wasm_rquickjs/

lib.rs

use crate::conversions::generate_conversions;
use crate::exports::generate_export_impls;
use crate::imports::generate_import_modules;
use crate::javascript::escape_js_ident;
use crate::skeleton::{copy_skeleton_lock, copy_skeleton_sources, generate_cargo_toml};
use crate::wit::{add_get_script_import, add_wizer_init_export};
use anyhow::{Context, anyhow};
use camino::{Utf8Path, Utf8PathBuf};
use heck::{ToLowerCamelCase, ToSnakeCase, ToUpperCamelCase};
use proc_macro2::{Ident, Span};
use std::cell::RefCell;
use std::collections::{BTreeMap, BTreeSet, VecDeque};
use wit_parser::{
    Function, Interface, InterfaceId, PackageId, PackageName, PackageSourceMap, Resolve, TypeDef,
    TypeDefKind, TypeId, TypeOwner, WorldId, WorldItem, WorldKey,
};

/// WASI package namespaces whose interfaces are remapped to `wasip2::` in the generated code.
/// These correspond to interfaces provided by the `wasip2` crate and are mapped via the
/// `with:` block in `wit_bindgen::generate!`.
const WASI_REMAP_NAMESPACES: &[(&str, &str)] = &[
    ("cli", "cli"),
    ("clocks", "clocks"),
    ("filesystem", "filesystem"),
    ("http", "http"),
    ("io", "io"),
    ("random", "random"),
    ("sockets", "sockets"),
];

/// WASI package namespaces remapped to `wasip3::` in the WASI Preview 3 generation path.
///
/// Only the clock interfaces are remapped for now (Phase 1); every other WASI interface
/// present in a Preview 3 world keeps the bindings generated by `wit-bindgen` (i.e. it is
/// *not* treated as remapped). This list must stay consistent with the `with:` remap table
/// (`WASI_REMAPS_P3`) in `exports.rs`.
const WASI_REMAP_NAMESPACES_P3: &[(&str, &str)] = &[("clocks", "clocks")];

/// Selects which WASI generation the wrapper crate targets.
///
/// The default ([`GenerationTarget::WasiP2`]) reproduces the historical behavior:
/// synchronous exports/imports backed by the full Preview 2 skeleton (`wasip2`,
/// `wstd`, `golem-wasi-http`, Node.js builtins, Wizer pre-initialization).
///
/// [`GenerationTarget::WasiP3`] is the opt-in Preview 3 path. It uses the
/// **same** skeleton crate compiled with the `p3` Cargo feature instead of `p2`: a
/// minimal async runtime spine depending only on `rquickjs`, `wasip3` and a renamed
/// `wit-bindgen` (so the Node.js builtins are not duplicated). It supports synchronous
/// and asynchronous WIT exports and imports, including Wizer pre-initialization.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum GenerationTarget {
    /// WASI Preview 2 (default).
    #[default]
    WasiP2,
    /// WASI Preview 3 (opt-in, async).
    WasiP3,
}

impl GenerationTarget {
    pub fn is_p3(&self) -> bool {
        matches!(self, GenerationTarget::WasiP3)
    }
}

mod async_values;
mod conversions;
mod exports;
mod imports;
mod inject;
mod javascript;
#[cfg(feature = "optimize")]
mod optimize;
mod rust_bindgen;
mod skeleton;
mod types;
mod typescript;
mod wit;

pub use inject::{SLOT_END_MAGIC, SLOT_MAGIC, create_marker_file, inject_js_into_component};
#[cfg(feature = "optimize")]
pub use optimize::optimize_component;

/// Write `contents` to `path` only if the file doesn't exist or its current content differs.
/// This preserves file timestamps when content hasn't changed, avoiding unnecessary recompilation.
pub(crate) fn write_if_changed(
    path: impl AsRef<std::path::Path>,
    contents: impl AsRef<[u8]>,
) -> std::io::Result<()> {
    let path = path.as_ref();
    let contents = contents.as_ref();
    if let Ok(existing) = std::fs::read(path)
        && existing == contents
    {
        return Ok(());
    }
    std::fs::write(path, contents)
}

/// Copy a file from `src` to `dst` only if the destination doesn't exist or its content differs.
pub(crate) fn copy_if_changed(
    src: impl AsRef<std::path::Path>,
    dst: impl AsRef<std::path::Path>,
) -> std::io::Result<()> {
    let src = src.as_ref();
    let dst = dst.as_ref();
    let src_contents = std::fs::read(src)?;
    if let Ok(existing) = std::fs::read(dst)
        && existing == src_contents
    {
        return Ok(());
    }
    std::fs::write(dst, src_contents)
}

/// Specifies how a given user-defined JS module gets embedded into the generated Rust crate.
#[derive(Debug, Clone)]
pub enum EmbeddingMode {
    /// Points to a JS module file that is going to be embedded into the generated Rust crate
    EmbedFile(Utf8PathBuf),
    /// The JS module is going to be fetched run-time through an imported WIT interface
    Composition,
    /// Embeds a small marker in the compiled WASM component.
    /// After compilation, JS source can be injected into the marker via `inject_js_into_component`
    /// without recompiling the Rust crate. The injected JS can be any size — the WASM component
    /// is structurally rewritten to accommodate the new data.
    BinarySlot,
}

impl EmbeddingMode {
    pub fn is_binary_slot(&self) -> bool {
        matches!(self, EmbeddingMode::BinarySlot)
    }
}

/// Specifies a JS module to be evaluated in the generated component.
#[derive(Debug, Clone)]
pub struct JsModuleSpec {
    pub name: String,
    pub mode: EmbeddingMode,
}

impl JsModuleSpec {
    pub fn file_name(&self) -> String {
        self.name.replace('/', "_") + ".js"
    }
}

/// Generates a Rust wrapper crate for a combination of a WIT package and a JavaScript module.
///
/// The `wit` parameter should point to a WIT root (holding the WIT package of the component, with
/// optionally a `deps` subdirectory with an arbitrary number of dependencies).
///
/// The `js_modules` parameter must point to at least one JavaScript module that implements the WIT package,
/// and optionally additional modules that get imported during the initialization of the component. It is
/// always the first in the list that is considered the one containing the implementation of the WIT exports.
///
/// The `output` parameter is the root directory where the generated Rust crate's source code and
/// Cargo manifest is placed.
///
/// If `world` is `None`, the default world is selected and used, otherwise the specified one.
///
/// This always targets WASI Preview 2. Use [`generate_wrapper_crate_with_target`] to select the
/// (opt-in) WASI Preview 3 generation path.
pub fn generate_wrapper_crate(
    wit: &Utf8Path,
    js_modules: &[JsModuleSpec],
    output: &Utf8Path,
    world: Option<&str>,
) -> anyhow::Result<()> {
    generate_wrapper_crate_with_target(wit, js_modules, output, world, GenerationTarget::WasiP2)
}

/// Generates a Rust wrapper crate, selecting the WASI generation via `target`.
///
/// See [`generate_wrapper_crate`] for the description of the common parameters and
/// [`GenerationTarget`] for the differences between the Preview 2 and Preview 3 paths.
pub fn generate_wrapper_crate_with_target(
    wit: &Utf8Path,
    js_modules: &[JsModuleSpec],
    output: &Utf8Path,
    world: Option<&str>,
    target: GenerationTarget,
) -> anyhow::Result<()> {
    if target.is_p3() && uses_composition(js_modules) {
        anyhow::bail!(
            "Composition (@composition) JS modules are not supported by the WASI Preview 3 generation path yet"
        );
    }

    // Making sure the target directories exists
    std::fs::create_dir_all(output).context("Failed to create output directory")?;
    std::fs::create_dir_all(output.join("src")).context("Failed to create output/src directory")?;
    std::fs::create_dir_all(output.join("src").join("modules"))
        .context("Failed to create output/src/modules directory")?;

    // Resolving the WIT package (initial parse for Cargo.toml generation)
    let context = GeneratorContext::new(output, wit, world, target)?;

    // Generating the Cargo.toml file
    generate_cargo_toml(&context)?;

    // Copying the skeleton's Cargo.lock for faster dependency resolution
    copy_skeleton_lock(context.output, &context.world_name)
        .context("Failed to copy skeleton Cargo.lock")?;

    // Copying the skeleton files
    copy_skeleton_sources(context.output).context("Failed to copy skeleton sources")?;

    // Copying the WIT package to the output directory
    copy_wit_directory(wit, &context.output.join("wit"))
        .context("Failed to copy WIT package to output directory")?;

    if uses_composition(js_modules) {
        add_get_script_import(&context.output.join("wit"), world)
            .context("Failed to add get-script import to the WIT world")?;
    }

    // Add wizer-initialize export for pre-initialization support.
    add_wizer_init_export(&context.output.join("wit"), world, target.is_p3())
        .context("Failed to add wizer-initialize export to the WIT world")?;

    // Re-resolve the WIT package after modifications (wizer-initialize export was added)
    let modified_wit = output.join("wit");
    let context = GeneratorContext::new(output, &modified_wit, world, target)?;

    // Copying the JavaScript module to the output directory
    copy_js_modules(js_modules, context.output)
        .context("Failed to copy JavaScript module to output directory")?;

    // Generating the lib.rs file implementing the component exports
    generate_export_impls(&context, js_modules)
        .context("Failed to generate the component export implementations")?;

    // Generating the native modules implementing the component imports
    generate_import_modules(&context).context("Failed to generate the component import modules")?;

    // Generating the conversions.rs file implementing the IntoJs and FromJs typeclass instances
    // This step must be done after `generate_export_impls` to ensure all visited types are registered.
    generate_conversions(&context)
        .context("Failed to generate the IntoJs and FromJs typeclass instances")?;

    Ok(())
}

/// Generates TypeScript module definitions for a given (or default) world of a WIT package.
///
/// Returns the list of generated files.
pub fn generate_dts(
    wit: &Utf8Path,
    output: &Utf8Path,
    world: Option<&str>,
) -> anyhow::Result<Vec<Utf8PathBuf>> {
    generate_dts_with_target(wit, output, world, GenerationTarget::WasiP2)
}

/// Generates TypeScript module definitions, selecting the WASI generation via `target`.
///
/// Preview 2 permits a Promise-returning JavaScript implementation for every export. Preview 3
/// follows the WIT function kind: plain functions are synchronous and `async func` functions
/// return Promises.
pub fn generate_dts_with_target(
    wit: &Utf8Path,
    output: &Utf8Path,
    world: Option<&str>,
    target: GenerationTarget,
) -> anyhow::Result<Vec<Utf8PathBuf>> {
    // Making sure the target directories exist
    std::fs::create_dir_all(output).context("Failed to create output directory")?;

    let context = GeneratorContext::new(output, wit, world, target)?;

    let mut result = Vec::new();
    result.extend(
        typescript::generate_export_module(&context)
            .context("Failed to generate the TypeScript module definition for the exports")?,
    );

    // Generating the native modules implementing the component imports
    result.extend(typescript::generate_import_modules(&context).context(
        "Failed to generate the TypeScript module definitions for the imported modules",
    )?);

    Ok(result)
}

struct GeneratorContext<'a> {
    output: &'a Utf8Path,
    #[allow(dead_code)]
    wit_source_path: &'a Utf8Path,
    resolve: Resolve,
    root_package: PackageId,
    world: WorldId,
    #[allow(dead_code)]
    source_map: PackageSourceMap,
    visited_types: RefCell<BTreeSet<TypeId>>,
    world_name: String,
    types: wit_bindgen_core::Types,
    target: GenerationTarget,
}

impl<'a> GeneratorContext<'a> {
    fn new(
        output: &'a Utf8Path,
        wit: &'a Utf8Path,
        world: Option<&str>,
        target: GenerationTarget,
    ) -> anyhow::Result<Self> {
        let mut resolve = Resolve::default();
        let (root_package, source_map) = resolve
            .push_path(wit)
            .context("Failed to resolve WIT package")?;
        let world = resolve
            .select_world(std::slice::from_ref(&root_package), world)
            .context("Failed to select WIT world")?;

        if target.is_p3() {
            let mut unsupported = Vec::new();
            for (key, item) in &resolve.worlds[world].imports {
                let is_unsupported = match item {
                    WorldItem::Function(_) => true,
                    WorldItem::Type { id, .. } => resolve.types[*id].kind == TypeDefKind::Resource,
                    WorldItem::Interface { .. } => false,
                };
                if is_unsupported {
                    unsupported.push(match key {
                        WorldKey::Name(name) => name.clone(),
                        WorldKey::Interface(_) => "<resource>".to_string(),
                    });
                }
            }
            if !unsupported.is_empty() {
                return Err(anyhow!(
                    "Functions or resources declared directly in the world are not supported by \
                     the WASI Preview 3 generation path ({}); declare them inside an imported \
                     interface instead",
                    unsupported.join(", ")
                ));
            }
        }

        let world_name = resolve.worlds[world].name.clone();

        let mut types = wit_bindgen_core::Types::default();
        types.analyze(&resolve);

        Ok(Self {
            output,
            wit_source_path: wit,
            resolve,
            root_package,
            world,
            source_map,
            visited_types: RefCell::new(BTreeSet::new()),
            world_name,
            types,
            target,
        })
    }

    fn root_package_name(&self) -> String {
        self.resolve.packages[self.root_package].name.to_string()
    }

    fn record_visited_type(&self, type_id: TypeId) {
        self.visited_types.borrow_mut().insert(type_id);
    }

    fn is_exported_interface(&self, interface_id: InterfaceId) -> bool {
        let world = &self.resolve.worlds[self.world];
        world
            .exports
            .iter()
            .any(|(_, item)| matches!(item, WorldItem::Interface { id, .. } if id == &interface_id))
    }

    fn exported_interface_js_name(
        &self,
        interface_id: InterfaceId,
        export_name: &str,
    ) -> anyhow::Result<String> {
        let names = self.exported_interface_js_names()?;
        names
            .get(&interface_id)
            .cloned()
            .ok_or_else(|| anyhow!("Interface export not found: {export_name}"))
    }

    fn exported_interface_js_names(&self) -> anyhow::Result<BTreeMap<InterfaceId, String>> {
        let world = &self.resolve.worlds[self.world];
        let mut exported_interfaces = Vec::new();

        for (key, export) in &world.exports {
            if let WorldItem::Interface { id, .. } = export {
                let interface = &self.resolve.interfaces[*id];
                let export_name = match key {
                    WorldKey::Name(name) => name.as_str(),
                    WorldKey::Interface(_) => interface
                        .name
                        .as_deref()
                        .ok_or_else(|| anyhow!("Interface export does not have a name"))?,
                };
                let short_name = exported_interface_short_js_name(export_name);
                exported_interfaces.push((*id, export_name.to_string(), short_name));
            }
        }

        let mut short_name_counts = BTreeMap::<String, usize>::new();
        for (_, _, short_name) in &exported_interfaces {
            *short_name_counts.entry(short_name.clone()).or_default() += 1;
        }

        let mut result = BTreeMap::new();
        let mut used_names = BTreeMap::<String, InterfaceId>::new();
        for (interface_id, export_name, short_name) in exported_interfaces {
            let js_name = if short_name_counts.get(&short_name).copied().unwrap_or(0) > 1 {
                let interface = &self.resolve.interfaces[interface_id];
                exported_interface_qualified_js_name(self, interface, &export_name)?
            } else {
                short_name
            };

            if let Some(previous_id) = used_names.insert(js_name.clone(), interface_id) {
                anyhow::bail!(
                    "Exported WIT interfaces {previous_id:?} and {interface_id:?} both map to JavaScript export name '{js_name}'"
                );
            }

            result.insert(interface_id, js_name);
        }

        Ok(result)
    }

    fn is_exported_type(&self, type_id: TypeId) -> bool {
        if let Some(typ) = self.resolve.types.get(type_id) {
            match &typ.owner {
                TypeOwner::World(world_id) => {
                    if world_id == &self.world {
                        let world = &self.resolve.worlds[self.world];
                        world
                            .exports
                            .iter()
                            .any(|(_, item)| matches!(item, WorldItem::Type { id, .. } if id == &type_id))
                    } else {
                        false
                    }
                }
                TypeOwner::Interface(interface_id) => self.is_exported_interface(*interface_id),
                TypeOwner::None => false,
            }
        } else {
            false
        }
    }

    fn bindgen_type_info(&self, type_id: TypeId) -> wit_bindgen_core::TypeInfo {
        self.types.get(type_id)
    }

    fn get_imported_interface(
        &self,
        interface_id: &InterfaceId,
    ) -> anyhow::Result<ImportedInterface<'_>> {
        let interface = &self.resolve.interfaces[*interface_id];
        let name = interface
            .name
            .as_ref()
            .ok_or_else(|| anyhow!("Interface import does not have a name"))?
            .as_str();

        let functions = interface
            .functions
            .iter()
            .map(|(name, f)| (name.as_str(), f))
            .collect();

        let package_id = interface
            .package
            .ok_or_else(|| anyhow!("Anonymous interface imports are not supported yet"))?;
        let package = self
            .resolve
            .packages
            .get(package_id)
            .ok_or_else(|| anyhow!("Could not find package of imported interface {name}"))?;
        let package_name = &package.name;

        Ok(ImportedInterface {
            package_name: Some(package_name),
            name: name.to_string(),
            functions,
            interface: Some(interface),
            interface_id: Some(*interface_id),
        })
    }

    fn typ(&self, type_id: TypeId) -> anyhow::Result<&TypeDef> {
        self.resolve
            .types
            .get(type_id)
            .ok_or_else(|| anyhow!("Unknown type id: {type_id:?}"))
    }

    /// Returns `true` if the given package is a WASI package whose interfaces are remapped
    /// to `wasip2::` via the `with:` block in `wit_bindgen::generate!`.
    fn is_wasi_remapped_package(&self, package_id: PackageId) -> bool {
        let package = &self.resolve.packages[package_id];
        if package.name.namespace != "wasi" {
            return false;
        }
        if !self
            .wasi_remap_namespaces()
            .iter()
            .any(|(pkg_name, _)| *pkg_name == package.name.name.as_str())
        {
            return false;
        }
        // The remap only applies when the imported package's version matches the WASI
        // generation the runtime crate provides. The `wasip3` crate implements the
        // `wasi:*@0.3.x` interfaces, so a Preview 2 versioned import (e.g.
        // `wasi:clocks/monotonic-clock@0.2.3`) in a Preview 3 world must NOT be remapped —
        // its API surface differs and the bindings for it are generated by `wit-bindgen`
        // instead (via `generate_all`).
        if self.target.is_p3() {
            matches!(&package.name.version, Some(v) if v.major == 0 && v.minor == 3)
        } else {
            true
        }
    }

    /// The WASI namespace remap table for the selected generation target.
    fn wasi_remap_namespaces(&self) -> &'static [(&'static str, &'static str)] {
        if self.target.is_p3() {
            WASI_REMAP_NAMESPACES_P3
        } else {
            WASI_REMAP_NAMESPACES
        }
    }

    /// The Rust crate that remapped WASI interfaces resolve to for the selected target
    /// (`wasip2` for Preview 2, `wasip3` for Preview 3).
    fn wasi_remap_crate_ident(&self) -> Ident {
        let name = if self.target.is_p3() {
            "wasip3"
        } else {
            "wasip2"
        };
        Ident::new(name, Span::call_site())
    }

    /// Path to the `wit-bindgen` runtime module for the selected target. The Preview 3 path
    /// uses a renamed `wit-bindgen` dependency (`wit-bindgen-p3`) so that the two major
    /// versions can coexist in one skeleton crate.
    fn wit_bindgen_rt_path(&self) -> proc_macro2::TokenStream {
        if self.target.is_p3() {
            quote::quote! { wit_bindgen_p3::rt }
        } else {
            quote::quote! { wit_bindgen_rt }
        }
    }

    /// For a WASI-remapped resource type, returns the import module path and resource class
    /// name (e.g., `crate::modules::wasi_io_0_2_3_poll::Pollable`).
    fn wasi_resource_module_path(
        &self,
        type_id: TypeId,
    ) -> Option<(proc_macro2::TokenStream, Ident)> {
        let typ = self.resolve.types.get(type_id)?;
        let resource_name = typ.name.as_ref()?;
        let resource_ident = Ident::new(&resource_name.to_upper_camel_case(), Span::call_site());

        let interface_id = match &typ.owner {
            TypeOwner::Interface(id) => *id,
            _ => return None,
        };
        let interface = self.resolve.interfaces.get(interface_id)?;
        let interface_name = interface.name.as_ref()?;
        let package_id = interface.package?;
        let package = self.resolve.packages.get(package_id)?;
        let package_name = &package.name;

        let module_name = format!(
            "{}_{}",
            package_name.to_string().to_snake_case(),
            interface_name.to_snake_case()
        );
        let module_ident = Ident::new(&module_name, Span::call_site());

        Some((
            quote::quote! { crate::modules::#module_ident },
            resource_ident,
        ))
    }

    /// Returns `true` if the given type belongs to a WASI-remapped interface.
    fn is_wasi_remapped_type(&self, type_id: TypeId) -> bool {
        if let Some(typ) = self.resolve.types.get(type_id) {
            match &typ.owner {
                TypeOwner::Interface(interface_id) => {
                    if let Some(interface) = self.resolve.interfaces.get(*interface_id)
                        && let Some(package_id) = interface.package
                    {
                        return self.is_wasi_remapped_package(package_id);
                    }
                    false
                }
                _ => false,
            }
        } else {
            false
        }
    }
}

fn exported_interface_short_js_name(export_name: &str) -> String {
    escape_js_ident(export_name.to_lower_camel_case())
}

fn exported_interface_qualified_js_name(
    context: &GeneratorContext<'_>,
    interface: &Interface,
    export_name: &str,
) -> anyhow::Result<String> {
    let package_id = interface
        .package
        .ok_or_else(|| anyhow!("Anonymous interface exports cannot be qualified: {export_name}"))?;
    let package = context
        .resolve
        .packages
        .get(package_id)
        .ok_or_else(|| anyhow!("Unknown owner package of interface export: {export_name}"))?;
    let interface_name = interface.name.as_deref().unwrap_or(export_name);
    let module_name = format!(
        "{}_{}",
        package.name.to_string().to_snake_case(),
        interface_name.to_snake_case()
    );

    Ok(escape_js_ident(module_name.to_lower_camel_case()))
}

pub struct ImportedInterface<'a> {
    package_name: Option<&'a PackageName>,
    name: String,
    functions: Vec<(&'a str, &'a Function)>,
    interface: Option<&'a Interface>,
    interface_id: Option<InterfaceId>,
}

impl<'a> ImportedInterface<'a> {
    pub fn module_name(&self) -> anyhow::Result<String> {
        let package_name = self
            .package_name
            .ok_or_else(|| anyhow!("imported interface has no package name"))?;
        let interface_name = &self.name;

        Ok(format!(
            "{}_{}",
            package_name.to_string().to_snake_case(),
            interface_name.to_snake_case()
        ))
    }

    pub fn rust_interface_name(&self) -> Ident {
        let interface_name = format!("Js{}Module", self.name.to_upper_camel_case());
        Ident::new(&interface_name, Span::call_site())
    }

    pub fn name_and_interface(&self) -> Option<(&str, &Interface)> {
        self.interface
            .map(|interface| (self.name.as_str(), interface))
    }

    pub fn fully_qualified_interface_name(&self) -> String {
        if let Some(package_name) = &self.package_name {
            package_name.interface_id(&self.name)
        } else {
            self.name.clone()
        }
    }

    pub fn interface_stack(&self) -> VecDeque<InterfaceId> {
        self.interface_id.iter().cloned().collect()
    }
}

/// Recursively copies a WIT directory to `<output>/wit`.
fn copy_wit_directory(wit: &Utf8Path, output: &Utf8Path) -> anyhow::Result<()> {
    std::fs::create_dir_all(output)?;
    copy_dir_if_changed(wit.as_std_path(), output.as_std_path())
        .context("Failed to copy WIT directory")?;
    Ok(())
}

fn copy_dir_if_changed(src: &std::path::Path, dst: &std::path::Path) -> std::io::Result<()> {
    std::fs::create_dir_all(dst)?;
    for entry in std::fs::read_dir(src)? {
        let entry = entry?;
        let src_path = entry.path();
        let dst_path = dst.join(entry.file_name());
        if src_path.is_dir() {
            copy_dir_if_changed(&src_path, &dst_path)?;
        } else {
            copy_if_changed(&src_path, &dst_path)?;
        }
    }
    Ok(())
}

/// Copies the JS module files to `<output>/src/<name>.js` or generates slot files.
fn copy_js_modules(js_modules: &[JsModuleSpec], output: &Utf8Path) -> anyhow::Result<()> {
    let mut slot_index: u32 = 0;
    for module in js_modules {
        match &module.mode {
            EmbeddingMode::EmbedFile(source) => {
                let filename = module.file_name();
                let js_dest = output.join("src").join(filename);
                copy_if_changed(source, js_dest)
                    .context(format!("Failed to copy JavaScript module {}", module.name))?;
            }
            EmbeddingMode::BinarySlot => {
                let slot_filename = module.name.replace('/', "_") + ".slot";
                let slot_dest = output.join("src").join(slot_filename);
                let slot_data = inject::create_marker_file(slot_index);
                write_if_changed(slot_dest, slot_data).context(format!(
                    "Failed to create marker file for module {}",
                    module.name
                ))?;
                slot_index += 1;
            }
            EmbeddingMode::Composition => {}
        }
    }
    Ok(())
}

/// Checks if any of the provided JS modules uses composition mode.
fn uses_composition(js_module_spec: &[JsModuleSpec]) -> bool {
    js_module_spec
        .iter()
        .any(|m| matches!(m.mode, EmbeddingMode::Composition))
}