Auto merge of #118457 - eholk:genfn, r=compiler-errors

Add support for `gen fn`

This builds on #116447 to add support for `gen fn` functions. For the most part we follow the same approach as desugaring `async fn`, but replacing `Future` with `Iterator` and `async {}` with `gen {}` for the body.

The version implemented here uses the return type of a `gen fn` as the yield type. For example:

```rust
gen fn count_to_three() -> i32 {
    yield 1;
    yield 2;
    yield 3;
}
```

In the future, I think we should experiment with a syntax like `gen fn count_to_three() yield i32 { ... }`, but that can go in another PR.

cc `@oli-obk` `@compiler-errors`

compiler/rustc_ast/src/ast.rs

@@ -1311,7 +1311,7 @@ pub struct Closure {
     pub binder: ClosureBinder,
     pub capture_clause: CaptureBy,
     pub constness: Const,
-    pub asyncness: Async,
+    pub coro_kind: Option<CoroutineKind>,
     pub movability: Movability,
     pub fn_decl: P<FnDecl>,
     pub body: P<Expr>,
@@ -2406,28 +2406,34 @@ pub enum Unsafe {
     No,
 }
 
+/// Describes what kind of coroutine markers, if any, a function has.
+///
+/// Coroutine markers are things that cause the function to generate a coroutine, such as `async`,
+/// which makes the function return `impl Future`, or `gen`, which makes the function return `impl
+/// Iterator`.
 #[derive(Copy, Clone, Encodable, Decodable, Debug)]
-pub enum Async {
+pub enum CoroutineKind {
-    Yes { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
+    /// `async`, which evaluates to `impl Future`
-    No,
+    Async { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
+    /// `gen`, which evaluates to `impl Iterator`
+    Gen { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
 }
 
-#[derive(Copy, Clone, Encodable, Decodable, Debug)]
+impl CoroutineKind {
-pub enum Gen {
-    Yes { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
-    No,
-}
-
-impl Async {
     pub fn is_async(self) -> bool {
-        matches!(self, Async::Yes { .. })
+        matches!(self, CoroutineKind::Async { .. })
     }
 
-    /// In this case this is an `async` return, the `NodeId` for the generated `impl Trait` item.
+    pub fn is_gen(self) -> bool {
-    pub fn opt_return_id(self) -> Option<(NodeId, Span)> {
+        matches!(self, CoroutineKind::Gen { .. })
+    }
+
+    /// In this case this is an `async` or `gen` return, the `NodeId` for the generated `impl Trait`
+    /// item.
+    pub fn return_id(self) -> (NodeId, Span) {
         match self {
-            Async::Yes { return_impl_trait_id, span, .. } => Some((return_impl_trait_id, span)),
+            CoroutineKind::Async { return_impl_trait_id, span, .. }
-            Async::No => None,
+            | CoroutineKind::Gen { return_impl_trait_id, span, .. } => (return_impl_trait_id, span),
         }
     }
 }
@@ -2831,8 +2837,8 @@ impl Extern {
 pub struct FnHeader {
     /// The `unsafe` keyword, if any
     pub unsafety: Unsafe,
-    /// The `async` keyword, if any
+    /// Whether this is `async`, `gen`, or nothing.
-    pub asyncness: Async,
+    pub coro_kind: Option<CoroutineKind>,
     /// The `const` keyword, if any
     pub constness: Const,
     /// The `extern` keyword and corresponding ABI string, if any
@@ -2842,9 +2848,9 @@ pub struct FnHeader {
 impl FnHeader {
     /// Does this function header have any qualifiers or is it empty?
     pub fn has_qualifiers(&self) -> bool {
-        let Self { unsafety, asyncness, constness, ext } = self;
+        let Self { unsafety, coro_kind, constness, ext } = self;
         matches!(unsafety, Unsafe::Yes(_))
-            || asyncness.is_async()
+            || coro_kind.is_some()
             || matches!(constness, Const::Yes(_))
             || !matches!(ext, Extern::None)
     }
@@ -2852,12 +2858,7 @@ impl FnHeader {
 
 impl Default for FnHeader {
     fn default() -> FnHeader {
-        FnHeader {
+        FnHeader { unsafety: Unsafe::No, coro_kind: None, constness: Const::No, ext: Extern::None }
-            unsafety: Unsafe::No,
-            asyncness: Async::No,
-            constness: Const::No,
-            ext: Extern::None,
-        }
     }
 }
 
@@ -3177,7 +3178,7 @@ mod size_asserts {
     static_assert_size!(Block, 32);
     static_assert_size!(Expr, 72);
     static_assert_size!(ExprKind, 40);
-    static_assert_size!(Fn, 152);
+    static_assert_size!(Fn, 160);
     static_assert_size!(ForeignItem, 96);
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericArg, 24);

compiler/rustc_ast/src/mut_visit.rs

@@ -121,8 +121,8 @@ pub trait MutVisitor: Sized {
         noop_visit_fn_decl(d, self);
     }
 
-    fn visit_asyncness(&mut self, a: &mut Async) {
+    fn visit_coro_kind(&mut self, a: &mut CoroutineKind) {
-        noop_visit_asyncness(a, self);
+        noop_visit_coro_kind(a, self);
     }
 
     fn visit_closure_binder(&mut self, b: &mut ClosureBinder) {
@@ -871,13 +871,14 @@ pub fn noop_visit_closure_binder<T: MutVisitor>(binder: &mut ClosureBinder, vis:
     }
 }
 
-pub fn noop_visit_asyncness<T: MutVisitor>(asyncness: &mut Async, vis: &mut T) {
+pub fn noop_visit_coro_kind<T: MutVisitor>(coro_kind: &mut CoroutineKind, vis: &mut T) {
-    match asyncness {
+    match coro_kind {
-        Async::Yes { span: _, closure_id, return_impl_trait_id } => {
+        CoroutineKind::Async { span, closure_id, return_impl_trait_id }
+        | CoroutineKind::Gen { span, closure_id, return_impl_trait_id } => {
+            vis.visit_span(span);
             vis.visit_id(closure_id);
             vis.visit_id(return_impl_trait_id);
         }
-        Async::No => {}
     }
 }
 
@@ -1170,9 +1171,9 @@ fn visit_const_item<T: MutVisitor>(
 }
 
 pub fn noop_visit_fn_header<T: MutVisitor>(header: &mut FnHeader, vis: &mut T) {
-    let FnHeader { unsafety, asyncness, constness, ext: _ } = header;
+    let FnHeader { unsafety, coro_kind, constness, ext: _ } = header;
     visit_constness(constness, vis);
-    vis.visit_asyncness(asyncness);
+    coro_kind.as_mut().map(|coro_kind| vis.visit_coro_kind(coro_kind));
     visit_unsafety(unsafety, vis);
 }
 
@@ -1406,7 +1407,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             binder,
             capture_clause,
             constness,
-            asyncness,
+            coro_kind,
             movability: _,
             fn_decl,
             body,
@@ -1415,7 +1416,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
         }) => {
             vis.visit_closure_binder(binder);
             visit_constness(constness, vis);
-            vis.visit_asyncness(asyncness);
+            coro_kind.as_mut().map(|coro_kind| vis.visit_coro_kind(coro_kind));
             vis.visit_capture_by(capture_clause);
             vis.visit_fn_decl(fn_decl);
             vis.visit_expr(body);

compiler/rustc_ast/src/visit.rs

@@ -861,7 +861,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
         ExprKind::Closure(box Closure {
             binder,
             capture_clause,
-            asyncness: _,
+            coro_kind: _,
             constness: _,
             movability: _,
             fn_decl,

compiler/rustc_ast_lowering/src/expr.rs

@@ -195,15 +195,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     binder,
                     capture_clause,
                     constness,
-                    asyncness,
+                    coro_kind,
                     movability,
                     fn_decl,
                     body,
                     fn_decl_span,
                     fn_arg_span,
-                }) => {
+                }) => match coro_kind {
-                    if let Async::Yes { closure_id, .. } = asyncness {
+                    Some(
-                        self.lower_expr_async_closure(
+                        CoroutineKind::Async { closure_id, .. }
+                        | CoroutineKind::Gen { closure_id, .. },
+                    ) => self.lower_expr_async_closure(
                         binder,
                         *capture_clause,
                         e.id,
@@ -213,9 +215,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         body,
                         *fn_decl_span,
                         *fn_arg_span,
-                        )
+                    ),
-                    } else {
+                    None => self.lower_expr_closure(
-                        self.lower_expr_closure(
                         binder,
                         *capture_clause,
                         e.id,
@@ -225,9 +226,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         body,
                         *fn_decl_span,
                         *fn_arg_span,
-                        )
+                    ),
-                    }
+                },
-                }
                 ExprKind::Block(blk, opt_label) => {
                     let opt_label = self.lower_label(*opt_label);
                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)

compiler/rustc_ast_lowering/src/item.rs

@@ -206,15 +206,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     // `impl Future<Output = T>` here because lower_body
                     // only cares about the input argument patterns in the function
                     // declaration (decl), not the return types.
-                    let asyncness = header.asyncness;
+                    let coro_kind = header.coro_kind;
-                    let body_id =
+                    let body_id = this.lower_maybe_coroutine_body(
-                        this.lower_maybe_async_body(span, hir_id, decl, asyncness, body.as_deref());
+                        span,
+                        hir_id,
+                        decl,
+                        coro_kind,
+                        body.as_deref(),
+                    );
 
                     let itctx = ImplTraitContext::Universal;
                     let (generics, decl) =
                         this.lower_generics(generics, header.constness, id, &itctx, |this| {
-                            let ret_id = asyncness.opt_return_id();
+                            this.lower_fn_decl(decl, id, *fn_sig_span, FnDeclKind::Fn, coro_kind)
-                            this.lower_fn_decl(decl, id, *fn_sig_span, FnDeclKind::Fn, ret_id)
                         });
                     let sig = hir::FnSig {
                         decl,
@@ -725,27 +729,30 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 (generics, kind, expr.is_some())
             }
             AssocItemKind::Fn(box Fn { sig, generics, body: None, .. }) => {
-                let asyncness = sig.header.asyncness;
                 let names = self.lower_fn_params_to_names(&sig.decl);
                 let (generics, sig) = self.lower_method_sig(
                     generics,
                     sig,
                     i.id,
                     FnDeclKind::Trait,
-                    asyncness.opt_return_id(),
+                    sig.header.coro_kind,
                 );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)), false)
             }
             AssocItemKind::Fn(box Fn { sig, generics, body: Some(body), .. }) => {
-                let asyncness = sig.header.asyncness;
+                let body_id = self.lower_maybe_coroutine_body(
-                let body_id =
+                    i.span,
-                    self.lower_maybe_async_body(i.span, hir_id, &sig.decl, asyncness, Some(body));
+                    hir_id,
+                    &sig.decl,
+                    sig.header.coro_kind,
+                    Some(body),
+                );
                 let (generics, sig) = self.lower_method_sig(
                     generics,
                     sig,
                     i.id,
                     FnDeclKind::Trait,
-                    asyncness.opt_return_id(),
+                    sig.header.coro_kind,
                 );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)), true)
             }
@@ -834,12 +841,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 },
             ),
             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
-                let asyncness = sig.header.asyncness;
+                let body_id = self.lower_maybe_coroutine_body(
-                let body_id = self.lower_maybe_async_body(
                     i.span,
                     hir_id,
                     &sig.decl,
-                    asyncness,
+                    sig.header.coro_kind,
                     body.as_deref(),
                 );
                 let (generics, sig) = self.lower_method_sig(
@@ -847,7 +853,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     sig,
                     i.id,
                     if self.is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
-                    asyncness.opt_return_id(),
+                    sig.header.coro_kind,
                 );
 
                 (generics, hir::ImplItemKind::Fn(sig, body_id))
@@ -1011,17 +1017,23 @@ impl<'hir> LoweringContext<'_, 'hir> {
         })
     }
 
-    fn lower_maybe_async_body(
+    /// Takes what may be the body of an `async fn` or a `gen fn` and wraps it in an `async {}` or
+    /// `gen {}` block as appropriate.
+    fn lower_maybe_coroutine_body(
         &mut self,
         span: Span,
         fn_id: hir::HirId,
         decl: &FnDecl,
-        asyncness: Async,
+        coro_kind: Option<CoroutineKind>,
         body: Option<&Block>,
     ) -> hir::BodyId {
-        let (closure_id, body) = match (asyncness, body) {
+        let (Some(coro_kind), Some(body)) = (coro_kind, body) else {
-            (Async::Yes { closure_id, .. }, Some(body)) => (closure_id, body),
+            return self.lower_fn_body_block(span, decl, body);
-            _ => return self.lower_fn_body_block(span, decl, body),
+        };
+        let closure_id = match coro_kind {
+            CoroutineKind::Async { closure_id, .. } | CoroutineKind::Gen { closure_id, .. } => {
+                closure_id
+            }
         };
 
         self.lower_body(|this| {
@@ -1163,21 +1175,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 parameters.push(new_parameter);
             }
 
-            let async_expr = this.make_async_expr(
+            let mkbody = |this: &mut LoweringContext<'_, 'hir>| {
-                CaptureBy::Value { move_kw: rustc_span::DUMMY_SP },
-                closure_id,
-                None,
-                body.span,
-                hir::CoroutineSource::Fn,
-                |this| {
                 // Create a block from the user's function body:
                 let user_body = this.lower_block_expr(body);
 
                 // Transform into `drop-temps { <user-body> }`, an expression:
                 let desugared_span =
                     this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
-                    let user_body =
+                let user_body = this.expr_drop_temps(desugared_span, this.arena.alloc(user_body));
-                        this.expr_drop_temps(desugared_span, this.arena.alloc(user_body));
 
                 // As noted above, create the final block like
                 //
@@ -1195,12 +1200,29 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 );
 
                 this.expr_block(body)
-                },
+            };
-            );
+            let coroutine_expr = match coro_kind {
+                CoroutineKind::Async { .. } => this.make_async_expr(
+                    CaptureBy::Value { move_kw: rustc_span::DUMMY_SP },
+                    closure_id,
+                    None,
+                    body.span,
+                    hir::CoroutineSource::Fn,
+                    mkbody,
+                ),
+                CoroutineKind::Gen { .. } => this.make_gen_expr(
+                    CaptureBy::Value { move_kw: rustc_span::DUMMY_SP },
+                    closure_id,
+                    None,
+                    body.span,
+                    hir::CoroutineSource::Fn,
+                    mkbody,
+                ),
+            };
 
             let hir_id = this.lower_node_id(closure_id);
             this.maybe_forward_track_caller(body.span, fn_id, hir_id);
-            let expr = hir::Expr { hir_id, kind: async_expr, span: this.lower_span(body.span) };
+            let expr = hir::Expr { hir_id, kind: coroutine_expr, span: this.lower_span(body.span) };
 
             (this.arena.alloc_from_iter(parameters), expr)
         })
@@ -1212,21 +1234,26 @@ impl<'hir> LoweringContext<'_, 'hir> {
         sig: &FnSig,
         id: NodeId,
         kind: FnDeclKind,
-        is_async: Option<(NodeId, Span)>,
+        coro_kind: Option<CoroutineKind>,
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
         let itctx = ImplTraitContext::Universal;
         let (generics, decl) =
             self.lower_generics(generics, sig.header.constness, id, &itctx, |this| {
-                this.lower_fn_decl(&sig.decl, id, sig.span, kind, is_async)
+                this.lower_fn_decl(&sig.decl, id, sig.span, kind, coro_kind)
             });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
     }
 
     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
+        let asyncness = if let Some(CoroutineKind::Async { span, .. }) = h.coro_kind {
+            hir::IsAsync::Async(span)
+        } else {
+            hir::IsAsync::NotAsync
+        };
         hir::FnHeader {
             unsafety: self.lower_unsafety(h.unsafety),
-            asyncness: self.lower_asyncness(h.asyncness),
+            asyncness: asyncness,
             constness: self.lower_constness(h.constness),
             abi: self.lower_extern(h.ext),
         }
@@ -1268,13 +1295,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         });
     }
 
-    fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
-        match a {
-            Async::Yes { span, .. } => hir::IsAsync::Async(span),
-            Async::No => hir::IsAsync::NotAsync,
-        }
-    }
-
     pub(super) fn lower_constness(&mut self, c: Const) -> hir::Constness {
         match c {
             Const::Yes(_) => hir::Constness::Const,

compiler/rustc_ast_lowering/src/lib.rs

@@ -1778,13 +1778,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }))
     }
 
-    // Lowers a function declaration.
+    /// Lowers a function declaration.
-    //
+    ///
-    // `decl`: the unlowered (AST) function declaration.
+    /// `decl`: the unlowered (AST) function declaration.
-    // `fn_node_id`: `impl Trait` arguments are lowered into generic parameters on the given `NodeId`.
+    ///
-    // `make_ret_async`: if `Some`, converts `-> T` into `-> impl Future<Output = T>` in the
+    /// `fn_node_id`: `impl Trait` arguments are lowered into generic parameters on the given
-    //      return type. This is used for `async fn` declarations. The `NodeId` is the ID of the
+    /// `NodeId`.
-    //      return type `impl Trait` item, and the `Span` points to the `async` keyword.
+    ///
+    /// `transform_return_type`: if `Some`, applies some conversion to the return type, such as is
+    /// needed for `async fn` and `gen fn`. See [`CoroutineKind`] for more details.
     #[instrument(level = "debug", skip(self))]
     fn lower_fn_decl(
         &mut self,
@@ -1792,7 +1794,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         fn_node_id: NodeId,
         fn_span: Span,
         kind: FnDeclKind,
-        make_ret_async: Option<(NodeId, Span)>,
+        coro: Option<CoroutineKind>,
     ) -> &'hir hir::FnDecl<'hir> {
         let c_variadic = decl.c_variadic();
 
@@ -1821,11 +1823,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             self.lower_ty_direct(&param.ty, &itctx)
         }));
 
-        let output = if let Some((ret_id, _span)) = make_ret_async {
+        let output = match coro {
+            Some(coro) => {
                 let fn_def_id = self.local_def_id(fn_node_id);
-            self.lower_async_fn_ret_ty(&decl.output, fn_def_id, ret_id, kind, fn_span)
+                self.lower_coroutine_fn_ret_ty(&decl.output, fn_def_id, coro, kind, fn_span)
-        } else {
+            }
-            match &decl.output {
+            None => match &decl.output {
                 FnRetTy::Ty(ty) => {
                     let context = if kind.return_impl_trait_allowed() {
                         let fn_def_id = self.local_def_id(fn_node_id);
@@ -1849,7 +1852,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     hir::FnRetTy::Return(self.lower_ty(ty, &context))
                 }
                 FnRetTy::Default(span) => hir::FnRetTy::DefaultReturn(self.lower_span(*span)),
-            }
+            },
         };
 
         self.arena.alloc(hir::FnDecl {
@@ -1888,17 +1891,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     // `fn_node_id`: `NodeId` of the parent function (used to create child impl trait definition)
     // `opaque_ty_node_id`: `NodeId` of the opaque `impl Trait` type that should be created
     #[instrument(level = "debug", skip(self))]
-    fn lower_async_fn_ret_ty(
+    fn lower_coroutine_fn_ret_ty(
         &mut self,
         output: &FnRetTy,
         fn_def_id: LocalDefId,
-        opaque_ty_node_id: NodeId,
+        coro: CoroutineKind,
         fn_kind: FnDeclKind,
         fn_span: Span,
     ) -> hir::FnRetTy<'hir> {
         let span = self.lower_span(fn_span);
         let opaque_ty_span = self.mark_span_with_reason(DesugaringKind::Async, span, None);
 
+        let opaque_ty_node_id = match coro {
+            CoroutineKind::Async { return_impl_trait_id, .. }
+            | CoroutineKind::Gen { return_impl_trait_id, .. } => return_impl_trait_id,
+        };
+
         let captured_lifetimes: Vec<_> = self
             .resolver
             .take_extra_lifetime_params(opaque_ty_node_id)
@@ -1914,15 +1922,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             span,
             opaque_ty_span,
             |this| {
-                let future_bound = this.lower_async_fn_output_type_to_future_bound(
+                let bound = this.lower_coroutine_fn_output_type_to_bound(
                     output,
+                    coro,
                     span,
                     ImplTraitContext::ReturnPositionOpaqueTy {
                         origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
                         fn_kind,
                     },
                 );
-                arena_vec![this; future_bound]
+                arena_vec![this; bound]
             },
         );
 
@@ -1931,9 +1940,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     /// Transforms `-> T` into `Future<Output = T>`.
-    fn lower_async_fn_output_type_to_future_bound(
+    fn lower_coroutine_fn_output_type_to_bound(
         &mut self,
         output: &FnRetTy,
+        coro: CoroutineKind,
         span: Span,
         nested_impl_trait_context: ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
@@ -1948,17 +1958,21 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             FnRetTy::Default(ret_ty_span) => self.arena.alloc(self.ty_tup(*ret_ty_span, &[])),
         };
 
-        // "<Output = T>"
+        // "<$assoc_ty_name = T>"
+        let (assoc_ty_name, trait_lang_item) = match coro {
+            CoroutineKind::Async { .. } => (hir::FN_OUTPUT_NAME, hir::LangItem::Future),
+            CoroutineKind::Gen { .. } => (hir::ITERATOR_ITEM_NAME, hir::LangItem::Iterator),
+        };
+
         let future_args = self.arena.alloc(hir::GenericArgs {
             args: &[],
-            bindings: arena_vec![self; self.output_ty_binding(span, output_ty)],
+            bindings: arena_vec![self; self.assoc_ty_binding(assoc_ty_name, span, output_ty)],
             parenthesized: hir::GenericArgsParentheses::No,
             span_ext: DUMMY_SP,
         });
 
         hir::GenericBound::LangItemTrait(
-            // ::std::future::Future<future_params>
+            trait_lang_item,
-            hir::LangItem::Future,
             self.lower_span(span),
             self.next_id(),
             future_args,

compiler/rustc_ast_lowering/src/path.rs

@@ -389,7 +389,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             FnRetTy::Default(_) => self.arena.alloc(self.ty_tup(*span, &[])),
         };
         let args = smallvec![GenericArg::Type(self.arena.alloc(self.ty_tup(*inputs_span, inputs)))];
-        let binding = self.output_ty_binding(output_ty.span, output_ty);
+        let binding = self.assoc_ty_binding(hir::FN_OUTPUT_NAME, output_ty.span, output_ty);
         (
             GenericArgsCtor {
                 args,
@@ -401,13 +401,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         )
     }
 
-    /// An associated type binding `Output = $ty`.
+    /// An associated type binding `$assoc_ty_name = $ty`.
-    pub(crate) fn output_ty_binding(
+    pub(crate) fn assoc_ty_binding(
         &mut self,
+        assoc_ty_name: rustc_span::Symbol,
         span: Span,
         ty: &'hir hir::Ty<'hir>,
     ) -> hir::TypeBinding<'hir> {
-        let ident = Ident::with_dummy_span(hir::FN_OUTPUT_NAME);
+        let ident = Ident::with_dummy_span(assoc_ty_name);
         let kind = hir::TypeBindingKind::Equality { term: ty.into() };
         let args = arena_vec![self;];
         let bindings = arena_vec![self;];

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1268,13 +1268,18 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
         self.check_c_variadic_type(fk);
 
-        // Functions cannot both be `const async`
+        // Functions cannot both be `const async` or `const gen`
         if let Some(&FnHeader {
             constness: Const::Yes(cspan),
-            asyncness: Async::Yes { span: aspan, .. },
+            coro_kind:
+                Some(
+                    CoroutineKind::Async { span: aspan, .. }
+                    | CoroutineKind::Gen { span: aspan, .. },
+                ),
             ..
         }) = fk.header()
         {
+            // FIXME(gen_blocks): Report a different error for `const gen`
             self.err_handler().emit_err(errors::ConstAndAsync {
                 spans: vec![cspan, aspan],
                 cspan,

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1490,11 +1490,16 @@ impl<'a> State<'a> {
         }
     }
 
-    fn print_asyncness(&mut self, asyncness: ast::Async) {
+    fn print_coro_kind(&mut self, coro_kind: ast::CoroutineKind) {
-        if asyncness.is_async() {
+        match coro_kind {
+            ast::CoroutineKind::Gen { .. } => {
+                self.word_nbsp("gen");
+            }
+            ast::CoroutineKind::Async { .. } => {
                 self.word_nbsp("async");
             }
         }
+    }
 
     pub fn print_type_bounds(&mut self, bounds: &[ast::GenericBound]) {
         let mut first = true;
@@ -1685,7 +1690,7 @@ impl<'a> State<'a> {
 
     fn print_fn_header_info(&mut self, header: ast::FnHeader) {
         self.print_constness(header.constness);
-        self.print_asyncness(header.asyncness);
+        header.coro_kind.map(|coro_kind| self.print_coro_kind(coro_kind));
         self.print_unsafety(header.unsafety);
 
         match header.ext {

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -413,7 +413,7 @@ impl<'a> State<'a> {
                 binder,
                 capture_clause,
                 constness,
-                asyncness,
+                coro_kind,
                 movability,
                 fn_decl,
                 body,
@@ -423,7 +423,7 @@ impl<'a> State<'a> {
                 self.print_closure_binder(binder);
                 self.print_constness(*constness);
                 self.print_movability(*movability);
-                self.print_asyncness(*asyncness);
+                coro_kind.map(|coro_kind| self.print_coro_kind(coro_kind));
                 self.print_capture_clause(*capture_clause);
 
                 self.print_fn_params_and_ret(fn_decl, true);

compiler/rustc_builtin_macros/src/test.rs

@@ -541,10 +541,14 @@ fn check_test_signature(
         return Err(sd.emit_err(errors::TestBadFn { span: i.span, cause: span, kind: "unsafe" }));
     }
 
-    if let ast::Async::Yes { span, .. } = f.sig.header.asyncness {
+    if let Some(ast::CoroutineKind::Async { span, .. }) = f.sig.header.coro_kind {
         return Err(sd.emit_err(errors::TestBadFn { span: i.span, cause: span, kind: "async" }));
     }
 
+    if let Some(ast::CoroutineKind::Gen { span, .. }) = f.sig.header.coro_kind {
+        return Err(sd.emit_err(errors::TestBadFn { span: i.span, cause: span, kind: "gen" }));
+    }
+
     // If the termination trait is active, the compiler will check that the output
     // type implements the `Termination` trait as `libtest` enforces that.
     let has_output = match &f.sig.decl.output {

compiler/rustc_expand/src/build.rs

@@ -547,7 +547,7 @@ impl<'a> ExtCtxt<'a> {
                 binder: ast::ClosureBinder::NotPresent,
                 capture_clause: ast::CaptureBy::Ref,
                 constness: ast::Const::No,
-                asyncness: ast::Async::No,
+                coro_kind: None,
                 movability: ast::Movability::Movable,
                 fn_decl,
                 body,

compiler/rustc_hir/src/hir.rs

@@ -2255,6 +2255,8 @@ pub enum ImplItemKind<'hir> {
 
 /// The name of the associated type for `Fn` return types.
 pub const FN_OUTPUT_NAME: Symbol = sym::Output;
+/// The name of the associated type for `Iterator` item types.
+pub const ITERATOR_ITEM_NAME: Symbol = sym::Item;
 
 /// Bind a type to an associated type (i.e., `A = Foo`).
 ///

compiler/rustc_hir_typeck/src/closure.rs

@@ -651,9 +651,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         },
                     )
                 }
-                Some(hir::CoroutineKind::Gen(hir::CoroutineSource::Fn)) => {
+                // For a `gen {}` block created as a `gen fn` body, we need the return type to be
-                    todo!("gen closures do not exist yet")
+                // ().
-                }
+                Some(hir::CoroutineKind::Gen(hir::CoroutineSource::Fn)) => self.tcx.types.unit,
 
                 _ => astconv.ty_infer(None, decl.output.span()),
             },

compiler/rustc_lint/src/early.rs

@@ -162,7 +162,11 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
         // Explicitly check for lints associated with 'closure_id', since
         // it does not have a corresponding AST node
         if let ast_visit::FnKind::Fn(_, _, sig, _, _, _) = fk {
-            if let ast::Async::Yes { closure_id, .. } = sig.header.asyncness {
+            if let Some(
+                ast::CoroutineKind::Async { closure_id, .. }
+                | ast::CoroutineKind::Gen { closure_id, .. },
+            ) = sig.header.coro_kind
+            {
                 self.check_id(closure_id);
             }
         }
@@ -223,7 +227,11 @@ impl<'a, T: EarlyLintPass> ast_visit::Visitor<'a> for EarlyContextAndPass<'a, T>
         // it does not have a corresponding AST node
         match e.kind {
             ast::ExprKind::Closure(box ast::Closure {
-                asyncness: ast::Async::Yes { closure_id, .. },
+                coro_kind:
+                    Some(
+                        ast::CoroutineKind::Async { closure_id, .. }
+                        | ast::CoroutineKind::Gen { closure_id, .. },
+                    ),
                 ..
             }) => self.check_id(closure_id),
             _ => {}

compiler/rustc_parse/messages.ftl

@@ -23,6 +23,8 @@ parse_async_block_in_2015 = `async` blocks are only allowed in Rust 2018 or late
 parse_async_fn_in_2015 = `async fn` is not permitted in Rust 2015
     .label = to use `async fn`, switch to Rust 2018 or later
 
+parse_async_gen_fn = `async gen` functions are not supported
+
 parse_async_move_block_in_2015 = `async move` blocks are only allowed in Rust 2018 or later
 
 parse_async_move_order_incorrect = the order of `move` and `async` is incorrect

compiler/rustc_parse/src/errors.rs

@@ -562,6 +562,13 @@ pub(crate) struct GenFn {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_async_gen_fn)]
+pub(crate) struct AsyncGenFn {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_comma_after_base_struct)]
 #[note]

compiler/rustc_parse/src/parser/expr.rs

@@ -10,7 +10,7 @@ use super::{
 use crate::errors;
 use crate::maybe_recover_from_interpolated_ty_qpath;
 use ast::mut_visit::{noop_visit_expr, MutVisitor};
-use ast::{GenBlockKind, Pat, Path, PathSegment};
+use ast::{CoroutineKind, GenBlockKind, Pat, Path, PathSegment};
 use core::mem;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
@@ -21,7 +21,7 @@ use rustc_ast::util::parser::{prec_let_scrutinee_needs_par, AssocOp, Fixity};
 use rustc_ast::visit::Visitor;
 use rustc_ast::{self as ast, AttrStyle, AttrVec, CaptureBy, ExprField, UnOp, DUMMY_NODE_ID};
 use rustc_ast::{AnonConst, BinOp, BinOpKind, FnDecl, FnRetTy, MacCall, Param, Ty, TyKind};
-use rustc_ast::{Arm, Async, BlockCheckMode, Expr, ExprKind, Label, Movability, RangeLimits};
+use rustc_ast::{Arm, BlockCheckMode, Expr, ExprKind, Label, Movability, RangeLimits};
 use rustc_ast::{ClosureBinder, MetaItemLit, StmtKind};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::stack::ensure_sufficient_stack;
@@ -2237,7 +2237,7 @@ impl<'a> Parser<'a> {
         let asyncness = if self.token.uninterpolated_span().at_least_rust_2018() {
             self.parse_asyncness(Case::Sensitive)
         } else {
-            Async::No
+            None
         };
 
         let capture_clause = self.parse_capture_clause()?;
@@ -2261,7 +2261,7 @@ impl<'a> Parser<'a> {
             }
         };
 
-        if let Async::Yes { span, .. } = asyncness {
+        if let Some(CoroutineKind::Async { span, .. }) = asyncness {
             // Feature-gate `async ||` closures.
             self.sess.gated_spans.gate(sym::async_closure, span);
         }
@@ -2284,7 +2284,7 @@ impl<'a> Parser<'a> {
                 binder,
                 capture_clause,
                 constness,
-                asyncness,
+                coro_kind: asyncness,
                 movability,
                 fn_decl,
                 body,

compiler/rustc_parse/src/parser/item.rs

@@ -11,8 +11,8 @@ use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
 use rustc_ast::util::case::Case;
 use rustc_ast::MacCall;
 use rustc_ast::{self as ast, AttrVec, Attribute, DUMMY_NODE_ID};
-use rustc_ast::{Async, Const, Defaultness, IsAuto, Mutability, Unsafe, UseTree, UseTreeKind};
 use rustc_ast::{BindingAnnotation, Block, FnDecl, FnSig, Param, SelfKind};
+use rustc_ast::{Const, Defaultness, IsAuto, Mutability, Unsafe, UseTree, UseTreeKind};
 use rustc_ast::{EnumDef, FieldDef, Generics, TraitRef, Ty, TyKind, Variant, VariantData};
 use rustc_ast::{FnHeader, ForeignItem, Path, PathSegment, Visibility, VisibilityKind};
 use rustc_ast_pretty::pprust;
@@ -2401,7 +2401,7 @@ impl<'a> Parser<'a> {
         let ext_start_sp = self.token.span;
         let ext = self.parse_extern(case);
 
-        if let Async::Yes { span, .. } = asyncness {
+        if let Some(CoroutineKind::Async { span, .. }) = asyncness {
             if span.is_rust_2015() {
                 self.sess.emit_err(errors::AsyncFnIn2015 {
                     span,
@@ -2410,8 +2410,16 @@ impl<'a> Parser<'a> {
             }
         }
 
-        if let Gen::Yes { span, .. } = genness {
+        if let Some(CoroutineKind::Gen { span, .. }) = genness {
-            self.sess.emit_err(errors::GenFn { span });
+            self.sess.gated_spans.gate(sym::gen_blocks, span);
+        }
+
+        if let (
+            Some(CoroutineKind::Async { span: async_span, .. }),
+            Some(CoroutineKind::Gen { span: gen_span, .. }),
+        ) = (asyncness, genness)
+        {
+            self.sess.emit_err(errors::AsyncGenFn { span: async_span.to(gen_span) });
         }
 
         if !self.eat_keyword_case(kw::Fn, case) {
@@ -2444,13 +2452,18 @@ impl<'a> Parser<'a> {
                         }
                     } else if self.check_keyword(kw::Async) {
                         match asyncness {
-                            Async::Yes { span, .. } => Some(WrongKw::Duplicated(span)),
+                            Some(CoroutineKind::Async { span, .. }) => {
-                            Async::No => {
+                                Some(WrongKw::Duplicated(span))
-                                recover_asyncness = Async::Yes {
+                            }
+                            Some(CoroutineKind::Gen { .. }) => {
+                                panic!("not sure how to recover here")
+                            }
+                            None => {
+                                recover_asyncness = Some(CoroutineKind::Async {
                                     span: self.token.span,
                                     closure_id: DUMMY_NODE_ID,
                                     return_impl_trait_id: DUMMY_NODE_ID,
-                                };
+                                });
                                 Some(WrongKw::Misplaced(unsafe_start_sp))
                             }
                         }
@@ -2531,6 +2544,8 @@ impl<'a> Parser<'a> {
                         }
                     }
 
+                    // FIXME(gen_blocks): add keyword recovery logic for genness
+
                     if wrong_kw.is_some()
                         && self.may_recover()
                         && self.look_ahead(1, |tok| tok.is_keyword_case(kw::Fn, case))
@@ -2542,7 +2557,7 @@ impl<'a> Parser<'a> {
                         return Ok(FnHeader {
                             constness: recover_constness,
                             unsafety: recover_unsafety,
-                            asyncness: recover_asyncness,
+                            coro_kind: recover_asyncness,
                             ext,
                         });
                     }
@@ -2552,7 +2567,13 @@ impl<'a> Parser<'a> {
             }
         }
 
-        Ok(FnHeader { constness, unsafety, asyncness, ext })
+        let coro_kind = match asyncness {
+            Some(CoroutineKind::Async { .. }) => asyncness,
+            Some(CoroutineKind::Gen { .. }) => unreachable!("asycness cannot be Gen"),
+            None => genness,
+        };
+
+        Ok(FnHeader { constness, unsafety, coro_kind, ext })
     }
 
     /// Parses the parameter list and result type of a function declaration.

compiler/rustc_parse/src/parser/mod.rs

@@ -11,7 +11,6 @@ mod stmt;
 mod ty;
 
 use crate::lexer::UnmatchedDelim;
-use ast::Gen;
 pub use attr_wrapper::AttrWrapper;
 pub use diagnostics::AttemptLocalParseRecovery;
 pub(crate) use expr::ForbiddenLetReason;
@@ -25,9 +24,10 @@ use rustc_ast::tokenstream::{AttributesData, DelimSpan, Spacing};
 use rustc_ast::tokenstream::{TokenStream, TokenTree, TokenTreeCursor};
 use rustc_ast::util::case::Case;
 use rustc_ast::AttrId;
+use rustc_ast::CoroutineKind;
 use rustc_ast::DUMMY_NODE_ID;
 use rustc_ast::{self as ast, AnonConst, Const, DelimArgs, Extern};
-use rustc_ast::{Async, AttrArgs, AttrArgsEq, Expr, ExprKind, Mutability, StrLit};
+use rustc_ast::{AttrArgs, AttrArgsEq, Expr, ExprKind, Mutability, StrLit};
 use rustc_ast::{HasAttrs, HasTokens, Unsafe, Visibility, VisibilityKind};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
@@ -1125,22 +1125,30 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses asyncness: `async` or nothing.
-    fn parse_asyncness(&mut self, case: Case) -> Async {
+    fn parse_asyncness(&mut self, case: Case) -> Option<CoroutineKind> {
         if self.eat_keyword_case(kw::Async, case) {
             let span = self.prev_token.uninterpolated_span();
-            Async::Yes { span, closure_id: DUMMY_NODE_ID, return_impl_trait_id: DUMMY_NODE_ID }
+            Some(CoroutineKind::Async {
+                span,
+                closure_id: DUMMY_NODE_ID,
+                return_impl_trait_id: DUMMY_NODE_ID,
+            })
         } else {
-            Async::No
+            None
         }
     }
 
     /// Parses genness: `gen` or nothing.
-    fn parse_genness(&mut self, case: Case) -> Gen {
+    fn parse_genness(&mut self, case: Case) -> Option<CoroutineKind> {
         if self.token.span.at_least_rust_2024() && self.eat_keyword_case(kw::Gen, case) {
             let span = self.prev_token.uninterpolated_span();
-            Gen::Yes { span, closure_id: DUMMY_NODE_ID, return_impl_trait_id: DUMMY_NODE_ID }
+            Some(CoroutineKind::Gen {
+                span,
+                closure_id: DUMMY_NODE_ID,
+                return_impl_trait_id: DUMMY_NODE_ID,
+            })
         } else {
-            Gen::No
+            None
         }
     }
 

compiler/rustc_parse/src/parser/ty.rs

@@ -596,7 +596,7 @@ impl<'a> Parser<'a> {
             tokens: None,
         };
         let span_start = self.token.span;
-        let ast::FnHeader { ext, unsafety, constness, asyncness } =
+        let ast::FnHeader { ext, unsafety, constness, coro_kind } =
             self.parse_fn_front_matter(&inherited_vis, Case::Sensitive)?;
         if self.may_recover() && self.token.kind == TokenKind::Lt {
             self.recover_fn_ptr_with_generics(lo, &mut params, param_insertion_point)?;
@@ -609,9 +609,10 @@ impl<'a> Parser<'a> {
             // cover it.
             self.sess.emit_err(FnPointerCannotBeConst { span: whole_span, qualifier: span });
         }
-        if let ast::Async::Yes { span, .. } = asyncness {
+        if let Some(ast::CoroutineKind::Async { span, .. }) = coro_kind {
             self.sess.emit_err(FnPointerCannotBeAsync { span: whole_span, qualifier: span });
         }
+        // FIXME(gen_blocks): emit a similar error for `gen fn()`
         let decl_span = span_start.to(self.token.span);
         Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl, decl_span })))
     }

compiler/rustc_resolve/src/def_collector.rs

@@ -156,7 +156,10 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
 
     fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
         if let FnKind::Fn(_, _, sig, _, generics, body) = fn_kind {
-            if let Async::Yes { closure_id, .. } = sig.header.asyncness {
+            if let Some(
+                CoroutineKind::Async { closure_id, .. } | CoroutineKind::Gen { closure_id, .. },
+            ) = sig.header.coro_kind
+            {
                 self.visit_generics(generics);
 
                 // For async functions, we need to create their inner defs inside of a
@@ -281,11 +284,12 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
                 // Async closures desugar to closures inside of closures, so
                 // we must create two defs.
                 let closure_def = self.create_def(expr.id, kw::Empty, DefKind::Closure, expr.span);
-                match closure.asyncness {
+                match closure.coro_kind {
-                    Async::Yes { closure_id, .. } => {
+                    Some(
-                        self.create_def(closure_id, kw::Empty, DefKind::Closure, expr.span)
+                        CoroutineKind::Async { closure_id, .. }
-                    }
+                        | CoroutineKind::Gen { closure_id, .. },
-                    Async::No => closure_def,
+                    ) => self.create_def(closure_id, kw::Empty, DefKind::Closure, expr.span),
+                    None => closure_def,
                 }
             }
             ExprKind::Gen(_, _, _) => {

compiler/rustc_resolve/src/late.rs

@@ -916,8 +916,10 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                             &sig.decl.output,
                         );
 
-                        if let Some((async_node_id, _)) = sig.header.asyncness.opt_return_id() {
+                        if let Some((coro_node_id, _)) =
-                            this.record_lifetime_params_for_impl_trait(async_node_id);
+                            sig.header.coro_kind.map(|coro_kind| coro_kind.return_id())
+                        {
+                            this.record_lifetime_params_for_impl_trait(coro_node_id);
                         }
                     },
                 );
@@ -940,12 +942,13 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                         this.visit_generics(generics);
 
                         let declaration = &sig.decl;
-                        let async_node_id = sig.header.asyncness.opt_return_id();
+                        let coro_node_id =
+                            sig.header.coro_kind.map(|coro_kind| coro_kind.return_id());
 
                         this.with_lifetime_rib(
                             LifetimeRibKind::AnonymousCreateParameter {
                                 binder: fn_id,
-                                report_in_path: async_node_id.is_some(),
+                                report_in_path: coro_node_id.is_some(),
                             },
                             |this| {
                                 this.resolve_fn_signature(
@@ -958,7 +961,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                                     &declaration.output,
                                 );
 
-                                if let Some((async_node_id, _)) = async_node_id {
+                                if let Some((async_node_id, _)) = coro_node_id {
                                     this.record_lifetime_params_for_impl_trait(async_node_id);
                                 }
                             },
@@ -4288,8 +4291,10 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             // `async |x| ...` gets desugared to `|x| async {...}`, so we need to
             // resolve the arguments within the proper scopes so that usages of them inside the
             // closure are detected as upvars rather than normal closure arg usages.
+            //
+            // Similarly, `gen |x| ...` gets desugared to `|x| gen {...}`, so we handle that too.
             ExprKind::Closure(box ast::Closure {
-                asyncness: Async::Yes { .. },
+                coro_kind: Some(_),
                 ref fn_decl,
                 ref body,
                 ..

src/tools/clippy/clippy_lints/src/doc/needless_doctest_main.rs

@@ -3,7 +3,7 @@ use std::{io, thread};
 
 use crate::doc::{NEEDLESS_DOCTEST_MAIN, TEST_ATTR_IN_DOCTEST};
 use clippy_utils::diagnostics::span_lint;
-use rustc_ast::{Async, Fn, FnRetTy, Item, ItemKind};
+use rustc_ast::{CoroutineKind, Fn, FnRetTy, Item, ItemKind};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::emitter::EmitterWriter;
 use rustc_errors::Handler;
@@ -69,7 +69,7 @@ pub fn check(
                                 if !ignore {
                                     get_test_spans(&item, &mut test_attr_spans);
                                 }
-                                let is_async = matches!(sig.header.asyncness, Async::Yes { .. });
+                                let is_async = matches!(sig.header.coro_kind, Some(CoroutineKind::Async { .. }));
                                 let returns_nothing = match &sig.decl.output {
                                     FnRetTy::Default(..) => true,
                                     FnRetTy::Ty(ty) if ty.kind.is_unit() => true,

src/tools/clippy/clippy_utils/src/ast_utils.rs

@@ -188,7 +188,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
             Closure(box ast::Closure {
                 binder: lb,
                 capture_clause: lc,
-                asyncness: la,
+                coro_kind: la,
                 movability: lm,
                 fn_decl: lf,
                 body: le,
@@ -197,7 +197,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
             Closure(box ast::Closure {
                 binder: rb,
                 capture_clause: rc,
-                asyncness: ra,
+                coro_kind: ra,
                 movability: rm,
                 fn_decl: rf,
                 body: re,
@@ -206,7 +206,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
         ) => {
             eq_closure_binder(lb, rb)
                 && lc == rc
-                && la.is_async() == ra.is_async()
+                && la.map_or(false, |la| la.is_async()) == ra.map_or(false, |ra| ra.is_async())
                 && lm == rm
                 && eq_fn_decl(lf, rf)
                 && eq_expr(le, re)
@@ -563,9 +563,18 @@ pub fn eq_fn_sig(l: &FnSig, r: &FnSig) -> bool {
     eq_fn_decl(&l.decl, &r.decl) && eq_fn_header(&l.header, &r.header)
 }
 
+fn eq_opt_coro_kind(l: Option<CoroutineKind>, r: Option<CoroutineKind>) -> bool {
+    match (l, r) {
+        (Some(CoroutineKind::Async { .. }), Some(CoroutineKind::Async { .. }))
+        | (Some(CoroutineKind::Gen { .. }), Some(CoroutineKind::Gen { .. })) => true,
+        (None, None) => true,
+        _ => false,
+    }
+}
+
 pub fn eq_fn_header(l: &FnHeader, r: &FnHeader) -> bool {
     matches!(l.unsafety, Unsafe::No) == matches!(r.unsafety, Unsafe::No)
-        && l.asyncness.is_async() == r.asyncness.is_async()
+        && eq_opt_coro_kind(l.coro_kind, r.coro_kind)
         && matches!(l.constness, Const::No) == matches!(r.constness, Const::No)
         && eq_ext(&l.ext, &r.ext)
 }

src/tools/rustfmt/src/closures.rs

@@ -29,7 +29,7 @@ pub(crate) fn rewrite_closure(
     binder: &ast::ClosureBinder,
     constness: ast::Const,
     capture: ast::CaptureBy,
-    is_async: &ast::Async,
+    coro_kind: &Option<ast::CoroutineKind>,
     movability: ast::Movability,
     fn_decl: &ast::FnDecl,
     body: &ast::Expr,
@@ -40,7 +40,7 @@ pub(crate) fn rewrite_closure(
     debug!("rewrite_closure {:?}", body);
 
     let (prefix, extra_offset) = rewrite_closure_fn_decl(
-        binder, constness, capture, is_async, movability, fn_decl, body, span, context, shape,
+        binder, constness, capture, coro_kind, movability, fn_decl, body, span, context, shape,
     )?;
     // 1 = space between `|...|` and body.
     let body_shape = shape.offset_left(extra_offset)?;
@@ -233,7 +233,7 @@ fn rewrite_closure_fn_decl(
     binder: &ast::ClosureBinder,
     constness: ast::Const,
     capture: ast::CaptureBy,
-    asyncness: &ast::Async,
+    coro_kind: &Option<ast::CoroutineKind>,
     movability: ast::Movability,
     fn_decl: &ast::FnDecl,
     body: &ast::Expr,
@@ -263,7 +263,11 @@ fn rewrite_closure_fn_decl(
     } else {
         ""
     };
-    let is_async = if asyncness.is_async() { "async " } else { "" };
+    let coro = match coro_kind {
+        Some(ast::CoroutineKind::Async { .. }) => "async ",
+        Some(ast::CoroutineKind::Gen { .. }) => "gen ",
+        None => "",
+    };
     let mover = if matches!(capture, ast::CaptureBy::Value { .. }) {
         "move "
     } else {
@@ -272,7 +276,7 @@ fn rewrite_closure_fn_decl(
     // 4 = "|| {".len(), which is overconservative when the closure consists of
     // a single expression.
     let nested_shape = shape
-        .shrink_left(binder.len() + const_.len() + immovable.len() + is_async.len() + mover.len())?
+        .shrink_left(binder.len() + const_.len() + immovable.len() + coro.len() + mover.len())?
         .sub_width(4)?;
 
     // 1 = |
@@ -310,7 +314,7 @@ fn rewrite_closure_fn_decl(
         .tactic(tactic)
         .preserve_newline(true);
     let list_str = write_list(&item_vec, &fmt)?;
-    let mut prefix = format!("{binder}{const_}{immovable}{is_async}{mover}|{list_str}|");
+    let mut prefix = format!("{binder}{const_}{immovable}{coro}{mover}|{list_str}|");
 
     if !ret_str.is_empty() {
         if prefix.contains('\n') {
@@ -339,7 +343,7 @@ pub(crate) fn rewrite_last_closure(
             ref binder,
             constness,
             capture_clause,
-            ref asyncness,
+            ref coro_kind,
             movability,
             ref fn_decl,
             ref body,
@@ -360,7 +364,7 @@ pub(crate) fn rewrite_last_closure(
             binder,
             constness,
             capture_clause,
-            asyncness,
+            coro_kind,
             movability,
             fn_decl,
             body,

src/tools/rustfmt/src/expr.rs

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

src/tools/rustfmt/src/items.rs

@@ -287,7 +287,7 @@ pub(crate) struct FnSig<'a> {
     decl: &'a ast::FnDecl,
     generics: &'a ast::Generics,
     ext: ast::Extern,
-    is_async: Cow<'a, ast::Async>,
+    coro_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,
-            is_async: Cow::Borrowed(&method_sig.header.asyncness),
+            coro_kind: Cow::Borrowed(&method_sig.header.coro_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,
-                is_async: Cow::Borrowed(&fn_sig.header.asyncness),
+                coro_kind: Cow::Borrowed(&fn_sig.header.coro_kind),
                 defaultness,
                 unsafety: fn_sig.header.unsafety,
                 visibility: vis,
@@ -343,7 +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));
-        result.push_str(format_async(&self.is_async));
+        self.coro_kind
+            .map(|coro_kind| result.push_str(format_coro(&coro_kind)));
         result.push_str(format_unsafety(self.unsafety));
         result.push_str(&format_extern(
             self.ext,

src/tools/rustfmt/src/utils.rs

@@ -75,10 +75,10 @@ pub(crate) fn format_visibility(
 }
 
 #[inline]
-pub(crate) fn format_async(is_async: &ast::Async) -> &'static str {
+pub(crate) fn format_coro(coro_kind: &ast::CoroutineKind) -> &'static str {
-    match is_async {
+    match coro_kind {
-        ast::Async::Yes { .. } => "async ",
+        ast::CoroutineKind::Async { .. } => "async ",
-        ast::Async::No => "",
+        ast::CoroutineKind::Gen { .. } => "gen ",
     }
 }
 

tests/ui-fulldeps/pprust-expr-roundtrip.rs

@@ -132,7 +132,7 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                         binder: ClosureBinder::NotPresent,
                         capture_clause: CaptureBy::Value { move_kw: DUMMY_SP },
                         constness: Const::No,
-                        asyncness: Async::No,
+                        coro_kind: None,
                         movability: Movability::Movable,
                         fn_decl: decl.clone(),
                         body: e,

tests/ui/coroutine/async_gen_fn.rs

@@ -0,0 +1,11 @@
+// edition: 2024
+// compile-flags: -Zunstable-options
+#![feature(gen_blocks)]
+
+// async generators are not yet supported, so this test makes sure they make some kind of reasonable
+// error.
+
+async gen fn foo() {}
+//~^ `async gen` functions are not supported
+
+fn main() {}

tests/ui/coroutine/async_gen_fn.stderr

@@ -0,0 +1,8 @@
+error: `async gen` functions are not supported
+  --> $DIR/async_gen_fn.rs:8:1
+   |
+LL | async gen fn foo() {}
+   | ^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/coroutine/gen_fn.e2024.stderr

@@ -1,10 +1,12 @@
-error: `gen` functions are not yet implemented
+error[E0658]: gen blocks are experimental
   --> $DIR/gen_fn.rs:4:1
    |
 LL | gen fn foo() {}
    | ^^^
    |
-   = help: for now you can use `gen {}` blocks and return `impl Iterator` instead
+   = note: see issue #117078 <https://github.com/rust-lang/rust/issues/117078> for more information
+   = help: add `#![feature(gen_blocks)]` to the crate attributes to enable
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/coroutine/gen_fn.rs

@@ -3,6 +3,6 @@
 
 gen fn foo() {}
 //[none]~^ ERROR: expected one of `#`, `async`, `const`, `default`, `extern`, `fn`, `pub`, `unsafe`, or `use`, found `gen`
-//[e2024]~^^ ERROR: `gen` functions are not yet implemented
+//[e2024]~^^ ERROR: gen blocks are experimental
 
 fn main() {}

tests/ui/coroutine/gen_fn_iter.rs

@@ -0,0 +1,20 @@
+// edition: 2024
+// compile-flags: -Zunstable-options
+// run-pass
+#![feature(gen_blocks)]
+
+// make sure that a ridiculously simple gen fn works as an iterator.
+
+gen fn foo() -> i32 {
+    yield 1;
+    yield 2;
+    yield 3;
+}
+
+fn main() {
+    let mut iter = foo();
+    assert_eq!(iter.next(), Some(1));
+    assert_eq!(iter.next(), Some(2));
+    assert_eq!(iter.next(), Some(3));
+    assert_eq!(iter.next(), None);
+}

tests/ui/coroutine/gen_fn_lifetime_capture.rs

@@ -0,0 +1,19 @@
+// edition: 2024
+// compile-flags: -Zunstable-options
+// check-pass
+#![feature(gen_blocks)]
+
+// make sure gen fn captures lifetimes in its signature
+
+gen fn foo<'a, 'b>(x: &'a i32, y: &'b i32, z: &'b i32) -> &'b i32 {
+    yield y;
+    yield z;
+}
+
+fn main() {
+    let z = 3;
+    let mut iter = foo(&1, &2, &z);
+    assert_eq!(iter.next(), Some(&2));
+    assert_eq!(iter.next(), Some(&3));
+    assert_eq!(iter.next(), None);
+}