bytecodealliance/wasmtime v46.0.0

on GitHub

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