Implement async gen blocks

compiler/rustc_hir/src/hir.rs

@@ -1339,12 +1339,16 @@ impl<'hir> Body<'hir> {
 /// The type of source expression that caused this coroutine to be created.
 #[derive(Clone, PartialEq, Eq, Debug, Copy, Hash, HashStable_Generic, Encodable, Decodable)]
 pub enum CoroutineKind {
-    /// An explicit `async` block or the body of an async function.
+    /// An explicit `async` block or the body of an `async` function.
     Async(CoroutineSource),
 
     /// An explicit `gen` block or the body of a `gen` function.
     Gen(CoroutineSource),
 
+    /// An explicit `async gen` block or the body of an `async gen` function,
+    /// which is able to both `yield` and `.await`.
+    AsyncGen(CoroutineSource),
+
     /// A coroutine literal created via a `yield` inside a closure.
     Coroutine,
 }
@@ -1369,6 +1373,14 @@ impl fmt::Display for CoroutineKind {
                 }
                 k.fmt(f)
             }
+            CoroutineKind::AsyncGen(k) => {
+                if f.alternate() {
+                    f.write_str("`async gen` ")?;
+                } else {
+                    f.write_str("async gen ")?
+                }
+                k.fmt(f)
+            }
         }
     }
 }
@@ -2064,17 +2076,6 @@ impl fmt::Display for YieldSource {
     }
 }
 
-impl From<CoroutineKind> for YieldSource {
-    fn from(kind: CoroutineKind) -> Self {
-        match kind {
-            // Guess based on the kind of the current coroutine.
-            CoroutineKind::Coroutine => Self::Yield,
-            CoroutineKind::Async(_) => Self::Await { expr: None },
-            CoroutineKind::Gen(_) => Self::Yield,
-        }
-    }
-}
-
 // N.B., if you change this, you'll probably want to change the corresponding
 // type structure in middle/ty.rs as well.
 #[derive(Debug, Clone, Copy, HashStable_Generic)]