Wasmtime Files

46.0.0

Released 2026-06-22.

Added

• Added opt-in support for the WebAssembly branch-hinting proposal: the metadata.code.branch_hint custom section is parsed and used to mark cold blocks during Cranelift compilation, behind Config::wasm_branch_hinting (off by default). #13459

• Wasmtime's C API now supports type reflection of GC values. #13268

• Wasmtime now supports being compiled to arm64_32 platforms. #13259

• The cranelift-frontend crate,the RISC-V Cranelift backend, and Wasmtime's component-model-async feature now support no_std. #13401 #13479 #13533

• Wasmtime has initial support for the component model (implements "...") feature. #13361 #13497 #13513

• The wasmtime CLI now supports configuring the initial-cwd property of WASI. #13468

• The wasmtime serve subcommand now supports --header to set headers for the guest. #13471

• Wasmtime supports WASI 0.2.12, which notably includes exit-with-code in stable. #13536

• Wasmtime's component-model-async-bytes feature was renamed to component-model-bytes and now lifting/lowering using Bytes and BytesMut is directly supported. #13366

• Wasmtime now exposes the async call stack of components through its public API which can be used to determine which root task performed an import call. #13510

• Wasmtime now supports WASI 0.3.0 by default and the component-model-async wasm feature is now enabled by default. #13612

Changed

• Performance of bulk-data-transfer instructions such as {array,table,memory}.{copy,fill,init} have been improved. #13312 #13367 #13368 #13382 #13407 #13424 #13438 #13460 #13524

• Codegen for conditions based on ctz or clz has been optimized. #13332 #13343

• Cranelift now optimizes conditional branches of constant conditions into unconditional jumps. #13267 #13391

• Wasmtime's copying collector now has an in-wasm fast path for its bump allocator. #13323

• Wasmtime's GC implementation has been hardened in the face of GC heap corruption to avoid panicking or aborting. Corruption is returned as a WasmtimeBug type for embedders to detect and safely tear down the store/instance. #13321 #13320

• Cranelift's MemFlags type is now renamed to MemFlagsData, and AliasRegions are now stored in the DFG. #13353 #13354

• Wasmtime now handles OOM gracefully in more situations. #13371 #13372 #13374 #13375 #13376 #13377 #13378 #13379 #13388 #13413 #13412 #13414

• Cranelift's egraph rewrite pass now uses a concept of fuel to avoid exponential blowup of rewrites. #13390

• Wasmtime now consumes fuel in bulk-data-transfer instructions proportional to the size of the transfer. #13393 #13448

• Cranelift's ISLE format now supports structs as well as tuple fields for structs/enums. #13319 #13335

• Wasmtime's implementation of passive element and data segments is now optimized to perform more work in wasm itself and has a refactored representation on the host. #13394 #13444

• Heuristics for triggering GC in the DRC collector have been adjusted to avoid blowups seen in the wild. #13422

• Wasmtime's default garbage collector is now the copying collector instead of the deferred-reference-counting collector. This collector should be more performant in most situations and additionally have the ability to collect cycles. #13439

• Wasmtime now traps if a waitable is being waited on synchronously and additionally added to a waitable-set. #13415

• Wasmtime's behavior with subtask.cancel is now adjusted to resume the cancelled task immediately instead of always yielding. #13443

• The wasmtime_wasi_http::handler module has had its interface overhauled to better handle configuring the lifecycle of a request as it flows through the system in terms of timeouts and such. #13404

• Most of Wasmtime's instance initialization is now compiled into a per-module initialization function rather than happening through the host in Wasmtime. #13487

• The InstanceExportLookup trait has been generalized into ExportLookup, and this is now optionally implemented for wit_parser::ItemName with the wit-parser crate feature. #13505

• Many of Cranelift's *_imm instructions have been removed in favor as they were just sugar over other opcodes. Builder-style methods remain, however. #13527 #13541 #13543 #13545 #13548 #13553

• Wasmtime's caching behavior is no longer gated on cfg(debug_assertions) and has been adjusted to handle a git source differently. #13535

• Bindings generation for store-using *WithStore traits now have a type parameter of the store on the trait itself instead of on every method. #13549

• Wasmtime now requires Rust 1.94.0 to compile. #13547

Fixed

• Wasmtime's copying collector has had a few bugs related to how it's translated to CLIF fixed. #13381

• Returning a ThrownException when there wasn't a pending exception within a store has been fixed. #13306

• A DRC corruption issue when overwriting an i31ref slot has been fixed. #13307

• Taking a store's exception from a debug handler has been fixed. #13310

• Alignment checks of atomics with Winch have been fixed. #13337

• GC barriers around managing the pending exception within a store have been fixed. #13330

• Cranelift's handling of short jumps on some architectures is now improved to handle very large basic blocks. #13392

• Cross-component stream copies have been fixed. #13418

• Cranelift-generated stack maps have been fixed in a few cases. #13449 #13466 #13498

• Component-to-component adapters which use resources in arguments and disable concurrency_support have been fixed. #13542

• A panic in substituted_component_type has been fixed when guests have exported resources. #13608