facet_serialize/

lib.rs

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

extern crate alloc;

use alloc::string::String;
use alloc::vec::Vec;

use facet_core::{Def, Facet, Field, PointerType, ShapeAttribute, StructKind, Type, UserType};
use facet_reflect::{
    FieldIter, FieldsForSerializeIter, HasFields, Peek, PeekListLikeIter, PeekMapIter, PeekSetIter,
    ScalarType,
};
use log::{debug, trace};

fn variant_is_newtype_like(variant: &facet_core::Variant) -> bool {
    variant.data.kind == facet_core::StructKind::Tuple && variant.data.fields.len() == 1
}

// --- Serializer Trait Definition ---

/// A trait for implementing format-specific serialization logic.
/// The core iterative serializer uses this trait to output data.
pub trait Serializer<'shape> {
    /// The error type returned by serialization methods
    type Error;

    /// Serialize an unsigned 64-bit integer.
    fn serialize_u64(&mut self, value: u64) -> Result<(), Self::Error>;

    /// Serialize an unsigned 128-bit integer.
    fn serialize_u128(&mut self, value: u128) -> Result<(), Self::Error>;

    /// Serialize a signed 64-bit integer.
    fn serialize_i64(&mut self, value: i64) -> Result<(), Self::Error>;

    /// Serialize a signed 128-bit integer.
    fn serialize_i128(&mut self, value: i128) -> Result<(), Self::Error>;

    /// Serialize a double-precision floating-point value.
    fn serialize_f64(&mut self, value: f64) -> Result<(), Self::Error>;

    /// Serialize a boolean value.
    fn serialize_bool(&mut self, value: bool) -> Result<(), Self::Error>;

    /// Serialize a character.
    fn serialize_char(&mut self, value: char) -> Result<(), Self::Error>;

    /// Serialize a UTF-8 string slice.
    fn serialize_str(&mut self, value: &str) -> Result<(), Self::Error>;

    /// Serialize a raw byte slice.
    fn serialize_bytes(&mut self, value: &[u8]) -> Result<(), Self::Error>;

    // Special values

    /// Serialize a `None` variant of an Option type.
    fn serialize_none(&mut self) -> Result<(), Self::Error>;

    /// Serialize a unit value `()`.
    fn serialize_unit(&mut self) -> Result<(), Self::Error>;

    // Enum specific values

    /// Serialize a unit variant of an enum (no data).
    ///
    /// # Arguments
    ///
    /// * `variant_index` - The index of the variant.
    /// * `variant_name` - The name of the variant.
    fn serialize_unit_variant(
        &mut self,
        variant_index: usize,
        variant_name: &'shape str,
    ) -> Result<(), Self::Error>;

    /// Begin serializing an object/map-like value.
    ///
    /// # Arguments
    ///
    /// * `len` - The number of fields, if known.
    fn start_object(&mut self, len: Option<usize>) -> Result<(), Self::Error>;

    /// Serialize a field name (for objects and maps).
    ///
    /// # Arguments
    ///
    /// * `name` - The field or key name to serialize.
    fn serialize_field_name(&mut self, name: &'shape str) -> Result<(), Self::Error>;

    /// Begin serializing an array/sequence-like value.
    ///
    /// # Arguments
    ///
    /// * `len` - The number of elements, if known.
    fn start_array(&mut self, len: Option<usize>) -> Result<(), Self::Error>;

    /// Begin serializing a map/dictionary-like value.
    ///
    /// # Arguments
    ///
    /// * `len` - The number of entries, if known.
    fn start_map(&mut self, len: Option<usize>) -> Result<(), Self::Error>;

    /// Serialize an unsigned 8-bit integer.
    #[inline(always)]
    fn serialize_u8(&mut self, value: u8) -> Result<(), Self::Error> {
        self.serialize_u64(value as u64)
    }

    /// Serialize an unsigned 16-bit integer.
    #[inline(always)]
    fn serialize_u16(&mut self, value: u16) -> Result<(), Self::Error> {
        self.serialize_u64(value as u64)
    }

    /// Serialize an unsigned 32-bit integer.
    #[inline(always)]
    fn serialize_u32(&mut self, value: u32) -> Result<(), Self::Error> {
        self.serialize_u64(value as u64)
    }

    /// Serialize a `usize` integer.
    #[inline(always)]
    fn serialize_usize(&mut self, value: usize) -> Result<(), Self::Error> {
        // We assume `usize` will never be >64 bits
        self.serialize_u64(value as u64)
    }

    /// Serialize a signed 8-bit integer.
    #[inline(always)]
    fn serialize_i8(&mut self, value: i8) -> Result<(), Self::Error> {
        self.serialize_i64(value as i64)
    }

    /// Serialize a signed 16-bit integer.
    #[inline(always)]
    fn serialize_i16(&mut self, value: i16) -> Result<(), Self::Error> {
        self.serialize_i64(value as i64)
    }

    /// Serialize a signed 32-bit integer.
    #[inline(always)]
    fn serialize_i32(&mut self, value: i32) -> Result<(), Self::Error> {
        self.serialize_i64(value as i64)
    }

    /// Serialize an `isize` integer.
    #[inline(always)]
    fn serialize_isize(&mut self, value: isize) -> Result<(), Self::Error> {
        // We assume `isize` will never be >64 bits
        self.serialize_i64(value as i64)
    }

    /// Serialize a single-precision floating-point value.
    #[inline(always)]
    fn serialize_f32(&mut self, value: f32) -> Result<(), Self::Error> {
        self.serialize_f64(value as f64)
    }

    /// Begin serializing a map key value.
    #[inline(always)]
    fn begin_map_key(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the end of serializing a map key value.
    #[inline(always)]
    fn end_map_key(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Begin serializing a map value.
    #[inline(always)]
    fn begin_map_value(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the end of serializing a map value.
    #[inline(always)]
    fn end_map_value(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the end of serializing an object/map-like value.
    #[inline(always)]
    fn end_object(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the end of serializing an array/sequence-like value.
    #[inline(always)]
    fn end_array(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the end of serializing a map/dictionary-like value.
    #[inline(always)]
    fn end_map(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the end of serializing a field.
    #[inline(always)]
    fn end_field(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    /// Signal the start of an enum variant
    #[inline(always)]
    fn start_enum_variant(&mut self, discriminant: u64) -> Result<(), Self::Error> {
        let _ = discriminant;
        Ok(())
    }
}

// --- Iterative Serialization Logic ---

/// Task items for the serialization stack.
enum SerializeTask<'mem, 'facet, 'shape> {
    Value(Peek<'mem, 'facet, 'shape>, Option<Field<'shape>>),
    Object {
        entries: FieldsForSerializeIter<'mem, 'facet, 'shape>,
        first: bool,
        len: usize,
    },
    Array {
        items: PeekListLikeIter<'mem, 'facet, 'shape>,
        first: bool,
    },
    Set {
        items: PeekSetIter<'mem, 'facet, 'shape>,
        first: bool,
        len: usize,
    },
    Map {
        entries: PeekMapIter<'mem, 'facet, 'shape>,
        first: bool,
        len: usize,
    },
    TupleStruct {
        items: FieldsForSerializeIter<'mem, 'facet, 'shape>,
        first: bool,
        len: usize,
    },
    Tuple {
        items: FieldIter<'mem, 'facet, 'shape>,
        first: bool,
    },
    // End markers
    EndObject,
    EndArray,
    EndMapKey,
    EndMapValue,
    EndField,
    // Field-related tasks
    SerializeFieldName(&'shape str),
    SerializeMapKey(Peek<'mem, 'facet, 'shape>),
    SerializeMapValue(Peek<'mem, 'facet, 'shape>),
}

/// Serializes a `Peek` value using the provided `Serializer`.
///
/// This function uses an iterative approach with a stack to avoid recursion depth limits.
pub fn serialize_iterative<'mem, 'facet, 'shape, S>(
    peek: Peek<'mem, 'facet, 'shape>,
    serializer: &mut S,
) -> Result<(), S::Error>
where
    S: Serializer<'shape>,
{
    let mut stack = Vec::new();
    stack.push(SerializeTask::Value(peek, None));

    while let Some(task) = stack.pop() {
        match task {
            SerializeTask::Value(mut cpeek, maybe_field) => {
                debug!("Serializing a value, shape is {}", cpeek.shape());

                if cpeek
                    .shape()
                    .attributes
                    .contains(&ShapeAttribute::Transparent)
                {
                    let old_shape = cpeek.shape();

                    // then serialize the inner shape instead
                    let ps = cpeek.into_struct().unwrap();
                    cpeek = ps.field(0).unwrap();

                    let new_shape = cpeek.shape();
                    debug!(
                        "{old_shape} is transparent, let's serialize the inner {new_shape} instead"
                    );
                }

                debug!(
                    "Matching def={:?}, ty={:?} for shape={}",
                    cpeek.shape().def,
                    cpeek.shape().ty,
                    cpeek.shape()
                );
                match (cpeek.shape().def, cpeek.shape().ty) {
                    (Def::Scalar, _) => {
                        let cpeek = cpeek.innermost_peek();

                        // Dispatch to appropriate scalar serialization method based on type
                        match cpeek.scalar_type() {
                            Some(ScalarType::Unit) => serializer.serialize_unit()?,
                            Some(ScalarType::Bool) => {
                                serializer.serialize_bool(*cpeek.get::<bool>().unwrap())?
                            }
                            Some(ScalarType::Char) => {
                                serializer.serialize_char(*cpeek.get::<char>().unwrap())?
                            }

                            // String types
                            Some(ScalarType::Str) => {
                                serializer.serialize_str(cpeek.get::<str>().unwrap())?
                            }
                            Some(ScalarType::String) => {
                                serializer.serialize_str(cpeek.get::<String>().unwrap())?
                            }
                            Some(ScalarType::CowStr) => serializer.serialize_str(
                                cpeek.get::<alloc::borrow::Cow<'_, str>>().unwrap().as_ref(),
                            )?,

                            // Float types
                            Some(ScalarType::F32) => {
                                serializer.serialize_f32(*cpeek.get::<f32>().unwrap())?
                            }
                            Some(ScalarType::F64) => {
                                serializer.serialize_f64(*cpeek.get::<f64>().unwrap())?
                            }

                            // Integer types
                            Some(ScalarType::U8) => {
                                serializer.serialize_u8(*cpeek.get::<u8>().unwrap())?
                            }
                            Some(ScalarType::U16) => {
                                serializer.serialize_u16(*cpeek.get::<u16>().unwrap())?
                            }
                            Some(ScalarType::U32) => {
                                serializer.serialize_u32(*cpeek.get::<u32>().unwrap())?
                            }
                            Some(ScalarType::U64) => {
                                serializer.serialize_u64(*cpeek.get::<u64>().unwrap())?
                            }
                            Some(ScalarType::U128) => {
                                serializer.serialize_u128(*cpeek.get::<u128>().unwrap())?
                            }
                            Some(ScalarType::USize) => {
                                serializer.serialize_usize(*cpeek.get::<usize>().unwrap())?
                            }
                            Some(ScalarType::I8) => {
                                serializer.serialize_i8(*cpeek.get::<i8>().unwrap())?
                            }
                            Some(ScalarType::I16) => {
                                serializer.serialize_i16(*cpeek.get::<i16>().unwrap())?
                            }
                            Some(ScalarType::I32) => {
                                serializer.serialize_i32(*cpeek.get::<i32>().unwrap())?
                            }
                            Some(ScalarType::I64) => {
                                serializer.serialize_i64(*cpeek.get::<i64>().unwrap())?
                            }
                            Some(ScalarType::I128) => {
                                serializer.serialize_i128(*cpeek.get::<i128>().unwrap())?
                            }
                            Some(ScalarType::ISize) => {
                                serializer.serialize_isize(*cpeek.get::<isize>().unwrap())?
                            }
                            Some(unsupported) => {
                                panic!("facet-serialize: unsupported scalar type: {unsupported:?}")
                            }
                            None => {
                                // For other scalar types that don't have a specific ScalarType variant,
                                // try to use Display formatting if available
                                if let Some(_display) =
                                    cpeek.shape().vtable.sized().and_then(|v| (v.display)())
                                {
                                    // Use display formatting if available
                                    serializer.serialize_str(&alloc::format!("{cpeek}"))?
                                } else {
                                    panic!("Unsupported shape (no display): {}", cpeek.shape())
                                }
                            }
                        }
                    }
                    (Def::List(ld), _) => {
                        if ld.t().is_type::<u8>() {
                            // Special case for Vec<u8> - serialize as bytes
                            if cpeek.shape().is_type::<Vec<u8>>() {
                                serializer.serialize_bytes(cpeek.get::<Vec<u8>>().unwrap())?
                            } else {
                                // For other list types with u8 elements (like Bytes/BytesMut),
                                // serialize as array
                                let peek_list = cpeek.into_list_like().unwrap();
                                stack.push(SerializeTask::Array {
                                    items: peek_list.iter(),
                                    first: true,
                                });
                            }
                        } else {
                            let peek_list = cpeek.into_list_like().unwrap();
                            stack.push(SerializeTask::Array {
                                items: peek_list.iter(),
                                first: true,
                            });
                        }
                    }
                    (Def::Array(ad), _) => {
                        if ad.t().is_type::<u8>() {
                            let bytes: Vec<u8> = cpeek
                                .into_list_like()
                                .unwrap()
                                .iter()
                                .map(|p| *p.get::<u8>().unwrap())
                                .collect();
                            serializer.serialize_bytes(&bytes)?;
                        } else {
                            let peek_list = cpeek.into_list_like().unwrap();
                            stack.push(SerializeTask::Array {
                                items: peek_list.iter(),
                                first: true,
                            });
                        }
                    }
                    (Def::Slice(sd), _) => {
                        if sd.t().is_type::<u8>() {
                            serializer.serialize_bytes(cpeek.get::<[u8]>().unwrap())?
                        } else {
                            let peek_list = cpeek.into_list_like().unwrap();
                            stack.push(SerializeTask::Array {
                                items: peek_list.iter(),
                                first: true,
                            });
                        }
                    }
                    (Def::Map(_), _) => {
                        let peek_map = cpeek.into_map().unwrap();
                        let len = peek_map.len();
                        stack.push(SerializeTask::Map {
                            entries: peek_map.iter(),
                            first: true,
                            len,
                        });
                    }
                    (Def::Set(_), _) => {
                        let peek_set = cpeek.into_set().unwrap();
                        stack.push(SerializeTask::Set {
                            items: peek_set.iter(),
                            first: true,
                            len: peek_set.len(),
                        });
                    }
                    (Def::Option(_), _) => {
                        let opt = cpeek.into_option().unwrap();
                        if let Some(inner_peek) = opt.value() {
                            stack.push(SerializeTask::Value(inner_peek, None));
                        } else {
                            serializer.serialize_none()?;
                        }
                    }
                    (Def::Pointer(_), _) => {
                        // For smart pointers, we need to borrow the inner value and serialize it
                        // This is similar to how transparent structs work - we serialize the inner value directly

                        let sp = cpeek.into_pointer().unwrap();
                        if let Some(inner_peek) = sp.borrow_inner() {
                            // Push the inner value to be serialized
                            stack.push(SerializeTask::Value(inner_peek, None));
                        } else {
                            // The smart pointer doesn't support borrowing or has an opaque pointee
                            // We can't serialize it
                            todo!(
                                "Smart pointer without borrow support or with opaque pointee cannot be serialized"
                            );
                        }
                    }
                    (_, Type::User(UserType::Struct(sd))) => {
                        debug!("Serializing struct: shape={}", cpeek.shape(),);
                        debug!(
                            "  Struct details: kind={:?}, field_count={}",
                            sd.kind,
                            sd.fields.len()
                        );

                        match sd.kind {
                            StructKind::Unit => {
                                debug!("  Handling unit struct (no fields)");
                                // Correctly handle unit struct type when encountered as a value
                                serializer.serialize_unit()?;
                            }
                            StructKind::Tuple => {
                                debug!("  Handling tuple with {} fields", sd.fields.len());
                                let peek_struct = cpeek.into_struct().unwrap();
                                let fields_iter = peek_struct.fields();
                                debug!("  Serializing {} fields as tuple", sd.fields.len());

                                stack.push(SerializeTask::Tuple {
                                    items: fields_iter,
                                    first: true,
                                });
                                trace!(
                                    "  Pushed TupleFields to stack for tuple, will handle {} fields",
                                    sd.fields.len()
                                );
                            }
                            StructKind::TupleStruct => {
                                debug!("  Handling tuple struct");
                                let peek_struct = cpeek.into_struct().unwrap();
                                let fields = peek_struct.fields_for_serialize().count();
                                debug!("  Serializing {fields} fields as array");

                                stack.push(SerializeTask::TupleStruct {
                                    items: peek_struct.fields_for_serialize(),
                                    first: true,
                                    len: fields,
                                });
                                trace!(
                                    "  Pushed TupleStructFields to stack, will handle {fields} fields"
                                );
                            }
                            StructKind::Struct => {
                                debug!("  Handling record struct");
                                let peek_struct = cpeek.into_struct().unwrap();
                                let fields = peek_struct.fields_for_serialize().count();
                                debug!("  Serializing {fields} fields as object");

                                stack.push(SerializeTask::Object {
                                    entries: peek_struct.fields_for_serialize(),
                                    first: true,
                                    len: fields,
                                });
                                trace!(
                                    "  Pushed ObjectFields to stack, will handle {fields} fields"
                                );
                            }
                        }
                    }
                    (_, Type::User(UserType::Enum(_))) => {
                        let peek_enum = cpeek.into_enum().unwrap();
                        let variant = peek_enum
                            .active_variant()
                            .expect("Failed to get active variant");
                        let variant_index = peek_enum
                            .variant_index()
                            .expect("Failed to get variant index");
                        trace!("Active variant index is {variant_index}, variant is {variant:?}");
                        let discriminant = variant
                            .discriminant
                            .map(|d| d as u64)
                            .unwrap_or(variant_index as u64);
                        serializer.start_enum_variant(discriminant)?;
                        let flattened = maybe_field.map(|f| f.flattened).unwrap_or_default();

                        if variant.data.fields.is_empty() {
                            // Unit variant
                            serializer.serialize_unit_variant(variant_index, variant.name)?;
                        } else {
                            if !flattened {
                                // For now, treat all enum variants with data as objects
                                serializer.start_object(Some(1))?;
                                stack.push(SerializeTask::EndObject);

                                // Serialize variant name as field name
                                serializer.serialize_field_name(variant.name)?;
                            }

                            if variant_is_newtype_like(variant) {
                                // Newtype variant - serialize the inner value directly
                                let fields = peek_enum.fields_for_serialize().collect::<Vec<_>>();
                                let (field, field_peek) = fields[0];
                                // TODO: error if `skip_serialize` is set?
                                stack.push(SerializeTask::Value(field_peek, Some(field)));
                            } else if variant.data.kind == StructKind::Tuple
                                || variant.data.kind == StructKind::TupleStruct
                            {
                                // Tuple variant - serialize as array
                                let fields = peek_enum.fields_for_serialize().count();
                                serializer.start_array(Some(fields))?;
                                stack.push(SerializeTask::EndArray);

                                // Push fields in reverse order for tuple variant
                                let fields_for_serialize =
                                    peek_enum.fields_for_serialize().collect::<Vec<_>>();
                                for (field, field_peek) in fields_for_serialize.into_iter().rev() {
                                    stack.push(SerializeTask::Value(field_peek, Some(field)));
                                }
                            } else {
                                // Struct variant - serialize as object
                                let fields = peek_enum.fields_for_serialize().count();
                                serializer.start_object(Some(fields))?;
                                stack.push(SerializeTask::EndObject);

                                // Push fields in reverse order for struct variant
                                let fields_for_serialize =
                                    peek_enum.fields_for_serialize().collect::<Vec<_>>();
                                for (field, field_peek) in fields_for_serialize.into_iter().rev() {
                                    stack.push(SerializeTask::EndField);
                                    stack.push(SerializeTask::Value(field_peek, Some(field)));
                                    stack.push(SerializeTask::SerializeFieldName(field.name));
                                }
                            }
                        }
                    }
                    (_, Type::Pointer(pointer_type)) => {
                        // Handle pointer types using our new safe abstraction
                        if let Some(str_value) = cpeek.as_str() {
                            // We have a string value, serialize it
                            serializer.serialize_str(str_value)?;
                        } else if let Some(bytes) = cpeek.as_bytes() {
                            // We have a byte slice, serialize it as bytes
                            serializer.serialize_bytes(bytes)?;
                        } else if let PointerType::Function(_) = pointer_type {
                            // Serialize function pointers as units
                            serializer.serialize_unit()?;
                        } else {
                            // Handle other pointer types with innermost_peek which is safe
                            let innermost = cpeek.innermost_peek();
                            if innermost.shape() != cpeek.shape() {
                                // We got a different inner value, serialize it
                                stack.push(SerializeTask::Value(innermost, None));
                            } else {
                                // Couldn't access inner value safely, fall back to unit
                                serializer.serialize_unit()?;
                            }
                        }
                    }
                    _ => {
                        // Default case for any other definitions
                        debug!(
                            "Unhandled type: {:?}, falling back to unit",
                            cpeek.shape().ty
                        );
                        serializer.serialize_unit()?;
                    }
                }
            }

            SerializeTask::Object {
                mut entries,
                first,
                len,
            } => {
                if first {
                    serializer.start_object(Some(len))?;
                }

                let Some((field, value)) = entries.next() else {
                    serializer.end_object()?;
                    continue;
                };

                stack.push(SerializeTask::Object {
                    entries,
                    first: false,
                    len,
                });
                stack.push(SerializeTask::EndField);
                stack.push(SerializeTask::Value(value, Some(field)));
                stack.push(SerializeTask::SerializeFieldName(field.name));
            }
            SerializeTask::Array { mut items, first } => {
                if first {
                    serializer.start_array(Some(items.len()))?;
                }

                let Some(value) = items.next() else {
                    serializer.end_array()?;
                    continue;
                };

                stack.push(SerializeTask::Array {
                    items,
                    first: false,
                });
                stack.push(SerializeTask::Value(value, None));
            }
            SerializeTask::Set {
                mut items,
                first,
                len,
            } => {
                if first {
                    serializer.start_array(Some(len))?;
                }

                let Some(value) = items.next() else {
                    serializer.end_array()?;
                    continue;
                };

                stack.push(SerializeTask::Set {
                    items,
                    first: false,
                    len,
                });
                stack.push(SerializeTask::Value(value, None));
            }
            SerializeTask::Map {
                mut entries,
                first,
                len,
            } => {
                if first {
                    serializer.start_map(Some(len))?;
                }

                let Some((key, value)) = entries.next() else {
                    serializer.end_map()?;
                    continue;
                };

                stack.push(SerializeTask::Map {
                    entries,
                    first: false,
                    len,
                });
                stack.push(SerializeTask::SerializeMapValue(value));
                stack.push(SerializeTask::SerializeMapKey(key));
            }
            SerializeTask::TupleStruct {
                mut items,
                first,
                len,
            } => {
                if first {
                    serializer.start_array(Some(len))?;
                }

                let Some((field, value)) = items.next() else {
                    serializer.end_array()?;
                    continue;
                };

                stack.push(SerializeTask::TupleStruct {
                    items,
                    first: false,
                    len,
                });
                stack.push(SerializeTask::Value(value, Some(field)));
            }
            SerializeTask::Tuple { mut items, first } => {
                if first {
                    serializer.start_array(Some(items.len()))?;
                }

                let Some((field, value)) = items.next() else {
                    serializer.end_array()?;
                    continue;
                };

                stack.push(SerializeTask::Tuple {
                    items,
                    first: false,
                });
                stack.push(SerializeTask::Value(value, Some(field)));
            }

            // --- Field name and map key/value handling ---
            SerializeTask::SerializeFieldName(name) => {
                serializer.serialize_field_name(name)?;
            }
            SerializeTask::SerializeMapKey(key_peek) => {
                stack.push(SerializeTask::EndMapKey);
                stack.push(SerializeTask::Value(key_peek, None));
                serializer.begin_map_key()?;
            }
            SerializeTask::SerializeMapValue(value_peek) => {
                stack.push(SerializeTask::EndMapValue);
                stack.push(SerializeTask::Value(value_peek, None));
                serializer.begin_map_value()?;
            }

            // --- End composite type tasks ---
            SerializeTask::EndObject => {
                serializer.end_object()?;
            }
            SerializeTask::EndArray => {
                serializer.end_array()?;
            }
            SerializeTask::EndMapKey => {
                serializer.end_map_key()?;
            }
            SerializeTask::EndMapValue => {
                serializer.end_map_value()?;
            }
            SerializeTask::EndField => {
                serializer.end_field()?;
            }
        }
    }

    // Successful completion
    Ok(())
}

// --- Helper Trait for Ergonomics ---

/// Extension trait to simplify calling the generic serializer.
pub trait Serialize<'a>: Facet<'a> {
    /// Serialize this value using the provided `Serializer`.
    fn serialize<'shape, S: Serializer<'shape>>(
        &'a self,
        serializer: &mut S,
    ) -> Result<(), S::Error>;
}

impl<'a, T> Serialize<'a> for T
where
    T: Facet<'a>,
{
    /// Serialize this value using the provided `Serializer`.
    fn serialize<'shape, S: Serializer<'shape>>(
        &'a self,
        serializer: &mut S,
    ) -> Result<(), S::Error> {
        let peek = Peek::new(self);
        serialize_iterative(peek, serializer)
    }
}