bytecodealliance/wasmtime v40.0.0

on GitHub

40.0.0

Released 2025-12-20.

Added

• WASIp3 support for wasi:http now implements Response::from_http to convert
  from standard Rust types to WASI types.
  #12063

• Cranelift now supports a "patchable" ABI which has a maximum number of
  arguments and clobbers no registers. This is paired as well with a new
  patchable_call instruction which supports being turned into NOPs at runtime.
  #12061
  #12101

Changed

• Support for the WebAssembly threads proposal is now classified as tier 2 by
  default. Additionally creation of SharedMemory is disabled by deafult behind
  a new config knob/CLI flag.
  #12036

• A variety of peephole-style optimizations have been added to Cranelift's
  optimization passes.
  #11994
  #11995
  #11996
  #11997
  #11998
  #11999
  #12000
  #12006
  #12008

• Component host functions have been slightly optimized to remove an Arc clone
  and reduce contention.
  #11987

• Support for component-model-async has been updated to account for the
  changes specified in WebAssembly/component-model#578.
  This means that historical binaries using WASIp3, for example, are no longer
  valid. Recompilation of historical components will be required and
  source-level changes may also be required in some circumstances.
  #12031
  #12043

• The UnsyncBoxBody type is now used everywhere in wasmtime-wasi-http instead
  of just in the wasip3 support.
  #12060

• Initial groundwork for gracefully handling OOM (e.g. returning an error
  instead of aborting) has been added.
  #12070
  #12089

• Wasmtime will create a private copy of code memory when guest debugging is
  enabled to assist with modifying code when adding/removing breakpoints.
  #12051

• The ResourceTable type will no longer use Tombstone when compiled in debug
  mode.
  #12114

• Intra-component future/stream reads/writes will now trap instead of
  accidentally being allowed.
  #12117

• Cranelift optimization rules have been tweaked after it was discovered that
  they could pessimize code containing long chains of computations.
  #12116

Fixed

• Compilation of i8x16.popcnt has been fixed in Winch for some potential
  inputs.
  #12010

• A panic in Instance::prepare_call for some component-model-async situations
  has been fixed.
  #12054

• An off-by-one error for lifting/lowering enums/variants with 255 cases has
  been fixed.
  #12066

• Restarting the read of a host future after cancellation has been fixed.
  #12093

• Compilation for OpenBSD on x86_64 has been fixed.
  #12097

• Components containing a module type which exoprts a tag are now supported.
  #12125