Skip to content

Tracking issue for wasm32-unknown-unknown's future-incompat warning for C ABI changes #138762

New issue
New issue
Closed
#133952
Closed
Tracking issue for wasm32-unknown-unknown's future-incompat warning for C ABI changes#138762
#133952
Labels
A-ABIArea: Concerning the application binary interface (ABI)C-future-incompatibilityCategory: Future-incompatibility lintsC-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCO-wasmTarget: WASM (WebAssembly), http://webassembly.org/T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

Description

@alexcrichton
alexcrichton
opened on Mar 21, 2025

This issue is intended to be a tracking issue for the future-incompat warning being added in #138601. This lint is notifying users of an upcoming change to the C ABI used by the wasm32-unknown-unknown target, notably around passing aggregates-by-value in parameter position. An exampe of code that will change is:

#[repr(C)]
pub struct Pair {
    x: u32,
    y: u32,
}

#[unsafe(no_mangle)]
pub extern "C" fn pair_add(pair: Pair) -> u32 {
    pair.x + pair.y
}

where today this generates:

(func $pair_add (param i32 i32) (result i32)
  local.get 1
  local.get 0
  i32.add
)

but in the future this will generate:

(func (param i32) (result i32)
  local.get 0
  i32.load offset=4
  local.get 0
  i32.load
  i32.add
)

More details about this change and its history can be found in the blog post associated with this change. In short though users need to do one of the following to resolve the warnings:

  1. Pin to a stable compiler and don't update until the default has changed.
  2. Update to nightly, pass -Zwasm-c-abi=spec, and then use that until the default changes. This means signatures will change immediately and work will be necessary to port external JS for example.
  3. Update to nightly, pass -Zwasm-c-abi=legacy. This will silence the warnings but be warned that code will still break in the future when the ABI changes.
  4. Update function signatures to not pass aggregates-by-value, but instead pass them by reference. This works both today and with the future ABI.

The current plan is to change the default ABI mid-summer 2025. This'll get updated with exact timelines as things happen. More background can be found in #122532

Implementation history

Activity

  1. rustbot
    added
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Mar 21, 2025
  2. alexcrichton
    changed the title [-]Tracking issue for wasm32-unknown-unknown's future-incompat warning[/-] [+]Tracking issue for wasm32-unknown-unknown's future-incompat warning for C ABI changes[/+] on Mar 21, 2025
  3. jieyouxu
    added
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    O-wasmTarget: WASM (WebAssembly), http://webassembly.org/
    C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFC
    C-future-incompatibilityCategory: Future-incompatibility lints
    A-ABIArea: Concerning the application binary interface (ABI)
    and removed
    needs-triageThis issue may need triage. Remove when done. See docs forge.rust-lang.org/release/issue-triaging
    on Mar 21, 2025
  4. tgross35
    added a commit that references this issue on Mar 22, 2025

    Rollup merge of rust-lang#138601 - RalfJung:wasm-abi-fcw, r=alexcrichton

    d7573cd
  5. matthiaskrgr
    added a commit that references this issue on Mar 24, 2025

    Rollup merge of rust-lang#138601 - RalfJung:wasm-abi-fcw, r=alexcrichton

    9c93d95
  6. bors
    added 4 commits that reference this issue on Mar 25, 2025

    Auto merge of rust-lang#138601 - RalfJung:wasm-abi-fcw, r=<try>

    cb87fb8

    Auto merge of rust-lang#138601 - RalfJung:wasm-abi-fcw, r=<try>

    fde8c77

    Auto merge of rust-lang#138601 - RalfJung:wasm-abi-fcw, r=<try>

    c19c3fb

    Auto merge of rust-lang#138601 - RalfJung:wasm-abi-fcw, r=alexcrichton

    068609c
  7. github-actions
    added a commit that references this issue on Mar 27, 2025

    Auto merge of #138601 - RalfJung:wasm-abi-fcw, r=alexcrichton

    e2dbc25

  8. 16 remaining items

  9. Zalathar
    added a commit that references this issue on Apr 24, 2025

    Rollup merge of rust-lang#139809 - alexcrichton:wasm-simd-safe, r=Ral…

    54c65e6
  10. matthiaskrgr
    added a commit that references this issue on Apr 24, 2025

    Rollup merge of rust-lang#139809 - alexcrichton:wasm-simd-safe, r=Ral…

    32b2428
  11. rust-timer
    added a commit that references this issue on Apr 24, 2025

    Unrolled build for rust-lang#139809

    3d6e33e
  12. cuviper
    added 2 commits that reference this issue on Apr 24, 2025

    Ignore zero-sized types in wasm future-compat warning

    39721fa

    Don't warn about `v128` in wasm ABI transition

    eaa7d3a
  13. github-actions
    added a commit that references this issue on Apr 25, 2025

    Rollup merge of #139809 - alexcrichton:wasm-simd-safe, r=RalfJung

    9bb232a
  14. fmease
    added a commit that references this issue on Jun 14, 2025

    Rollup merge of rust-lang#133952 - bjorn3:remove_wasm_legacy_abi, r=a…

    760b63d
  15. matthiaskrgr
    added 2 commits that reference this issue on Jun 15, 2025

    Rollup merge of rust-lang#133952 - bjorn3:remove_wasm_legacy_abi, r=a…

    d9cddf0

    Rollup merge of rust-lang#133952 - bjorn3:remove_wasm_legacy_abi, r=a…

    81824fd
  16. fmease
    added 2 commits that reference this issue on Jun 15, 2025

    Rollup merge of rust-lang#133952 - bjorn3:remove_wasm_legacy_abi, r=a…

    d3c83f3

    Rollup merge of rust-lang#133952 - bjorn3:remove_wasm_legacy_abi, r=a…

    d6dc965
  17. bors
    closed this as completedin d6dc965on Jun 16, 2025
  18. rust-timer
    added a commit that references this issue on Jun 16, 2025

    Unrolled build for #133952

    6c0e0a2
  19. github-actions
    added a commit that references this issue on Jun 18, 2025

    Rollup merge of rust-lang#133952 - bjorn3:remove_wasm_legacy_abi, r=a…

    871de6e
  20. antoyo
    added a commit that references this issue on Jun 28, 2025

    Rollup merge of #133952 - bjorn3:remove_wasm_legacy_abi, r=alexcrichton

    29e0e4f
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Metadata

Metadata

Assignees

No one assigned

    Labels

    A-ABIArea: Concerning the application binary interface (ABI)C-future-incompatibilityCategory: Future-incompatibility lintsC-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCO-wasmTarget: WASM (WebAssembly), http://webassembly.org/T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

    Type

    No type

    Projects

    No projects

      Milestone

      No milestone

      Relationships

      None yet

      Development

      No branches or pull requests

      Issue actions