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`?