Auto merge of rust-lang#118420 - compiler-errors:async-gen, r=eholk

Introduce support for `async gen` blocks

I'm delighted to demonstrate that `async gen` block are not very difficult to support. They're simply coroutines that yield `Poll<Option<T>>` and return `()`.

**This PR is WIP and in draft mode for now** -- I'm mostly putting it up to show folks that it's possible. This PR needs a lang-team experiment associated with it or possible an RFC, since I don't think it falls under the jurisdiction of the `gen` RFC that was recently authored by oli (rust-lang/rfcs#3513, rust-lang#117078).

### Technical note on the pre-generator-transform yield type:

The reason that the underlying coroutines yield `Poll<Option<T>>` and not `Poll<T>` (which would make more sense, IMO, for the pre-transformed coroutine), is because the `TransformVisitor` that is used to turn coroutines into built-in state machine functions would have to destructure and reconstruct the latter into the former, which requires at least inserting a new basic block (for a `switchInt` terminator, to match on the `Poll` discriminant).

This does mean that the desugaring (at the `rustc_ast_lowering` level) of `async gen` blocks is a bit more involved. However, since we already need to intercept both `.await` and `yield` operators, I don't consider it much of a technical burden.

r? `@ghost`

Author: bors

src/closures.rs

@@ -29,7 +29,7 @@ pub(crate) fn rewrite_closure(
     binder: &ast::ClosureBinder,
     constness: ast::Const,
     capture: ast::CaptureBy,
-    coro_kind: &Option<ast::CoroutineKind>,
+    coroutine_kind: &Option<ast::CoroutineKind>,
     movability: ast::Movability,
     fn_decl: &ast::FnDecl,
     body: &ast::Expr,
@@ -40,7 +40,16 @@ pub(crate) fn rewrite_closure(
     debug!("rewrite_closure {:?}", body);
 
     let (prefix, extra_offset) = rewrite_closure_fn_decl(
-        binder, constness, capture, coro_kind, movability, fn_decl, body, span, context, shape,
+        binder,
+        constness,
+        capture,
+        coroutine_kind,
+        movability,
+        fn_decl,
+        body,
+        span,
+        context,
+        shape,
     )?;
     // 1 = space between `|...|` and body.
     let body_shape = shape.offset_left(extra_offset)?;
@@ -233,7 +242,7 @@ fn rewrite_closure_fn_decl(
     binder: &ast::ClosureBinder,
     constness: ast::Const,
     capture: ast::CaptureBy,
-    coro_kind: &Option<ast::CoroutineKind>,
+    coroutine_kind: &Option<ast::CoroutineKind>,
     movability: ast::Movability,
     fn_decl: &ast::FnDecl,
     body: &ast::Expr,
@@ -263,9 +272,10 @@ fn rewrite_closure_fn_decl(
     } else {
         ""
     };
-    let coro = match coro_kind {
+    let coro = match coroutine_kind {
         Some(ast::CoroutineKind::Async { .. }) => "async ",
         Some(ast::CoroutineKind::Gen { .. }) => "gen ",
+        Some(ast::CoroutineKind::AsyncGen { .. }) => "async gen ",
         None => "",
     };
     let mover = if matches!(capture, ast::CaptureBy::Value { .. }) {
@@ -343,7 +353,7 @@ pub(crate) fn rewrite_last_closure(
             ref binder,
             constness,
             capture_clause,
-            ref coro_kind,
+            ref coroutine_kind,
             movability,
             ref fn_decl,
             ref body,
@@ -364,7 +374,7 @@ pub(crate) fn rewrite_last_closure(
             binder,
             constness,
             capture_clause,
-            coro_kind,
+            coroutine_kind,
             movability,
             fn_decl,
             body,

src/expr.rs

@@ -212,7 +212,7 @@ pub(crate) fn format_expr(
             &cl.binder,
             cl.constness,
             cl.capture_clause,
-            &cl.coro_kind,
+            &cl.coroutine_kind,
             cl.movability,
             &cl.fn_decl,
             &cl.body,

src/items.rs

@@ -287,7 +287,7 @@ pub(crate) struct FnSig<'a> {
     decl: &'a ast::FnDecl,
     generics: &'a ast::Generics,
     ext: ast::Extern,
-    coro_kind: Cow<'a, Option<ast::CoroutineKind>>,
+    coroutine_kind: Cow<'a, Option<ast::CoroutineKind>>,
     constness: ast::Const,
     defaultness: ast::Defaultness,
     unsafety: ast::Unsafe,
@@ -302,7 +302,7 @@ impl<'a> FnSig<'a> {
     ) -> FnSig<'a> {
         FnSig {
             unsafety: method_sig.header.unsafety,
-            coro_kind: Cow::Borrowed(&method_sig.header.coro_kind),
+            coroutine_kind: Cow::Borrowed(&method_sig.header.coroutine_kind),
             constness: method_sig.header.constness,
             defaultness: ast::Defaultness::Final,
             ext: method_sig.header.ext,
@@ -328,7 +328,7 @@ impl<'a> FnSig<'a> {
                 generics,
                 ext: fn_sig.header.ext,
                 constness: fn_sig.header.constness,
-                coro_kind: Cow::Borrowed(&fn_sig.header.coro_kind),
+                coroutine_kind: Cow::Borrowed(&fn_sig.header.coroutine_kind),
                 defaultness,
                 unsafety: fn_sig.header.unsafety,
                 visibility: vis,
@@ -343,8 +343,8 @@ impl<'a> FnSig<'a> {
         result.push_str(&*format_visibility(context, self.visibility));
         result.push_str(format_defaultness(self.defaultness));
         result.push_str(format_constness(self.constness));
-        self.coro_kind
-            .map(|coro_kind| result.push_str(format_coro(&coro_kind)));
+        self.coroutine_kind
+            .map(|coroutine_kind| result.push_str(format_coro(&coroutine_kind)));
         result.push_str(format_unsafety(self.unsafety));
         result.push_str(&format_extern(
             self.ext,

src/utils.rs

@@ -75,10 +75,11 @@ pub(crate) fn format_visibility(
 }
 
 #[inline]
-pub(crate) fn format_coro(coro_kind: &ast::CoroutineKind) -> &'static str {
-    match coro_kind {
+pub(crate) fn format_coro(coroutine_kind: &ast::CoroutineKind) -> &'static str {
+    match coroutine_kind {
         ast::CoroutineKind::Async { .. } => "async ",
         ast::CoroutineKind::Gen { .. } => "gen ",
+        ast::CoroutineKind::AsyncGen { .. } => "async gen ",
     }
 }