wasmparser/validator/

core.rs

//! State relating to validating a WebAssembly module.
//!

mod canonical;
pub(crate) use canonical::InternRecGroup;

use self::arc::MaybeOwned;
use super::{
    check_max, combine_type_sizes,
    operators::{OperatorValidator, OperatorValidatorAllocations, ty_to_str},
    types::{CoreTypeId, EntityType, RecGroupId, TypeAlloc, TypeList},
};
#[cfg(feature = "simd")]
use crate::VisitSimdOperator;
use crate::{
    BinaryReaderError, ConstExpr, Data, DataKind, Element, ElementKind, ExternalKind, FuncType,
    Global, GlobalType, HeapType, MemoryType, RecGroup, RefType, Result, SubType, Table, TableInit,
    TableType, TagType, TypeRef, UnpackedIndex, ValType, VisitOperator, WasmFeatures,
    WasmModuleResources, limits::*,
};
use crate::{CompositeInnerType, prelude::*};
use alloc::sync::Arc;
use core::mem;

pub(crate) struct ModuleState {
    /// Internal state that is incrementally built-up for the module being
    /// validated. This houses type information for all wasm items, like
    /// functions. Note that this starts out as a solely owned `Arc<T>` so we can
    /// get mutable access, but after we get to the code section this is never
    /// mutated to we can clone it cheaply and hand it to sub-validators.
    pub module: arc::MaybeOwned<Module>,

    const_expr_allocs: OperatorValidatorAllocations,

    /// When parsing the code section, represents the current index in the section.
    code_section_index: Option<usize>,
}

impl ModuleState {
    pub fn new(features: WasmFeatures) -> ModuleState {
        ModuleState {
            module: arc::MaybeOwned::new(Module::new(features)),
            const_expr_allocs: OperatorValidatorAllocations::default(),
            code_section_index: None,
        }
    }

    pub fn next_code_index_and_type(&mut self) -> (u32, u32) {
        let index = self
            .code_section_index
            .get_or_insert(self.module.num_imported_functions as usize);

        assert!(*index < self.module.functions.len());

        let ty = self.module.functions[*index];
        *index += 1;

        ((*index - 1) as u32, ty)
    }

    pub fn add_global(
        &mut self,
        mut global: Global,
        types: &TypeList,
        offset: usize,
    ) -> Result<()> {
        self.module
            .check_global_type(&mut global.ty, types, offset)?;
        self.check_const_expr(&global.init_expr, global.ty.content_type, types)?;
        self.module.assert_mut().globals.push(global.ty);
        Ok(())
    }

    pub fn add_table(
        &mut self,
        mut table: Table<'_>,
        types: &TypeList,
        offset: usize,
    ) -> Result<()> {
        self.module.check_table_type(&mut table.ty, types, offset)?;

        match &table.init {
            TableInit::RefNull => {
                if !table.ty.element_type.is_nullable() {
                    bail!(offset, "type mismatch: non-defaultable element type");
                }
            }
            TableInit::Expr(expr) => {
                if !self.module.features.function_references() {
                    bail!(
                        offset,
                        "tables with expression initializers require \
                         the function-references proposal"
                    );
                }
                self.check_const_expr(expr, table.ty.element_type.into(), types)?;
            }
        }
        self.module.assert_mut().tables.push(table.ty);
        Ok(())
    }

    pub fn add_data_segment(&mut self, data: Data, types: &TypeList, offset: usize) -> Result<()> {
        match data.kind {
            DataKind::Passive => {
                if !self.module.features.bulk_memory() {
                    bail!(
                        offset,
                        "passive data segments require the bulk-memory proposal"
                    );
                }
                Ok(())
            }
            DataKind::Active {
                memory_index,
                offset_expr,
            } => {
                let ty = self.module.memory_at(memory_index, offset)?.index_type();
                self.check_const_expr(&offset_expr, ty, types)
            }
        }
    }

    pub fn add_element_segment(
        &mut self,
        mut e: Element,
        types: &TypeList,
        offset: usize,
    ) -> Result<()> {
        // the `funcref` value type is allowed all the way back to the MVP, so
        // don't check it here
        let element_ty = match &mut e.items {
            crate::ElementItems::Functions(_) => RefType::FUNC,
            crate::ElementItems::Expressions(ty, _) => {
                self.module.check_ref_type(ty, offset)?;
                *ty
            }
        };

        match e.kind {
            ElementKind::Active {
                table_index,
                offset_expr,
            } => {
                let table = self.module.table_at(table_index.unwrap_or(0), offset)?;
                if !types.reftype_is_subtype(element_ty, table.element_type) {
                    return Err(BinaryReaderError::new(
                        format!(
                            "type mismatch: invalid element type `{}` for table type `{}`",
                            ty_to_str(element_ty.into()),
                            ty_to_str(table.element_type.into()),
                        ),
                        offset,
                    ));
                }

                self.check_const_expr(&offset_expr, table.index_type(), types)?;
            }
            ElementKind::Passive | ElementKind::Declared => {
                if !self.module.features.bulk_memory() {
                    return Err(BinaryReaderError::new(
                        "bulk memory must be enabled",
                        offset,
                    ));
                }
            }
        }

        let validate_count = |count: u32| -> Result<(), BinaryReaderError> {
            if count > MAX_WASM_TABLE_ENTRIES as u32 {
                Err(BinaryReaderError::new(
                    "number of elements is out of bounds",
                    offset,
                ))
            } else {
                Ok(())
            }
        };
        match e.items {
            crate::ElementItems::Functions(reader) => {
                let count = reader.count();
                validate_count(count)?;
                for f in reader.into_iter_with_offsets() {
                    let (offset, f) = f?;
                    self.module.get_func_type(f, types, offset)?;
                    self.module.assert_mut().function_references.insert(f);
                }
            }
            crate::ElementItems::Expressions(ty, reader) => {
                validate_count(reader.count())?;
                for expr in reader {
                    self.check_const_expr(&expr?, ValType::Ref(ty), types)?;
                }
            }
        }
        self.module.assert_mut().element_types.push(element_ty);
        Ok(())
    }

    fn check_const_expr(
        &mut self,
        expr: &ConstExpr<'_>,
        expected_ty: ValType,
        types: &TypeList,
    ) -> Result<()> {
        let mut validator = VisitConstOperator {
            offset: 0,
            uninserted_funcref: false,
            ops: OperatorValidator::new_const_expr(
                &self.module.features,
                expected_ty,
                mem::take(&mut self.const_expr_allocs),
            ),
            resources: OperatorValidatorResources {
                types,
                module: &mut self.module,
            },
        };

        let mut ops = expr.get_operators_reader();
        while !ops.eof() {
            validator.offset = ops.original_position();
            ops.visit_operator(&mut validator)??;
        }

        // See comment in `RefFunc` below for why this is an assert.
        assert!(!validator.uninserted_funcref);

        self.const_expr_allocs = validator.ops.into_allocations();

        return Ok(());

        struct VisitConstOperator<'a> {
            offset: usize,
            uninserted_funcref: bool,
            ops: OperatorValidator,
            resources: OperatorValidatorResources<'a>,
        }

        impl VisitConstOperator<'_> {
            fn validator(&mut self) -> impl VisitOperator<'_, Output = Result<()>> {
                self.ops.with_resources(&self.resources, self.offset)
            }

            fn validate_extended_const(&mut self, op: &str) -> Result<()> {
                if self.ops.features.extended_const() {
                    Ok(())
                } else {
                    Err(BinaryReaderError::new(
                        format!("constant expression required: non-constant operator: {op}"),
                        self.offset,
                    ))
                }
            }

            fn validate_gc(&mut self, op: &str) -> Result<()> {
                if self.ops.features.gc() {
                    Ok(())
                } else {
                    Err(BinaryReaderError::new(
                        format!("constant expression required: non-constant operator: {op}"),
                        self.offset,
                    ))
                }
            }

            fn validate_shared_everything_threads(&mut self, op: &str) -> Result<()> {
                if self.ops.features.shared_everything_threads() {
                    Ok(())
                } else {
                    Err(BinaryReaderError::new(
                        format!("constant expression required: non-constant operator: {op}"),
                        self.offset,
                    ))
                }
            }

            fn validate_global(&mut self, index: u32) -> Result<()> {
                let module = &self.resources.module;
                let global = module.global_at(index, self.offset)?;

                if index >= module.num_imported_globals && !self.ops.features.gc() {
                    return Err(BinaryReaderError::new(
                        "constant expression required: global.get of locally defined global",
                        self.offset,
                    ));
                }
                if global.mutable {
                    return Err(BinaryReaderError::new(
                        "constant expression required: global.get of mutable global",
                        self.offset,
                    ));
                }
                Ok(())
            }

            // Functions in initialization expressions are only valid in
            // element segment initialization expressions and globals. In
            // these contexts we want to record all function references.
            //
            // Initialization expressions can also be found in the data
            // section, however. A `RefFunc` instruction in those situations
            // is always invalid and needs to produce a validation error. In
            // this situation, though, we can no longer modify
            // the state since it's been "snapshot" already for
            // parallel validation of functions.
            //
            // If we cannot modify the function references then this function
            // *should* result in a validation error, but we defer that
            // validation error to happen later. The `uninserted_funcref`
            // boolean here is used to track this and will cause a panic
            // (aka a fuzz bug) if we somehow forget to emit an error somewhere
            // else.
            fn insert_ref_func(&mut self, index: u32) {
                if self.resources.module.as_mut().is_none() {
                    self.uninserted_funcref = true;
                } else {
                    self.resources
                        .module
                        .assert_mut()
                        .function_references
                        .insert(index);
                }
            }

            fn not_const(&self, instr: &str) -> BinaryReaderError {
                BinaryReaderError::new(
                    format!("constant expression required: non-constant operator: {instr}"),
                    self.offset,
                )
            }
        }

        #[cfg_attr(not(feature = "simd"), allow(unused_macro_rules))]
        macro_rules! define_visit_operator {
            ($(@$proposal:ident $op:ident $({ $($arg:ident: $argty:ty),* })? => $visit:ident ($($ann:tt)*))*) => {
                $(
                    #[allow(unused_variables)]
                    fn $visit(&mut self $($(,$arg: $argty)*)?) -> Self::Output {
                        define_visit_operator!(@visit self $visit $($($arg)*)?)
                    }
                )*
            };

            // These are always valid in const expressions
            (@visit $self:ident visit_i32_const $val:ident) => {{
                $self.validator().visit_i32_const($val)
            }};
            (@visit $self:ident visit_i64_const $val:ident) => {{
                $self.validator().visit_i64_const($val)
            }};
            (@visit $self:ident visit_f32_const $val:ident) => {{
                $self.validator().visit_f32_const($val)
            }};
            (@visit $self:ident visit_f64_const $val:ident) => {{
                $self.validator().visit_f64_const($val)
            }};
            (@visit $self:ident visit_v128_const $val:ident) => {{
                $self.validator().simd_visitor().unwrap().visit_v128_const($val)
            }};
            (@visit $self:ident visit_ref_null $val:ident) => {{
                $self.validator().visit_ref_null($val)
            }};
            (@visit $self:ident visit_end) => {{
                $self.validator().visit_end()
            }};


            // These are valid const expressions when the extended-const proposal is enabled.
            (@visit $self:ident visit_i32_add) => {{
                $self.validate_extended_const("i32.add")?;
                $self.validator().visit_i32_add()
            }};
            (@visit $self:ident visit_i32_sub) => {{
                $self.validate_extended_const("i32.sub")?;
                $self.validator().visit_i32_sub()
            }};
            (@visit $self:ident visit_i32_mul) => {{
                $self.validate_extended_const("i32.mul")?;
                $self.validator().visit_i32_mul()
            }};
            (@visit $self:ident visit_i64_add) => {{
                $self.validate_extended_const("i64.add")?;
                $self.validator().visit_i64_add()
            }};
            (@visit $self:ident visit_i64_sub) => {{
                $self.validate_extended_const("i64.sub")?;
                $self.validator().visit_i64_sub()
            }};
            (@visit $self:ident visit_i64_mul) => {{
                $self.validate_extended_const("i64.mul")?;
                $self.validator().visit_i64_mul()
            }};

            // These are valid const expressions with the gc proposal is
            // enabled.
            (@visit $self:ident visit_struct_new $type_index:ident) => {{
                $self.validate_gc("struct.new")?;
                $self.validator().visit_struct_new($type_index)
            }};
            (@visit $self:ident visit_struct_new_default $type_index:ident) => {{
                $self.validate_gc("struct.new_default")?;
                $self.validator().visit_struct_new_default($type_index)
            }};
            (@visit $self:ident visit_array_new $type_index:ident) => {{
                $self.validate_gc("array.new")?;
                $self.validator().visit_array_new($type_index)
            }};
            (@visit $self:ident visit_array_new_default $type_index:ident) => {{
                $self.validate_gc("array.new_default")?;
                $self.validator().visit_array_new_default($type_index)
            }};
            (@visit $self:ident visit_array_new_fixed $type_index:ident $n:ident) => {{
                $self.validate_gc("array.new_fixed")?;
                $self.validator().visit_array_new_fixed($type_index, $n)
            }};
            (@visit $self:ident visit_ref_i31) => {{
                $self.validate_gc("ref.i31")?;
                $self.validator().visit_ref_i31()
            }};
            (@visit $self:ident visit_extern_convert_any) => {{
                $self.validate_gc("extern.convert_any")?;
                $self.validator().visit_extern_convert_any()
            }};
            (@visit $self:ident visit_any_convert_extern) => {{
                $self.validate_gc("any.convert_extern")?;
                $self.validator().visit_any_convert_extern()
            }};
            (@visit $self:ident visit_ref_i31_shared) => {{
                $self.validate_shared_everything_threads("ref.i31_shared")?;
                $self.validator().visit_ref_i31_shared()
            }};

            // `global.get` is a valid const expression for imported, immutable
            // globals.
            (@visit $self:ident visit_global_get $idx:ident) => {{
                $self.validate_global($idx)?;
                $self.validator().visit_global_get($idx)
            }};
            // `ref.func`, if it's in a `global` initializer, will insert into
            // the set of referenced functions so it's processed here.
            (@visit $self:ident visit_ref_func $idx:ident) => {{
                $self.insert_ref_func($idx);
                $self.validator().visit_ref_func($idx)
            }};

            (@visit $self:ident $op:ident $($args:tt)*) => {{
                Err($self.not_const(stringify!($op)))
            }}
        }

        impl<'a> VisitOperator<'a> for VisitConstOperator<'a> {
            type Output = Result<()>;

            #[cfg(feature = "simd")]
            fn simd_visitor(
                &mut self,
            ) -> Option<&mut dyn crate::VisitSimdOperator<'a, Output = Self::Output>> {
                Some(self)
            }

            crate::for_each_visit_operator!(define_visit_operator);
        }

        #[cfg(feature = "simd")]
        impl<'a> VisitSimdOperator<'a> for VisitConstOperator<'a> {
            crate::for_each_visit_simd_operator!(define_visit_operator);
        }
    }
}

#[derive(Debug)]
pub(crate) struct Module {
    // This is set once the code section starts.
    // `WasmModuleResources` implementations use the snapshot to
    // enable parallel validation of functions.
    pub snapshot: Option<Arc<TypeList>>,
    // Stores indexes into the validator's types list.
    pub types: Vec<CoreTypeId>,
    pub tables: Vec<TableType>,
    pub memories: Vec<MemoryType>,
    pub globals: Vec<GlobalType>,
    pub element_types: Vec<RefType>,
    pub data_count: Option<u32>,
    // Stores indexes into `types`.
    pub functions: Vec<u32>,
    pub tags: Vec<CoreTypeId>,
    pub function_references: Set<u32>,
    pub imports: IndexMap<(String, String), Vec<EntityType>>,
    pub exports: IndexMap<String, EntityType>,
    pub type_size: u32,
    num_imported_globals: u32,
    num_imported_functions: u32,
    features: WasmFeatures,
}

impl Module {
    pub fn new(features: WasmFeatures) -> Self {
        Self {
            snapshot: Default::default(),
            types: Default::default(),
            tables: Default::default(),
            memories: Default::default(),
            globals: Default::default(),
            element_types: Default::default(),
            data_count: Default::default(),
            functions: Default::default(),
            tags: Default::default(),
            function_references: Default::default(),
            imports: Default::default(),
            exports: Default::default(),
            type_size: 1,
            num_imported_globals: Default::default(),
            num_imported_functions: Default::default(),
            features,
        }
    }

    pub(crate) fn add_types(
        &mut self,
        rec_group: RecGroup,
        types: &mut TypeAlloc,
        offset: usize,
        check_limit: bool,
    ) -> Result<()> {
        if check_limit {
            check_max(
                self.types.len(),
                rec_group.types().len() as u32,
                MAX_WASM_TYPES,
                "types",
                offset,
            )?;
        }
        self.canonicalize_and_intern_rec_group(types, rec_group, offset)
    }

    pub fn add_import(
        &mut self,
        mut import: crate::Import,
        types: &TypeList,
        offset: usize,
    ) -> Result<()> {
        let entity = self.check_type_ref(&mut import.ty, types, offset)?;

        let (len, max, desc) = match import.ty {
            TypeRef::Func(type_index) => {
                self.functions.push(type_index);
                self.num_imported_functions += 1;
                (self.functions.len(), MAX_WASM_FUNCTIONS, "functions")
            }
            TypeRef::Table(ty) => {
                self.tables.push(ty);
                (self.tables.len(), self.max_tables(), "tables")
            }
            TypeRef::Memory(ty) => {
                self.memories.push(ty);
                (self.memories.len(), self.max_memories(), "memories")
            }
            TypeRef::Tag(ty) => {
                self.tags.push(self.types[ty.func_type_idx as usize]);
                (self.tags.len(), MAX_WASM_TAGS, "tags")
            }
            TypeRef::Global(ty) => {
                if !self.features.mutable_global() && ty.mutable {
                    return Err(BinaryReaderError::new(
                        "mutable global support is not enabled",
                        offset,
                    ));
                }
                self.globals.push(ty);
                self.num_imported_globals += 1;
                (self.globals.len(), MAX_WASM_GLOBALS, "globals")
            }
        };

        check_max(len, 0, max, desc, offset)?;

        self.type_size = combine_type_sizes(self.type_size, entity.info(types).size(), offset)?;

        self.imports
            .entry((import.module.to_string(), import.name.to_string()))
            .or_default()
            .push(entity);

        Ok(())
    }

    pub fn add_export(
        &mut self,
        name: &str,
        ty: EntityType,
        offset: usize,
        check_limit: bool,
        types: &TypeList,
    ) -> Result<()> {
        if !self.features.mutable_global() {
            if let EntityType::Global(global_type) = ty {
                if global_type.mutable {
                    return Err(BinaryReaderError::new(
                        "mutable global support is not enabled",
                        offset,
                    ));
                }
            }
        }

        if check_limit {
            check_max(self.exports.len(), 1, MAX_WASM_EXPORTS, "exports", offset)?;
        }

        self.type_size = combine_type_sizes(self.type_size, ty.info(types).size(), offset)?;

        match self.exports.insert(name.to_string(), ty) {
            Some(_) => Err(format_err!(
                offset,
                "duplicate export name `{name}` already defined"
            )),
            None => Ok(()),
        }
    }

    pub fn add_function(&mut self, type_index: u32, types: &TypeList, offset: usize) -> Result<()> {
        self.func_type_at(type_index, types, offset)?;
        self.functions.push(type_index);
        Ok(())
    }

    pub fn add_memory(&mut self, ty: MemoryType, offset: usize) -> Result<()> {
        self.check_memory_type(&ty, offset)?;
        self.memories.push(ty);
        Ok(())
    }

    pub fn add_tag(&mut self, ty: TagType, types: &TypeList, offset: usize) -> Result<()> {
        self.check_tag_type(&ty, types, offset)?;
        self.tags.push(self.types[ty.func_type_idx as usize]);
        Ok(())
    }

    fn sub_type_at<'a>(&self, types: &'a TypeList, idx: u32, offset: usize) -> Result<&'a SubType> {
        let id = self.type_id_at(idx, offset)?;
        Ok(&types[id])
    }

    fn func_type_at<'a>(
        &self,
        type_index: u32,
        types: &'a TypeList,
        offset: usize,
    ) -> Result<&'a FuncType> {
        match &self
            .sub_type_at(types, type_index, offset)?
            .composite_type
            .inner
        {
            CompositeInnerType::Func(f) => Ok(f),
            _ => bail!(offset, "type index {type_index} is not a function type"),
        }
    }

    pub fn check_type_ref(
        &self,
        type_ref: &mut TypeRef,
        types: &TypeList,
        offset: usize,
    ) -> Result<EntityType> {
        Ok(match type_ref {
            TypeRef::Func(type_index) => {
                self.func_type_at(*type_index, types, offset)?;
                EntityType::Func(self.types[*type_index as usize])
            }
            TypeRef::Table(t) => {
                self.check_table_type(t, types, offset)?;
                EntityType::Table(*t)
            }
            TypeRef::Memory(t) => {
                self.check_memory_type(t, offset)?;
                EntityType::Memory(*t)
            }
            TypeRef::Tag(t) => {
                self.check_tag_type(t, types, offset)?;
                EntityType::Tag(self.types[t.func_type_idx as usize])
            }
            TypeRef::Global(t) => {
                self.check_global_type(t, types, offset)?;
                EntityType::Global(*t)
            }
        })
    }

    fn check_table_type(&self, ty: &mut TableType, types: &TypeList, offset: usize) -> Result<()> {
        // The `funcref` value type is allowed all the way back to the MVP, so
        // don't check it here.
        if ty.element_type != RefType::FUNCREF {
            self.check_ref_type(&mut ty.element_type, offset)?
        }

        self.check_limits(ty.initial, ty.maximum, offset)?;
        if ty.table64 && !self.features().memory64() {
            bail!(offset, "memory64 must be enabled for 64-bit tables");
        }
        if ty.shared && !self.features().shared_everything_threads() {
            bail!(
                offset,
                "shared tables require the shared-everything-threads proposal"
            );
        }

        let true_maximum = if ty.table64 {
            u64::MAX
        } else {
            u32::MAX as u64
        };
        let err = format!("table size must be at most {true_maximum:#x} entries");
        if ty.initial > true_maximum {
            return Err(BinaryReaderError::new(err, offset));
        }
        if let Some(maximum) = ty.maximum {
            if maximum > true_maximum {
                return Err(BinaryReaderError::new(err, offset));
            }
        }
        if ty.shared && !types.reftype_is_shared(ty.element_type) {
            return Err(BinaryReaderError::new(
                "shared tables must have a shared element type",
                offset,
            ));
        }
        Ok(())
    }

    fn check_memory_type(&self, ty: &MemoryType, offset: usize) -> Result<()> {
        self.check_limits(ty.initial, ty.maximum, offset)?;

        if ty.memory64 && !self.features().memory64() {
            bail!(offset, "memory64 must be enabled for 64-bit memories");
        }
        if ty.shared && !self.features().threads() {
            bail!(offset, "threads must be enabled for shared memories");
        }

        let page_size = if let Some(page_size_log2) = ty.page_size_log2 {
            if !self.features().custom_page_sizes() {
                return Err(BinaryReaderError::new(
                    "the custom page sizes proposal must be enabled to customize a memory's page size",
                    offset,
                ));
            }
            // Currently 2**0 and 2**16 are the only valid page sizes, but this
            // may be relaxed to allow any power of two in the future.
            if page_size_log2 != 0 && page_size_log2 != 16 {
                return Err(BinaryReaderError::new("invalid custom page size", offset));
            }
            let page_size = 1_u64 << page_size_log2;
            debug_assert!(page_size.is_power_of_two());
            debug_assert!(page_size == DEFAULT_WASM_PAGE_SIZE || page_size == 1);
            page_size
        } else {
            let page_size_log2 = 16;
            debug_assert_eq!(DEFAULT_WASM_PAGE_SIZE, 1 << page_size_log2);
            DEFAULT_WASM_PAGE_SIZE
        };
        let true_maximum = if ty.memory64 {
            max_wasm_memory64_pages(page_size)
        } else {
            max_wasm_memory32_pages(page_size)
        };
        let err = format!("memory size must be at most {true_maximum:#x} {page_size}-byte pages");
        if ty.initial > true_maximum {
            return Err(BinaryReaderError::new(err, offset));
        }
        if let Some(maximum) = ty.maximum {
            if maximum > true_maximum {
                return Err(BinaryReaderError::new(err, offset));
            }
        }
        if ty.shared && ty.maximum.is_none() {
            return Err(BinaryReaderError::new(
                "shared memory must have maximum size",
                offset,
            ));
        }
        Ok(())
    }

    #[cfg(feature = "component-model")]
    pub(crate) fn imports_for_module_type(
        &self,
        offset: usize,
    ) -> Result<IndexMap<(String, String), EntityType>> {
        // Ensure imports are unique, which is a requirement of the component model:
        // https://github.com/WebAssembly/component-model/blob/d09f907/design/mvp/Explainer.md#import-and-export-definitions
        self.imports
            .iter()
            .map(|((module, name), types)| {
                if types.len() != 1 {
                    bail!(
                        offset,
                        "module has a duplicate import name `{module}:{name}` \
                         that is not allowed in components",
                    );
                }
                Ok(((module.clone(), name.clone()), types[0]))
            })
            .collect::<Result<_>>()
    }

    fn check_value_type(&self, ty: &mut ValType, offset: usize) -> Result<()> {
        // The above only checks the value type for features.
        // We must check it if it's a reference.
        match ty {
            ValType::Ref(rt) => self.check_ref_type(rt, offset),
            _ => self
                .features
                .check_value_type(*ty)
                .map_err(|e| BinaryReaderError::new(e, offset)),
        }
    }

    fn check_ref_type(&self, ty: &mut RefType, offset: usize) -> Result<()> {
        self.features
            .check_ref_type(*ty)
            .map_err(|e| BinaryReaderError::new(e, offset))?;
        let mut hty = ty.heap_type();
        self.check_heap_type(&mut hty, offset)?;
        *ty = RefType::new(ty.is_nullable(), hty).unwrap();
        Ok(())
    }

    fn check_heap_type(&self, ty: &mut HeapType, offset: usize) -> Result<()> {
        // Check that the heap type is valid.
        let type_index = match ty {
            HeapType::Abstract { .. } => return Ok(()),
            HeapType::Concrete(type_index) => type_index,
        };
        match type_index {
            UnpackedIndex::Module(idx) => {
                let id = self.type_id_at(*idx, offset)?;
                *type_index = UnpackedIndex::Id(id);
                Ok(())
            }
            // Types at this stage should not be canonicalized. All
            // canonicalized types should already be validated meaning they
            // shouldn't be double-checked here again.
            UnpackedIndex::RecGroup(_) | UnpackedIndex::Id(_) => unreachable!(),
        }
    }

    fn check_tag_type(&self, ty: &TagType, types: &TypeList, offset: usize) -> Result<()> {
        if !self.features().exceptions() {
            bail!(offset, "exceptions proposal not enabled");
        }
        let ty = self.func_type_at(ty.func_type_idx, types, offset)?;
        if !ty.results().is_empty() && !self.features.stack_switching() {
            return Err(BinaryReaderError::new(
                "invalid exception type: non-empty tag result type",
                offset,
            ));
        }
        Ok(())
    }

    fn check_global_type(
        &self,
        ty: &mut GlobalType,
        types: &TypeList,
        offset: usize,
    ) -> Result<()> {
        self.check_value_type(&mut ty.content_type, offset)?;
        if ty.shared && !self.features.shared_everything_threads() {
            bail!(
                offset,
                "shared globals require the shared-everything-threads proposal"
            );
        }
        if ty.shared && !types.valtype_is_shared(ty.content_type) {
            return Err(BinaryReaderError::new(
                "shared globals must have a shared value type",
                offset,
            ));
        }
        Ok(())
    }

    fn check_limits<T>(&self, initial: T, maximum: Option<T>, offset: usize) -> Result<()>
    where
        T: Into<u64>,
    {
        if let Some(max) = maximum {
            if initial.into() > max.into() {
                return Err(BinaryReaderError::new(
                    "size minimum must not be greater than maximum",
                    offset,
                ));
            }
        }
        Ok(())
    }

    pub fn max_tables(&self) -> usize {
        if self.features.reference_types() {
            MAX_WASM_TABLES
        } else {
            1
        }
    }

    pub fn max_memories(&self) -> usize {
        if self.features.multi_memory() {
            MAX_WASM_MEMORIES
        } else {
            1
        }
    }

    pub fn export_to_entity_type(
        &mut self,
        export: &crate::Export,
        offset: usize,
    ) -> Result<EntityType> {
        let check = |ty: &str, index: u32, total: usize| {
            if index as usize >= total {
                Err(format_err!(
                    offset,
                    "unknown {ty} {index}: exported {ty} index out of bounds",
                ))
            } else {
                Ok(())
            }
        };

        Ok(match export.kind {
            ExternalKind::Func => {
                check("function", export.index, self.functions.len())?;
                self.function_references.insert(export.index);
                EntityType::Func(self.types[self.functions[export.index as usize] as usize])
            }
            ExternalKind::Table => {
                check("table", export.index, self.tables.len())?;
                EntityType::Table(self.tables[export.index as usize])
            }
            ExternalKind::Memory => {
                check("memory", export.index, self.memories.len())?;
                EntityType::Memory(self.memories[export.index as usize])
            }
            ExternalKind::Global => {
                check("global", export.index, self.globals.len())?;
                EntityType::Global(self.globals[export.index as usize])
            }
            ExternalKind::Tag => {
                check("tag", export.index, self.tags.len())?;
                EntityType::Tag(self.tags[export.index as usize])
            }
        })
    }

    pub fn get_func_type<'a>(
        &self,
        func_idx: u32,
        types: &'a TypeList,
        offset: usize,
    ) -> Result<&'a FuncType> {
        match self.functions.get(func_idx as usize) {
            Some(idx) => self.func_type_at(*idx, types, offset),
            None => Err(format_err!(
                offset,
                "unknown function {func_idx}: func index out of bounds",
            )),
        }
    }

    fn global_at(&self, idx: u32, offset: usize) -> Result<&GlobalType> {
        match self.globals.get(idx as usize) {
            Some(t) => Ok(t),
            None => Err(format_err!(
                offset,
                "unknown global {idx}: global index out of bounds"
            )),
        }
    }

    fn table_at(&self, idx: u32, offset: usize) -> Result<&TableType> {
        match self.tables.get(idx as usize) {
            Some(t) => Ok(t),
            None => Err(format_err!(
                offset,
                "unknown table {idx}: table index out of bounds"
            )),
        }
    }

    fn memory_at(&self, idx: u32, offset: usize) -> Result<&MemoryType> {
        match self.memories.get(idx as usize) {
            Some(t) => Ok(t),
            None => Err(format_err!(
                offset,
                "unknown memory {idx}: memory index out of bounds"
            )),
        }
    }
}

impl InternRecGroup for Module {
    fn features(&self) -> &WasmFeatures {
        &self.features
    }

    fn add_type_id(&mut self, id: CoreTypeId) {
        self.types.push(id);
    }

    fn type_id_at(&self, idx: u32, offset: usize) -> Result<CoreTypeId> {
        self.types
            .get(idx as usize)
            .copied()
            .ok_or_else(|| format_err!(offset, "unknown type {idx}: type index out of bounds"))
    }

    fn types_len(&self) -> u32 {
        u32::try_from(self.types.len()).unwrap()
    }
}

struct OperatorValidatorResources<'a> {
    module: &'a mut MaybeOwned<Module>,
    types: &'a TypeList,
}

impl WasmModuleResources for OperatorValidatorResources<'_> {
    fn table_at(&self, at: u32) -> Option<TableType> {
        self.module.tables.get(at as usize).cloned()
    }

    fn memory_at(&self, at: u32) -> Option<MemoryType> {
        self.module.memories.get(at as usize).cloned()
    }

    fn tag_at(&self, at: u32) -> Option<&FuncType> {
        let type_id = *self.module.tags.get(at as usize)?;
        Some(self.types[type_id].unwrap_func())
    }

    fn global_at(&self, at: u32) -> Option<GlobalType> {
        self.module.globals.get(at as usize).cloned()
    }

    fn sub_type_at(&self, at: u32) -> Option<&SubType> {
        let id = *self.module.types.get(at as usize)?;
        Some(&self.types[id])
    }

    fn sub_type_at_id(&self, at: CoreTypeId) -> &SubType {
        &self.types[at]
    }

    fn type_id_of_function(&self, at: u32) -> Option<CoreTypeId> {
        let type_index = self.module.functions.get(at as usize)?;
        self.module.types.get(*type_index as usize).copied()
    }

    fn type_index_of_function(&self, at: u32) -> Option<u32> {
        self.module.functions.get(at as usize).copied()
    }

    fn check_heap_type(&self, t: &mut HeapType, offset: usize) -> Result<()> {
        self.module.check_heap_type(t, offset)
    }

    fn top_type(&self, heap_type: &HeapType) -> HeapType {
        self.types.top_type(heap_type)
    }

    fn element_type_at(&self, at: u32) -> Option<RefType> {
        self.module.element_types.get(at as usize).cloned()
    }

    fn is_subtype(&self, a: ValType, b: ValType) -> bool {
        self.types.valtype_is_subtype(a, b)
    }

    fn is_shared(&self, ty: RefType) -> bool {
        self.types.reftype_is_shared(ty)
    }

    fn element_count(&self) -> u32 {
        self.module.element_types.len() as u32
    }

    fn data_count(&self) -> Option<u32> {
        self.module.data_count
    }

    fn is_function_referenced(&self, idx: u32) -> bool {
        self.module.function_references.contains(&idx)
    }
}

/// The implementation of [`WasmModuleResources`] used by
/// [`Validator`](crate::Validator).
#[derive(Debug)]
pub struct ValidatorResources(pub(crate) Arc<Module>);

impl WasmModuleResources for ValidatorResources {
    fn table_at(&self, at: u32) -> Option<TableType> {
        self.0.tables.get(at as usize).cloned()
    }

    fn memory_at(&self, at: u32) -> Option<MemoryType> {
        self.0.memories.get(at as usize).cloned()
    }

    fn tag_at(&self, at: u32) -> Option<&FuncType> {
        let id = *self.0.tags.get(at as usize)?;
        let types = self.0.snapshot.as_ref().unwrap();
        match &types[id].composite_type.inner {
            CompositeInnerType::Func(f) => Some(f),
            _ => None,
        }
    }

    fn global_at(&self, at: u32) -> Option<GlobalType> {
        self.0.globals.get(at as usize).cloned()
    }

    fn sub_type_at(&self, at: u32) -> Option<&SubType> {
        let id = *self.0.types.get(at as usize)?;
        let types = self.0.snapshot.as_ref().unwrap();
        Some(&types[id])
    }

    fn sub_type_at_id(&self, at: CoreTypeId) -> &SubType {
        let types = self.0.snapshot.as_ref().unwrap();
        &types[at]
    }

    fn type_id_of_function(&self, at: u32) -> Option<CoreTypeId> {
        let type_index = *self.0.functions.get(at as usize)?;
        self.0.types.get(type_index as usize).copied()
    }

    fn type_index_of_function(&self, at: u32) -> Option<u32> {
        self.0.functions.get(at as usize).copied()
    }

    fn check_heap_type(&self, t: &mut HeapType, offset: usize) -> Result<()> {
        self.0.check_heap_type(t, offset)
    }

    fn top_type(&self, heap_type: &HeapType) -> HeapType {
        self.0.snapshot.as_ref().unwrap().top_type(heap_type)
    }

    fn element_type_at(&self, at: u32) -> Option<RefType> {
        self.0.element_types.get(at as usize).cloned()
    }

    fn is_subtype(&self, a: ValType, b: ValType) -> bool {
        self.0.snapshot.as_ref().unwrap().valtype_is_subtype(a, b)
    }

    fn is_shared(&self, ty: RefType) -> bool {
        self.0.snapshot.as_ref().unwrap().reftype_is_shared(ty)
    }

    fn element_count(&self) -> u32 {
        self.0.element_types.len() as u32
    }

    fn data_count(&self) -> Option<u32> {
        self.0.data_count
    }

    fn is_function_referenced(&self, idx: u32) -> bool {
        self.0.function_references.contains(&idx)
    }
}

const _: () = {
    const fn assert_send<T: Send>() {}

    // Assert that `ValidatorResources` is Send so function validation
    // can be parallelizable
    assert_send::<ValidatorResources>();
};

mod arc {
    use alloc::sync::Arc;
    use core::ops::Deref;

    enum Inner<T> {
        Owned(T),
        Shared(Arc<T>),

        Empty, // Only used for swapping from owned to shared.
    }

    pub struct MaybeOwned<T> {
        inner: Inner<T>,
    }

    impl<T> MaybeOwned<T> {
        pub fn new(val: T) -> MaybeOwned<T> {
            MaybeOwned {
                inner: Inner::Owned(val),
            }
        }

        #[inline]
        pub(crate) fn as_mut(&mut self) -> Option<&mut T> {
            match &mut self.inner {
                Inner::Owned(x) => Some(x),
                Inner::Shared(_) => None,
                Inner::Empty => Self::unreachable(),
            }
        }

        #[inline]
        pub fn assert_mut(&mut self) -> &mut T {
            self.as_mut().unwrap()
        }

        pub fn arc(&mut self) -> &Arc<T> {
            self.make_shared();
            match &self.inner {
                Inner::Shared(x) => x,
                _ => Self::unreachable(),
            }
        }

        #[inline]
        fn make_shared(&mut self) {
            if let Inner::Shared(_) = self.inner {
                return;
            }

            let inner = core::mem::replace(&mut self.inner, Inner::Empty);
            let x = match inner {
                Inner::Owned(x) => x,
                _ => Self::unreachable(),
            };
            let x = Arc::new(x);
            self.inner = Inner::Shared(x);
        }

        #[cold]
        #[inline(never)]
        fn unreachable() -> ! {
            unreachable!()
        }
    }

    impl<T: Default> Default for MaybeOwned<T> {
        fn default() -> MaybeOwned<T> {
            MaybeOwned::new(T::default())
        }
    }

    impl<T> Deref for MaybeOwned<T> {
        type Target = T;

        fn deref(&self) -> &T {
            match &self.inner {
                Inner::Owned(x) => x,
                Inner::Shared(x) => x,
                Inner::Empty => Self::unreachable(),
            }
        }
    }
}