            }));
                        }
                    }
                    _ => {
                        return Err(self.err(DeserErrorKind::UnsupportedType {
                            got: shape,
                            wanted: "array, list, tuple, or slice",
                        }));
                    }
                }

                trace!("Beginning pushback");
                self.stack.push(Instruction::ListItemOrListClose);
                wip = wip.begin_pushback().map_err(|e| self.reflect_err(e))?;
            }
            Outcome::ListEnded => {
                trace!("List closing");
                wip = wip.pop().map_err(|e| self.reflect_err(e))?;
            }
            Outcome::ObjectStarted => {
                let shape = wip.innermost_shape();
                match shape.def {
                    Def::Map(_md) => {
                        trace!("Object starting for map value ({})!", shape.blue());
                        wip = wip.put_default().map_err(|e| self.reflect_err(e))?;
                    }
                    Def::Enum(_ed) => {
                        trace!("Object starting for enum value ({})!", shape.blue());
                        // nothing to do here
                    }
                    Def::Struct(_) => {
                        trace!("Object starting for struct value ({})!", shape.blue());
                        // nothing to do here
                    }
                    _ => {
                        return Err(self.err(DeserErrorKind::UnsupportedType {
                            got: shape,
                            wanted: "map, enum, or struct",
                        }));
                    }
                }

                self.stack.push(Instruction::ObjectKeyOrObjectClose);
            }
            Outcome::ObjectEnded => todo!(),
        }
        Ok(wip)
    }

    fn object_key_or_object_close<'facet>(
        &mut self,
        mut wip: Wip<'facet>,
        outcome: Spanned<Outcome<'input>>,
    ) -> Result<Wip<'facet>, DeserError<'input>>
    where
        'input: 'facet,
    {
        match outcome.node {
            Outcome::Scalar(Scalar::String(key)) => {
                trace!("Parsed object key: {}", key);

                let mut ignore = false;
                let mut needs_pop = true;
                let mut handled_by_flatten = false;

                match wip.innermost_shape().def {
                    Def::Struct(sd) => {
                        // First try to find a direct field match
                        if let Some(index) = wip.field_index(&key) {
                            trace!("It's a struct field");
                            wip = wip.field(index).map_err(|e| self.reflect_err(e))?;
                        } else {
                            // Check for flattened fields
                            let mut found_in_flatten = false;
                            for (index, field) in sd.fields.iter().enumerate() {
                                if field.flags.contains(FieldFlags::FLATTEN) {
                                    trace!("Found flattened field #{}", index);
                                    // Enter the flattened field
                                    wip = wip.field(index).map_err(|e| self.reflect_err(e))?;

                                    // Check if this flattened field has the requested key
                                    if let Some(subfield_index) = wip.field_index(&key) {
                                        trace!("Found key {} in flattened field", key);
                                        wip = wip
                                            .field(subfield_index)
                                            .map_err(|e| self.reflect_err(e))?;
                                        found_in_flatten = true;
                                        handled_by_flatten = true;
                                        break;
                                    } else if let Some((_variant_index, _variant)) =
                                        wip.find_variant(&key)
                                    {
                                        trace!("Found key {} in flattened field", key);
                                        wip = wip
                                            .variant_named(&key)
                                            .map_err(|e| self.reflect_err(e))?;
                                        found_in_flatten = true;
                                        break;
                                    } else {
                                        // Key not in this flattened field, go back up
                                        wip = wip.pop().map_err(|e| self.reflect_err(e))?;
                                    }
                                }
                            }

                            if !found_in_flatten {
                                if wip.shape().has_deny_unknown_fields_attr() {
                                    trace!(
                                        "It's not a struct field AND we're denying unknown fields"
                                    );
                                    return Err(self.err(DeserErrorKind::UnknownField {
                                        field_name: key.to_string(),
                                        shape: wip.shape(),
                                    }));
                                } else {
                                    trace!(
                                        "It's not a struct field and we're ignoring unknown fields"
                                    );
                                    ignore = true;
                                }
                            }
                        }
                    }
                    Def::Enum(_ed) => match wip.find_variant(&key) {
                        Some((index, variant)) => {
                            trace!("Variant {} selected", variant.name.blue());
                            wip = wip.variant(index).map_err(|e| self.reflect_err(e))?;
                            needs_pop = false;
                        }
                        None => {
                            if let Some(_variant_index) = wip.selected_variant() {
                                trace!(
                                    "Already have a variant selected, treating key as struct field of variant"
                                );
                                // Try to find the field index of the key within the selected variant
                                if let Some(index) = wip.field_index(&key) {
                                    trace!("Found field {} in selected variant", key.blue());
                                    wip = wip.field(index).map_err(|e| self.reflect_err(e))?;
                                } else if wip.shape().has_deny_unknown_fields_attr() {
                                    trace!("Unknown field in variant and denying unknown fields");
                                    return Err(self.err(DeserErrorKind::UnknownField {
                                        field_name: key.to_string(),
                                        shape: wip.shape(),
                                    }));
                                } else {
                                    trace!("Ignoring unknown field in variant");
                                    ignore = true;
                                }
                            } else {
                                return Err(self.err(DeserErrorKind::NoSuchVariant {
                                    name: key.to_string(),
                                    enum_shape: wip.shape(),
                                }));
                            }
                        }
                    },
                    Def::Map(_) => {
                        wip = wip.push_map_key().map_err(|e| self.reflect_err(e))?;
                        wip = wip.put(key.to_string()).map_err(|e| self.reflect_err(e))?;
                        wip = wip.push_map_value().map_err(|e| self.reflect_err(e))?;
                    }
                    _ => {
                        return Err(self.err(DeserErrorKind::Unimplemented(
                            "object key for non-struct/map",
                        )));
                    }
                }

                self.stack.push(Instruction::ObjectKeyOrObjectClose);
                if ignore {
                    self.stack.push(Instruction::SkipValue);
                } else {
                    if needs_pop && !handled_by_flatten {
                        trace!("Pushing Pop insn to stack (ObjectVal)");
                        self.stack.push(Instruction::Pop(PopReason::ObjectVal));
                    } else if handled_by_flatten {
                        // We need two pops for flattened fields - one for the field itself,
                        // one for the containing struct
                        trace!("Pushing Pop insn to stack (ObjectVal) for flattened field");
                        self.stack.push(Instruction::Pop(PopReason::ObjectVal));
                        self.stack.push(Instruction::Pop(PopReason::ObjectVal));
                    }
                    self.stack.push(Instruction::Value(ValueReason::ObjectVal));
                }
                Ok(wip)
            }
            Outcome::ObjectEnded => {
                trace!("Object closing");
                Ok(wip)
            }
            _ => Err(self.err(DeserErrorKind::UnexpectedOutcome {
                got: outcome.node.into_owned(),
                wanted: "scalar or object close",
            })),
        }
    }

    fn list_item_or_list_close<'facet>(
        &mut self,
        mut wip: Wip<'facet>,
        outcome: Spanned<Outcome<'input>>,
    ) -> Result<Wip<'facet>, DeserError<'input>>
    where
        'input: 'facet,
    {
        match outcome.node {
            Outcome::ListEnded => {
                trace!("List close");
                Ok(wip)
            }
            _ => {
                self.stack.push(Instruction::ListItemOrListClose);
                self.stack.push(Instruction::Pop(PopReason::ListVal));

                trace!(
                    "Expecting list item, doing a little push before doing value with outcome {}",
                    outcome.magenta()
                );
                trace!("Before push, wip.shape is {}", wip.shape().blue());
                wip = wip.push().map_err(|e| self.reflect_err(e))?;
                trace!(" After push, wip.shape is {}", wip.shape().cyan());
                wip = self.value(wip, outcome)?;
                Ok(wip)
            }
        }
    }
}