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Rollup merge of #134618 - RalfJung:coroutine-clone-comments, r=lqd

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coroutine_clone: add comments

I was very surprised to learn that coroutines can be cloned. This has non-trivial semantic consequences that I do not think have been considered. Lucky enough, it's still unstable. Let's add some comments and pointers so we hopefully become aware when a MIR opt actually is in conflict with this.

Cc `@rust-lang/wg-mir-opt`
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Matthias Krüger 2024-12-22 03:49:44 +01:00 • committed by GitHub
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5
compiler/rustc_middle/src/mir/syntax.rs
View file

@ -821,6 +821,11 @@ pub enum TerminatorKind<'tcx> {
/// continues at the `resume` basic block, with the second argument written to the `resume_arg`
/// place. If the coroutine is dropped before then, the `drop` basic block is invoked.
///
/// Note that coroutines can be (unstably) cloned under certain conditions, which means that
/// this terminator can **return multiple times**! MIR optimizations that reorder code into
/// different basic blocks needs to be aware of that.
/// See <https://github.com/rust-lang/rust/issues/95360>.
///
/// Not permitted in bodies that are not coroutine bodies, or after coroutine lowering.
///
/// **Needs clarification**: What about the evaluation order of the `resume_arg` and `value`?

2
tests/ui/coroutine/clone-impl-static.rs
View file

@ -1,6 +1,8 @@
//@compile-flags: --diagnostic-width=300
// gate-test-coroutine_clone
// Verifies that static coroutines cannot be cloned/copied.
// This is important: the cloned coroutine would reference state of the original
// coroutine, leading to semantic nonsense.
#![feature(coroutines, coroutine_clone, stmt_expr_attributes)]

16
tests/ui/coroutine/clone-impl-static.stderr
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@ -1,27 +1,27 @@
error[E0277]: the trait bound `{static coroutine@$DIR/clone-impl-static.rs:9:5: 9:19}: Copy` is not satisfied
--> $DIR/clone-impl-static.rs:12:16
error[E0277]: the trait bound `{static coroutine@$DIR/clone-impl-static.rs:11:5: 11:19}: Copy` is not satisfied
--> $DIR/clone-impl-static.rs:14:16
|
LL | check_copy(&gen);
| ---------- ^^^^ the trait `Copy` is not implemented for `{static coroutine@$DIR/clone-impl-static.rs:9:5: 9:19}`
| ---------- ^^^^ the trait `Copy` is not implemented for `{static coroutine@$DIR/clone-impl-static.rs:11:5: 11:19}`
| |
| required by a bound introduced by this call
|
note: required by a bound in `check_copy`
--> $DIR/clone-impl-static.rs:18:18
--> $DIR/clone-impl-static.rs:20:18
|
LL | fn check_copy<T: Copy>(_x: &T) {}
| ^^^^ required by this bound in `check_copy`
error[E0277]: the trait bound `{static coroutine@$DIR/clone-impl-static.rs:9:5: 9:19}: Clone` is not satisfied
--> $DIR/clone-impl-static.rs:14:17
error[E0277]: the trait bound `{static coroutine@$DIR/clone-impl-static.rs:11:5: 11:19}: Clone` is not satisfied
--> $DIR/clone-impl-static.rs:16:17
|
LL | check_clone(&gen);
| ----------- ^^^^ the trait `Clone` is not implemented for `{static coroutine@$DIR/clone-impl-static.rs:9:5: 9:19}`
| ----------- ^^^^ the trait `Clone` is not implemented for `{static coroutine@$DIR/clone-impl-static.rs:11:5: 11:19}`
| |
| required by a bound introduced by this call
|
note: required by a bound in `check_clone`
--> $DIR/clone-impl-static.rs:19:19
--> $DIR/clone-impl-static.rs:21:19
|
LL | fn check_clone<T: Clone>(_x: &T) {}
| ^^^^^ required by this bound in `check_clone`