corotuine_clone: add comments

Author: RalfJung

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)]