Remove the `CoroutineWitness` type, follow-up to #144157 #151078

emil-dietrich · 2026-01-13T19:40:13Z

Based upon the work in #144157. Last commit says Anyone is welcome to pick this back up. I hope I understood correctly and effort/time to rebase was the blocker on that PR. My first PR on rust-lang - so let me know if I could've done something better.

rustbot · 2026-01-13T19:40:16Z

changes to the core type system

cc @lcnr

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in exhaustiveness checking

cc @Nadrieril

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_sanitizers

cc @rcvalle

rustbot · 2026-01-13T19:40:18Z

r? @WaffleLapkin

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

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emil-dietrich · 2026-01-13T19:45:39Z

r? @compiler-errors
I hope this is reasonable

rustbot · 2026-01-13T19:46:00Z

Failed to set assignee to compiler-errors: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

compiler-errors · 2026-01-13T19:47:57Z

I don’t work on Rust anymore unfortunately.

r? lcnr

rust-log-analyzer · 2026-01-13T20:20:40Z

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [ui] tests/ui/imports/ambiguous-6.rs ... ok
test [ui] tests/ui/imports/ambiguous-4.rs ... ok
test [ui] tests/ui/imports/ambiguous-7.rs ... ok
test [ui] tests/ui/imports/ambiguous-9.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-glob-vs-multiouter.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-globvsglob.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-no-implicit-prelude.rs ... ok
test [ui] tests/ui/imports/ambiguous-8.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-non-prelude-core-glob.rs ... ok
test [ui] tests/ui/imports/ambiguous-glob-vs-expanded-extern.rs ... ok
test [ui] tests/ui/imports/ambiguous-panic-non-prelude-std-glob.rs ... ok
---
To only update this specific test, also pass `--test-args async-await/partial-drop-partial-reinit.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/async-await/partial-drop-partial-reinit.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/async-await/partial-drop-partial-reinit" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2021"
stdout: none
--- stderr -------------------------------
error[E0277]: `NotSend` cannot be sent between threads safely
##[error]  --> /checkout/tests/ui/async-await/partial-drop-partial-reinit.rs:6:16
   |
LL |     gimme_send(foo());
   |     ---------- ^^^^^ `NotSend` cannot be sent between threads safely
   |     |
   |     required by a bound introduced by this call
...
LL | async fn foo() {
   |               - within this `impl Future<Output = ()>`
---
  --> /checkout/tests/ui/async-await/partial-drop-partial-reinit.rs:26:16
   |
LL |   async fn foo() {
   |  ________________^
LL | |     //~^ NOTE used within this `async` fn body
LL | |     //~| NOTE within this `impl Future
LL | |     let mut x = (NotSend {},);
...  |
LL | |     bar().await;
LL | | }
   | |_^
note: required by a bound in `gimme_send`
  --> /checkout/tests/ui/async-await/partial-drop-partial-reinit.rs:12:18
   |
LL | fn gimme_send<T: Send>(t: T) {
   |                  ^^^^ required by this bound in `gimme_send`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.
------------------------------------------

---- [ui] tests/ui/async-await/partial-drop-partial-reinit.rs stdout end ----
---- [ui] tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2024"
stdout: none
--- stderr -------------------------------
error[E0275]: overflow evaluating the requirement `Unique<impl Future<Output = ()>>: Send`
##[error]  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:16:5
   |
LL |     require_send(process(&1));
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`recursive_async_auto_trait_overflow_only_parent_args`)
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:9:34
   |
LL |   async fn process<'a>(x: &'a u32) {
   |  __________________________________^
LL | |     Box::pin(process(x)).await;
LL | | }
   | |_^
note: required by a bound in `require_send`
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs:13:25
   |
LL | fn require_send(_: impl Send) {}
   |                         ^^^^ required by this bound in `require_send`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0275`.
------------------------------------------

---- [ui] tests/ui/async-await/recursive-async-auto-trait-overflow-only-parent-args.rs stdout end ----
---- [ui] tests/ui/async-await/recursive-async-auto-trait-overflow.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/async-await/recursive-async-auto-trait-overflow" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2024"
stdout: none
--- stderr -------------------------------
error[E0275]: overflow evaluating the requirement `Unique<impl Future<Output = ()>>: Send`
##[error]  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:13:5
   |
LL |     require_send(process());
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`recursive_async_auto_trait_overflow`)
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
LL | |     Box::pin(process()).await;
LL | | }
   | |_^
   = note: required for `Unique<impl Future<Output = ()>>` to implement `Send`
note: required because it appears within the type `Box<impl Future<Output = ()>>`
  --> /rustc/FAKE_PREFIX/library/alloc/src/boxed.rs:234:11
note: required because it appears within the type `Pin<Box<impl Future<Output = ()>>>`
  --> /rustc/FAKE_PREFIX/library/core/src/pin.rs:1094:11
note: required because it's used within this `async` fn body
  --> /checkout/tests/ui/async-await/recursive-async-auto-trait-overflow.rs:6:24
   |
LL |   async fn process<'a>() {
   |  ________________________^
---
diff of run.stdout:

10                 offset: 1,
11             },
12             Field {
-                 ty: TypeId(0x41223169ff28813ba79b7268a2a968d9),
+                 ty: TypeId(0xe09322dd03745d1d9f5ce3532baab234),
14                 offset: 2,
15             },
16         ],


The actual run.stdout differed from the expected run.stdout
---
--- stdout -------------------------------
Tuple(
    Tuple {
        fields: [
            Field {
                ty: TypeId(0x0596b48cc04376e64d5c788c2aa46bdb),
                offset: 0,
            },
            Field {
                ty: TypeId(0x0596b48cc04376e64d5c788c2aa46bdb),
                offset: 1,
            },
            Field {
                ty: TypeId(0xe09322dd03745d1d9f5ce3532baab234),
                offset: 2,
            },
        ],
    },
)
Other

Kivooeo · 2026-01-13T22:37:54Z

hi, thanks for picking this up, but i guess it wasn't finished back then for some reason, and it seems to me because of exactly same problem that your pr hit here with overflow evaluating

so it seems like this problem is more complex and im not really sure if that's a good first issue (don't get me wrong i'm not trying to discourage you on this)

rust-bors · 2026-01-15T01:01:53Z

☔ The latest upstream changes (presumably #151144) made this pull request unmergeable. Please resolve the merge conflicts.

lcnr · 2026-01-15T10:19:48Z

The remaining work is quite technically involved #144157 (comment). @BoxyUwU is currently looking into introducing where-bounds on binders in general, which will supersede this. Alternatively, we can finish this once the new trait solver is stable #107374

lcnr · 2026-01-15T10:20:38Z

Thank you for trying to continue this work.

compiler-errors and others added 3 commits January 13, 2026 20:16

Remove coroutine witness type

673a1ad

Fix tools

47fb6e7

rebase fixup

3b6e091

rustbot assigned WaffleLapkin Jan 13, 2026

rustbot assigned lcnr and unassigned WaffleLapkin Jan 13, 2026

lcnr closed this Jan 15, 2026

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 15, 2026

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Remove the `CoroutineWitness` type, follow-up to #144157 #151078

Remove the `CoroutineWitness` type, follow-up to #144157 #151078

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Remove the CoroutineWitness type, follow-up to #144157 #151078

Remove the CoroutineWitness type, follow-up to #144157 #151078

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Remove the `CoroutineWitness` type, follow-up to #144157 #151078

Remove the `CoroutineWitness` type, follow-up to #144157 #151078

emil-dietrich commented Jan 13, 2026 •

edited by rustbot

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