LangItem-ify Coroutine trait in solvers

core/src/ops/coroutine.rs

@@ -76,6 +76,7 @@ pub trait Coroutine<R = ()> {
     /// values which are allowed to be returned each time a coroutine yields.
     /// For example an iterator-as-a-coroutine would likely have this type as
     /// `T`, the type being iterated over.
+    #[cfg_attr(not(bootstrap), lang = "coroutine_yield")]
     type Yield;
 
     /// The type of value this coroutine returns.
@@ -84,6 +85,7 @@ pub trait Coroutine<R = ()> {
     /// `return` statement or implicitly as the last expression of a coroutine
     /// literal. For example futures would use this as `Result<T, E>` as it
     /// represents a completed future.
+    #[cfg_attr(not(bootstrap), lang = "coroutine_return")]
     type Return;
 
     /// Resumes the execution of this coroutine.