Change type of async context parameter after state transform. by cjgillot · Pull Request #157166 · rust-lang/rust

cjgillot · 2026-05-30T17:36:29Z

Future::poll expects a &mut std::task::Context<'_>. Meanwhile, async coroutines use std::future::ResumeTy as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>.

In the coroutine state transform, we need to change the signature from ResumeTy to the proper &mut Context<'_>. The current code attempts to find locals that have type ResumeTy to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder.

Instead, this PR proposes to replace the ResumeTy argument with a new local, with value ResumeTy(transmute(context)).

Based on #156875.

rustbot · 2026-05-30T17:36:32Z

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-05-30T17:36:35Z

r? @tiif

rustbot has assigned @tiif.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: compiler
compiler expanded to 73 candidates
Random selection from 17 candidates

rustbot · 2026-06-04T12:12:55Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

oli-obk · 2026-06-09T15:36:02Z

+/// HIR uses `get_context` to unwrap a `&mut Context<'_>` from a `ResumeTy`.
+/// Both types are just a single pointer, but liveness analysis does not know that and
+/// supposes that the operand and the destination are live at the same time.
+/// Forcibly inline those calls to avoid this.


preexisting, but: it feels a bit funny to generate a function call in ast lowering only to desugar it here manually. The function body is effectively dead code.

View changes since the review

oli-obk

@bors r+

View changes since this review

rust-bors · 2026-06-09T15:36:44Z

📌 Commit b7e26ac has been approved by oli-obk

It is now in the queue for this repository.

…li-obk Change type of async context parameter after state transform. `Future::poll` expects a `&mut std::task::Context<'_>`. Meanwhile, async coroutines use `std::future::ResumeTy` as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement `for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>`. In the coroutine state transform, we need to change the signature from `ResumeTy` to the proper `&mut Context<'_>`. The current code attempts to find locals that have type `ResumeTy` to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder. Instead, this PR proposes to replace the `ResumeTy` argument with a new local, with value `ResumeTy(transmute(context))`. Based on rust-lang#156875.

Rollup of 18 pull requests Successful merges: - #152852 (Remove driver_lint_caps) - #157166 (Change type of async context parameter after state transform.) - #157335 (bootstrap: Handle dotted table keys when parsing bootstrap.toml) - #157503 (Disable `tests/debuginfo/pretty-std.rs` `OsString` cdb check) - #157571 (Remove ProcMacro enum from proc macro ABI) - #148183 (rustdoc: Test & document `test_harness` code block attribute) - #153847 (Fix marker trait winnowing depending on impl order) - #156067 (Fix async drop glue for Box<T>) - #156399 (fix improper ctypes in Znext solver) - #157338 (Make `Literal::byte_character_value` work with bytes as well) - #157410 (Implement rustc_public::CrateDef{,Type} for FieldDef) - #157605 (Arg splat experiment - syntax impl) - #157630 (Add multibyte JSON diagnostic regression test) - #157633 (Reorder `impl` restriction rendering and add bottom margin) - #157642 (Report duplicate relaxed bounds during ast lowering) - #157652 (fix doc for unicode normalization faq on `casefold` APIs) - #157661 (Update to ar_archive_writer v0.5.2) - #157668 (Add test for matches in `rustc_must_match_exhaustively`) Failed merges: - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N))

Rollup of 17 pull requests Successful merges: - #157166 (Change type of async context parameter after state transform.) - #157335 (bootstrap: Handle dotted table keys when parsing bootstrap.toml) - #157503 (Disable `tests/debuginfo/pretty-std.rs` `OsString` cdb check) - #157571 (Remove ProcMacro enum from proc macro ABI) - #148183 (rustdoc: Test & document `test_harness` code block attribute) - #153847 (Fix marker trait winnowing depending on impl order) - #156067 (Fix async drop glue for Box<T>) - #156399 (fix improper ctypes in Znext solver) - #157338 (Make `Literal::byte_character_value` work with bytes as well) - #157410 (Implement rustc_public::CrateDef{,Type} for FieldDef) - #157605 (Arg splat experiment - syntax impl) - #157630 (Add multibyte JSON diagnostic regression test) - #157633 (Reorder `impl` restriction rendering and add bottom margin) - #157642 (Report duplicate relaxed bounds during ast lowering) - #157652 (fix doc for unicode normalization faq on `casefold` APIs) - #157661 (Update to ar_archive_writer v0.5.2) - #157668 (Add test for matches in `rustc_must_match_exhaustively`) Failed merges: - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N))

rust-bors · 2026-06-09T21:49:38Z

⌛ Testing commit b7e26ac with merge 5103141...

Workflow: https://github.com/rust-lang/rust/actions/runs/27238086714

Change type of async context parameter after state transform. `Future::poll` expects a `&mut std::task::Context<'_>`. Meanwhile, async coroutines use `std::future::ResumeTy` as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement `for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>`. In the coroutine state transform, we need to change the signature from `ResumeTy` to the proper `&mut Context<'_>`. The current code attempts to find locals that have type `ResumeTy` to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder. Instead, this PR proposes to replace the `ResumeTy` argument with a new local, with value `ResumeTy(transmute(context))`. Based on #156875.

jhpratt · 2026-06-09T21:51:41Z

yielding to a rollup=never PR

@bors yield

rust-bors · 2026-06-09T21:51:44Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/27238086714

The next pull request likely to be tested is #129543.

Rollup of 17 pull requests Successful merges: - #157166 (Change type of async context parameter after state transform.) - #157335 (bootstrap: Handle dotted table keys when parsing bootstrap.toml) - #157503 (Disable `tests/debuginfo/pretty-std.rs` `OsString` cdb check) - #157571 (Remove ProcMacro enum from proc macro ABI) - #148183 (rustdoc: Test & document `test_harness` code block attribute) - #153847 (Fix marker trait winnowing depending on impl order) - #156067 (Fix async drop glue for Box<T>) - #156399 (fix improper ctypes in Znext solver) - #157338 (Make `Literal::byte_character_value` work with bytes as well) - #157410 (Implement rustc_public::CrateDef{,Type} for FieldDef) - #157605 (Arg splat experiment - syntax impl) - #157630 (Add multibyte JSON diagnostic regression test) - #157633 (Reorder `impl` restriction rendering and add bottom margin) - #157642 (Report duplicate relaxed bounds during ast lowering) - #157652 (fix doc for unicode normalization faq on `casefold` APIs) - #157661 (Update to ar_archive_writer v0.5.2) - #157668 (Add test for matches in `rustc_must_match_exhaustively`) Failed merges: - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N))

rust-bors · 2026-06-10T02:25:28Z

⌛ Testing commit b7e26ac with merge 39465ed...

Workflow: https://github.com/rust-lang/rust/actions/runs/27248950736

Change type of async context parameter after state transform. `Future::poll` expects a `&mut std::task::Context<'_>`. Meanwhile, async coroutines use `std::future::ResumeTy` as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement `for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>`. In the coroutine state transform, we need to change the signature from `ResumeTy` to the proper `&mut Context<'_>`. The current code attempts to find locals that have type `ResumeTy` to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder. Instead, this PR proposes to replace the `ResumeTy` argument with a new local, with value `ResumeTy(transmute(context))`. Based on #156875.

jhpratt · 2026-06-10T02:31:08Z

same thing

@bors yield

rust-bors · 2026-06-10T02:31:11Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/27248950736

The next pull request likely to be tested is #157683.

Rollup of 17 pull requests Successful merges: - #157166 (Change type of async context parameter after state transform.) - #157335 (bootstrap: Handle dotted table keys when parsing bootstrap.toml) - #157503 (Disable `tests/debuginfo/pretty-std.rs` `OsString` cdb check) - #157571 (Remove ProcMacro enum from proc macro ABI) - #148183 (rustdoc: Test & document `test_harness` code block attribute) - #153847 (Fix marker trait winnowing depending on impl order) - #156067 (Fix async drop glue for Box<T>) - #156399 (fix improper ctypes in Znext solver) - #157338 (Make `Literal::byte_character_value` work with bytes as well) - #157410 (Implement rustc_public::CrateDef{,Type} for FieldDef) - #157605 (Arg splat experiment - syntax impl) - #157630 (Add multibyte JSON diagnostic regression test) - #157633 (Reorder `impl` restriction rendering and add bottom margin) - #157642 (Report duplicate relaxed bounds during ast lowering) - #157652 (fix doc for unicode normalization faq on `casefold` APIs) - #157661 (Update to ar_archive_writer v0.5.2) - #157668 (Add test for matches in `rustc_must_match_exhaustively`) Failed merges: - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N))

rust-bors · 2026-06-10T02:37:33Z

⌛ Testing commit b7e26ac with merge e1e4985...

Workflow: https://github.com/rust-lang/rust/actions/runs/27249369474

Change type of async context parameter after state transform. `Future::poll` expects a `&mut std::task::Context<'_>`. Meanwhile, async coroutines use `std::future::ResumeTy` as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement `for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>`. In the coroutine state transform, we need to change the signature from `ResumeTy` to the proper `&mut Context<'_>`. The current code attempts to find locals that have type `ResumeTy` to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder. Instead, this PR proposes to replace the `ResumeTy` argument with a new local, with value `ResumeTy(transmute(context))`. Based on #156875.

jhpratt · 2026-06-10T02:42:45Z

repeated network failures; I've closed the tree elsewhere

@bors yield

rust-bors · 2026-06-10T02:42:48Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/27249369474

The next pull request likely to be tested is #157683.

Rollup of 17 pull requests Successful merges: - #157166 (Change type of async context parameter after state transform.) - #157335 (bootstrap: Handle dotted table keys when parsing bootstrap.toml) - #157503 (Disable `tests/debuginfo/pretty-std.rs` `OsString` cdb check) - #157571 (Remove ProcMacro enum from proc macro ABI) - #148183 (rustdoc: Test & document `test_harness` code block attribute) - #153847 (Fix marker trait winnowing depending on impl order) - #156067 (Fix async drop glue for Box<T>) - #156399 (fix improper ctypes in Znext solver) - #157338 (Make `Literal::byte_character_value` work with bytes as well) - #157410 (Implement rustc_public::CrateDef{,Type} for FieldDef) - #157605 (Arg splat experiment - syntax impl) - #157630 (Add multibyte JSON diagnostic regression test) - #157633 (Reorder `impl` restriction rendering and add bottom margin) - #157642 (Report duplicate relaxed bounds during ast lowering) - #157652 (fix doc for unicode normalization faq on `casefold` APIs) - #157661 (Update to ar_archive_writer v0.5.2) - #157668 (Add test for matches in `rustc_must_match_exhaustively`) Failed merges: - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N))

Rollup merge of #157166 - cjgillot:retype-context-after, r=oli-obk Change type of async context parameter after state transform. `Future::poll` expects a `&mut std::task::Context<'_>`. Meanwhile, async coroutines use `std::future::ResumeTy` as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement `for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>`. In the coroutine state transform, we need to change the signature from `ResumeTy` to the proper `&mut Context<'_>`. The current code attempts to find locals that have type `ResumeTy` to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder. Instead, this PR proposes to replace the `ResumeTy` argument with a new local, with value `ResumeTy(transmute(context))`. Based on #156875.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 30, 2026

rustbot assigned tiif May 30, 2026