Replace wasmer with wasmtime for SWC plugin WASM engine#91533
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This was referenced Mar 17, 2026
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Swap out swc_plugin_backend_wasmer (wasmer 6.1.0-rc) with a new swc_plugin_backend_wasmtime crate to reduce native binary size. - Create crates/swc-plugin-backend-wasmtime implementing the swc_plugin_runner Runtime/Instance/Caller traits via wasmtime - Update workspace Cargo.toml: add wasmtime deps, remove wasmer deps and release profiles - Wire WasmtimeRuntime into both code paths: - Turbopack: turbopack-ecmascript-plugins - Webpack/SWC NAPI: next-napi-bindings transform.rs - Remove swc_core/plugin_backend_wasmer feature flag usage - Integration points designed for future swappability (e.g. Node NAPI WASM engine) Fix compilation errors in wasmtime backend implementation - Remove unused import (std::sync::Arc) - Use raw pointer approach for Caller trait implementation to work around Rust lifetime constraints with wasmtime::Caller parameter - Implement runtime::Caller trait methods using unsafe pointer access, which is safe because the pointer lifetime is controlled within the host function closure The key challenge was that wasmtime's closure signature provides a Caller parameter with a specific lifetime that doesn't satisfy the trait object requirements for runtime::Caller. The solution uses unsafe pointer casting with careful lifetime management to bridge the gap between wasmtime's API and swc_plugin_runner's trait interface. Remove wasmer from compilation graph Replace swc_core/plugin_transform_host_native with granular internal features (__plugin_transform_host + __plugin_transform_host_schema_v1) to bypass __plugin_transform_env_native, which is the only feature path that pulls in wasmer via swc -> swc_plugin_backend_wasmer. Since we provide WasmtimeRuntime explicitly, we don't need swc's env-native backend wiring. This removes wasmer and all its sub-crates from cargo tree entirely, eliminating unnecessary compilation time.
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What?
Replace wasmer with wasmtime as the WASM runtime for SWC plugins.
Why?
wasmer/defaultvswasmer/js-default) and large binary sizeswc_plugin_backend_wasmtimeon crates.io, so we can use it directly instead of maintaining our own implementationHow?
swc_plugin_backend_wasmtimev9.0.0 crate from crates.io instead of a custom local implementationswc_core/plugin_transform_host_native(which pulls in wasmer via__plugin_transform_env_native) with granular features:__plugin_transform_host+__plugin_transform_host_schema_v1WasmtimeRuntimeexplicitly in both the NAPI bindings and turbopack plugin pathswasmtime/wasmtime-wasiworkspace dependencies — the upstream crate manages its own wasmtime versionswc_plugin_backend_wasmtime,swc_plugin_runner,wasmtime,wasmtime-internal-cranelift)vergen = "=9.0.6"to avoid avergen-libversion conflict withvergen-gitcl1.0.8Binary size impact (darwin-arm64, compared against current canary)