Add explicit syntax for coroutines instead of relying on closures having yield expressions

Author: oli-obk

Diff for: compiler/rustc_ast_lowering/src/expr.rs

@@ -217,6 +217,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         binder,
                         *capture_clause,
                         e.id,
+                        hir_id,
                         *constness,
                         *movability,
                         fn_decl,
@@ -955,6 +956,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         binder: &ClosureBinder,
         capture_clause: CaptureBy,
         closure_id: NodeId,
+        closure_hir_id: hir::HirId,
         constness: Const,
         movability: Movability,
         decl: &FnDecl,
@@ -965,7 +967,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 
         let (body_id, closure_kind) = self.with_new_scopes(fn_decl_span, move |this| {
-            let mut coroutine_kind = None;
+            let mut coroutine_kind = if this
+                .attrs
+                .get(&closure_hir_id.local_id)
+                .is_some_and(|attrs| attrs.iter().any(|attr| attr.has_name(sym::coroutine)))
+            {
+                Some(hir::CoroutineKind::Coroutine(Movability::Movable))
+            } else {
+                None
+            };
             let body_id = this.lower_fn_body(decl, |this| {
                 let e = this.lower_expr_mut(body);
                 coroutine_kind = this.coroutine_kind;

Diff for: compiler/rustc_feature/src/builtin_attrs.rs

@@ -534,6 +534,12 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         EncodeCrossCrate::Yes, experimental!(cfi_encoding)
     ),
 
+    // `#[coroutine]` attribute to be applied to closures to make them coroutines instead
+    gated!(
+        coroutine, Normal, template!(Word), ErrorFollowing,
+        EncodeCrossCrate::No, coroutines, experimental!(coroutines)
+    ),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================

Diff for: tests/ui/coroutine/derived-drop-parent-expr.rs

@@ -10,7 +10,7 @@ pub struct Client { pub nickname: String }
 
 fn main() {
     let g = move || match drop(Client { ..Client::default() }) {
-        _status => yield,
-    };
+            _status => yield,
+        };
     assert_send(g);
 }