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

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`

compiler/rustc_middle/src/mir/syntax.rs

@@ -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`?

tests/ui/coroutine/clone-impl-static.rs

@@ -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)]
 

tests/ui/coroutine/clone-impl-static.stderr

@@ -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`