diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 4ef8d64c4c0..bf648388f4e 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/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: CoroutineKind,
     pub movability: Movability,
     pub fn_decl: P<FnDecl>,
     pub body: P<Expr>,
@@ -2406,28 +2406,38 @@ 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 {
-    Yes { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
-    No,
+pub enum CoroutineKind {
+    /// `async`, which evaluates to `impl Future`
+    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 },
+    /// Neither `async` nor `gen`
+    None,
 }
 
-#[derive(Copy, Clone, Encodable, Decodable, Debug)]
-pub enum Gen {
-    Yes { span: Span, closure_id: NodeId, return_impl_trait_id: NodeId },
-    No,
-}
-
-impl Async {
+impl CoroutineKind {
     pub fn is_async(self) -> bool {
-        matches!(self, Async::Yes { .. })
+        matches!(self, CoroutineKind::Async { .. })
+    }
+
+    pub fn is_gen(self) -> bool {
+        matches!(self, CoroutineKind::Gen { .. })
     }
 
     /// In this case this is an `async` return, the `NodeId` for the generated `impl Trait` item.
     pub fn opt_return_id(self) -> Option<(NodeId, Span)> {
         match self {
-            Async::Yes { return_impl_trait_id, span, .. } => Some((return_impl_trait_id, span)),
-            Async::No => None,
+            CoroutineKind::Async { return_impl_trait_id, span, .. }
+            | CoroutineKind::Gen { return_impl_trait_id, span, .. } => {
+                Some((return_impl_trait_id, span))
+            }
+            CoroutineKind::None => None,
         }
     }
 }
@@ -2831,25 +2841,22 @@ impl Extern {
 pub struct FnHeader {
     /// The `unsafe` keyword, if any
     pub unsafety: Unsafe,
-    /// The `async` keyword, if any
-    pub asyncness: Async,
+    /// Whether this is `async`, `gen`, or nothing.
+    pub coro_kind: CoroutineKind,
     /// The `const` keyword, if any
     pub constness: Const,
     /// The `extern` keyword and corresponding ABI string, if any
     pub ext: Extern,
-    /// The `gen` keyword, if any
-    pub genness: Gen,
 }
 
 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, genness } = self;
+        let Self { unsafety, coro_kind, constness, ext } = self;
         matches!(unsafety, Unsafe::Yes(_))
-            || asyncness.is_async()
+            || !matches!(coro_kind, CoroutineKind::None)
             || matches!(constness, Const::Yes(_))
             || !matches!(ext, Extern::None)
-            || matches!(genness, Gen::Yes { .. })
     }
 }
 
@@ -2857,10 +2864,9 @@ impl Default for FnHeader {
     fn default() -> FnHeader {
         FnHeader {
             unsafety: Unsafe::No,
-            asyncness: Async::No,
+            coro_kind: CoroutineKind::None,
             constness: Const::No,
             ext: Extern::None,
-            genness: Gen::No,
         }
     }
 }
@@ -3181,7 +3187,7 @@ mod size_asserts {
     static_assert_size!(Block, 32);
     static_assert_size!(Expr, 72);
     static_assert_size!(ExprKind, 40);
-    static_assert_size!(Fn, 168);
+    static_assert_size!(Fn, 160);
     static_assert_size!(ForeignItem, 96);
     static_assert_size!(ForeignItemKind, 24);
     static_assert_size!(GenericArg, 24);
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 7cdbb1a8c61..f9f767862f5 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -121,12 +121,8 @@ pub trait MutVisitor: Sized {
         noop_visit_fn_decl(d, self);
     }
 
-    fn visit_asyncness(&mut self, a: &mut Async) {
-        noop_visit_asyncness(a, self);
-    }
-
-    fn visit_genness(&mut self, a: &mut Gen) {
-        noop_visit_genness(a, self);
+    fn visit_coro_kind(&mut self, a: &mut CoroutineKind) {
+        noop_visit_coro_kind(a, self);
     }
 
     fn visit_closure_binder(&mut self, b: &mut ClosureBinder) {
@@ -875,23 +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) {
-    match asyncness {
-        Async::Yes { span: _, closure_id, return_impl_trait_id } => {
+pub fn noop_visit_coro_kind<T: MutVisitor>(coro_kind: &mut CoroutineKind, vis: &mut T) {
+    match coro_kind {
+        CoroutineKind::Async { span: _, closure_id, return_impl_trait_id }
+        | CoroutineKind::Gen { span: _, closure_id, return_impl_trait_id } => {
             vis.visit_id(closure_id);
             vis.visit_id(return_impl_trait_id);
         }
-        Async::No => {}
-    }
-}
-
-pub fn noop_visit_genness<T: MutVisitor>(genness: &mut Gen, vis: &mut T) {
-    match genness {
-        Gen::Yes { span: _, closure_id, return_impl_trait_id } => {
-            vis.visit_id(closure_id);
-            vis.visit_id(return_impl_trait_id);
-        }
-        Gen::No => {}
+        CoroutineKind::None => {}
     }
 }
 
@@ -1184,10 +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: _, genness } = header;
+    let FnHeader { unsafety, coro_kind, constness, ext: _ } = header;
     visit_constness(constness, vis);
-    vis.visit_asyncness(asyncness);
-    vis.visit_genness(genness);
+    vis.visit_coro_kind(coro_kind);
     visit_unsafety(unsafety, vis);
 }
 
@@ -1421,7 +1407,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
             binder,
             capture_clause,
             constness,
-            asyncness,
+            coro_kind,
             movability: _,
             fn_decl,
             body,
@@ -1430,7 +1416,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
         }) => {
             vis.visit_closure_binder(binder);
             visit_constness(constness, vis);
-            vis.visit_asyncness(asyncness);
+            vis.visit_coro_kind(coro_kind);
             vis.visit_capture_by(capture_clause);
             vis.visit_fn_decl(fn_decl);
             vis.visit_expr(body);
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index 9dbadcb49d3..a303d6584f4 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/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,
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index eb1a1d15027..5846f17c539 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -195,39 +195,37 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     binder,
                     capture_clause,
                     constness,
-                    asyncness,
+                    coro_kind,
                     movability,
                     fn_decl,
                     body,
                     fn_decl_span,
                     fn_arg_span,
-                }) => {
-                    if let Async::Yes { closure_id, .. } = asyncness {
-                        self.lower_expr_async_closure(
-                            binder,
-                            *capture_clause,
-                            e.id,
-                            hir_id,
-                            *closure_id,
-                            fn_decl,
-                            body,
-                            *fn_decl_span,
-                            *fn_arg_span,
-                        )
-                    } else {
-                        self.lower_expr_closure(
-                            binder,
-                            *capture_clause,
-                            e.id,
-                            *constness,
-                            *movability,
-                            fn_decl,
-                            body,
-                            *fn_decl_span,
-                            *fn_arg_span,
-                        )
-                    }
-                }
+                }) => match coro_kind {
+                    CoroutineKind::Async { closure_id, .. }
+                    | CoroutineKind::Gen { closure_id, .. } => self.lower_expr_async_closure(
+                        binder,
+                        *capture_clause,
+                        e.id,
+                        hir_id,
+                        *closure_id,
+                        fn_decl,
+                        body,
+                        *fn_decl_span,
+                        *fn_arg_span,
+                    ),
+                    CoroutineKind::None => self.lower_expr_closure(
+                        binder,
+                        *capture_clause,
+                        e.id,
+                        *constness,
+                        *movability,
+                        fn_decl,
+                        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)
@@ -935,7 +933,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
         // Lower outside new scope to preserve `is_in_loop_condition`.
-        let fn_decl = self.lower_fn_decl(decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
+        let fn_decl = self.lower_fn_decl(
+            decl,
+            closure_id,
+            fn_decl_span,
+            FnDeclKind::Closure,
+            CoroutineKind::None,
+        );
 
         let c = self.arena.alloc(hir::Closure {
             def_id: self.local_def_id(closure_id),
@@ -1050,8 +1054,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
         // We need to lower the declaration outside the new scope, because we
         // have to conserve the state of being inside a loop condition for the
         // closure argument types.
-        let fn_decl =
-            self.lower_fn_decl(&outer_decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
+        let fn_decl = self.lower_fn_decl(
+            &outer_decl,
+            closure_id,
+            fn_decl_span,
+            FnDeclKind::Closure,
+            CoroutineKind::None,
+        );
 
         let c = self.arena.alloc(hir::Closure {
             def_id: self.local_def_id(closure_id),
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 852555048fe..5d32e1a78f1 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -1,5 +1,3 @@
-use crate::FnReturnTransformation;
-
 use super::errors::{InvalidAbi, InvalidAbiReason, InvalidAbiSuggestion, MisplacedRelaxTraitBound};
 use super::ResolverAstLoweringExt;
 use super::{AstOwner, ImplTraitContext, ImplTraitPosition};
@@ -208,35 +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 genness = header.genness;
+                    let coro_kind = header.coro_kind;
                     let body_id = this.lower_maybe_coroutine_body(
                         span,
                         hir_id,
                         decl,
-                        asyncness,
-                        genness,
+                        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()
-                                .map(|(node_id, span)| {
-                                    crate::FnReturnTransformation::Async(node_id, span)
-                                })
-                                .or_else(|| match genness {
-                                    Gen::Yes { span, closure_id: _, return_impl_trait_id } => {
-                                        Some(crate::FnReturnTransformation::Iterator(
-                                            return_impl_trait_id,
-                                            span,
-                                        ))
-                                    }
-                                    _ => None,
-                                });
-                            this.lower_fn_decl(decl, id, *fn_sig_span, FnDeclKind::Fn, ret_id)
+                            this.lower_fn_decl(decl, id, *fn_sig_span, FnDeclKind::Fn, coro_kind)
                         });
                     let sig = hir::FnSig {
                         decl,
@@ -620,7 +602,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                                     i.id,
                                     sig.span,
                                     FnDeclKind::ExternFn,
-                                    None,
+                                    CoroutineKind::None,
                                 ),
                                 this.lower_fn_params_to_names(fdec),
                             )
@@ -747,28 +729,22 @@ 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()
-                        .map(|(node_id, span)| crate::FnReturnTransformation::Async(node_id, span)),
+                    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 genness = sig.header.genness;
                 let body_id = self.lower_maybe_coroutine_body(
                     i.span,
                     hir_id,
                     &sig.decl,
-                    asyncness,
-                    genness,
+                    sig.header.coro_kind,
                     Some(body),
                 );
                 let (generics, sig) = self.lower_method_sig(
@@ -776,9 +752,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     sig,
                     i.id,
                     FnDeclKind::Trait,
-                    asyncness
-                        .opt_return_id()
-                        .map(|(node_id, span)| crate::FnReturnTransformation::Async(node_id, span)),
+                    sig.header.coro_kind,
                 );
                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)), true)
             }
@@ -867,14 +841,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 },
             ),
             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
-                let asyncness = sig.header.asyncness;
-                let genness = sig.header.genness;
+                self.current_item = Some(i.span);
                 let body_id = self.lower_maybe_coroutine_body(
                     i.span,
                     hir_id,
                     &sig.decl,
-                    asyncness,
-                    genness,
+                    sig.header.coro_kind,
                     body.as_deref(),
                 );
                 let (generics, sig) = self.lower_method_sig(
@@ -882,9 +854,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     sig,
                     i.id,
                     if self.is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
-                    asyncness
-                        .opt_return_id()
-                        .map(|(node_id, span)| crate::FnReturnTransformation::Async(node_id, span)),
+                    sig.header.coro_kind,
                 );
 
                 (generics, hir::ImplItemKind::Fn(sig, body_id))
@@ -1055,15 +1025,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
         fn_id: hir::HirId,
         decl: &FnDecl,
-        asyncness: Async,
-        genness: Gen,
+        coro_kind: CoroutineKind,
         body: Option<&Block>,
     ) -> hir::BodyId {
-        let (closure_id, body) = match (asyncness, genness, body) {
-            // FIXME(eholk): do something reasonable for `async gen fn`. Probably that's an error
-            // for now since it's not supported.
-            (Async::Yes { closure_id, .. }, _, Some(body))
-            | (_, Gen::Yes { closure_id, .. }, Some(body)) => (closure_id, body),
+        let (closure_id, body) = match (coro_kind, body) {
+            (
+                CoroutineKind::Async { closure_id, .. } | CoroutineKind::Gen { closure_id, .. },
+                Some(body),
+            ) => (closure_id, body),
             _ => return self.lower_fn_body_block(span, decl, body),
         };
 
@@ -1232,8 +1201,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
                 this.expr_block(body)
             };
-            let coroutine_expr = match (asyncness, genness) {
-                (Async::Yes { .. }, _) => this.make_async_expr(
+            let coroutine_expr = match coro_kind {
+                CoroutineKind::Async { .. } => this.make_async_expr(
                     CaptureBy::Value { move_kw: rustc_span::DUMMY_SP },
                     closure_id,
                     None,
@@ -1241,7 +1210,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::CoroutineSource::Fn,
                     mkbody,
                 ),
-                (_, Gen::Yes { .. }) => this.make_gen_expr(
+                CoroutineKind::Gen { .. } => this.make_gen_expr(
                     CaptureBy::Value { move_kw: rustc_span::DUMMY_SP },
                     closure_id,
                     None,
@@ -1249,7 +1218,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::CoroutineSource::Fn,
                     mkbody,
                 ),
-                _ => unreachable!("we must have either an async fn or a gen fn"),
+                CoroutineKind::None => unreachable!("we must have either an async fn or a gen fn"),
             };
 
             let hir_id = this.lower_node_id(closure_id);
@@ -1266,21 +1235,26 @@ impl<'hir> LoweringContext<'_, 'hir> {
         sig: &FnSig,
         id: NodeId,
         kind: FnDeclKind,
-        transform_return_type: Option<FnReturnTransformation>,
+        coro_kind: 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, transform_return_type)
+                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 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),
         }
@@ -1322,13 +1296,6 @@ 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,
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 96a413e9f73..92650f0c47e 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -493,21 +493,6 @@ enum ParenthesizedGenericArgs {
     Err,
 }
 
-/// Describes a return type transformation that can be performed by `LoweringContext::lower_fn_decl`
-#[derive(Debug)]
-enum FnReturnTransformation {
-    /// Replaces a return type `T` with `impl Future<Output = T>`.
-    ///
-    /// The `NodeId` is the ID of the return type `impl Trait` item, and the `Span` points to the
-    /// `async` keyword.
-    Async(NodeId, Span),
-    /// Replaces a return type `T` with `impl Iterator<Item = T>`.
-    ///
-    /// The `NodeId` is the ID of the return type `impl Trait` item, and the `Span` points to the
-    /// `gen` keyword.
-    Iterator(NodeId, Span),
-}
-
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn create_def(
         &mut self,
@@ -1374,7 +1359,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     generic_params,
                     unsafety: self.lower_unsafety(f.unsafety),
                     abi: self.lower_extern(f.ext),
-                    decl: self.lower_fn_decl(&f.decl, t.id, t.span, FnDeclKind::Pointer, None),
+                    decl: self.lower_fn_decl(
+                        &f.decl,
+                        t.id,
+                        t.span,
+                        FnDeclKind::Pointer,
+                        CoroutineKind::None,
+                    ),
                     param_names: self.lower_fn_params_to_names(&f.decl),
                 }))
             }
@@ -1809,7 +1800,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         fn_node_id: NodeId,
         fn_span: Span,
         kind: FnDeclKind,
-        transform_return_type: Option<FnReturnTransformation>,
+        coro: CoroutineKind,
     ) -> &'hir hir::FnDecl<'hir> {
         let c_variadic = decl.c_variadic();
 
@@ -1838,12 +1829,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             self.lower_ty_direct(&param.ty, &itctx)
         }));
 
-        let output = match transform_return_type {
-            Some(transform) => {
+        let output = match coro {
+            CoroutineKind::Async { .. } | CoroutineKind::Gen { .. } => {
                 let fn_def_id = self.local_def_id(fn_node_id);
-                self.lower_coroutine_fn_ret_ty(&decl.output, fn_def_id, transform, kind, fn_span)
+                self.lower_coroutine_fn_ret_ty(&decl.output, fn_def_id, coro, kind, fn_span)
             }
-            None => match &decl.output {
+            CoroutineKind::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);
@@ -1910,16 +1901,19 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         &mut self,
         output: &FnRetTy,
         fn_def_id: LocalDefId,
-        transform: FnReturnTransformation,
+        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 transform {
-            FnReturnTransformation::Async(opaque_ty_node_id, _)
-            | FnReturnTransformation::Iterator(opaque_ty_node_id, _) => opaque_ty_node_id,
+        let opaque_ty_node_id = match coro {
+            CoroutineKind::Async { return_impl_trait_id, .. }
+            | CoroutineKind::Gen { return_impl_trait_id, .. } => return_impl_trait_id,
+            CoroutineKind::None => {
+                unreachable!("lower_coroutine_fn_ret_ty must be called with either Async or Gen")
+            }
         };
 
         let captured_lifetimes: Vec<_> = self
@@ -1939,7 +1933,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             |this| {
                 let future_bound = this.lower_coroutine_fn_output_type_to_future_bound(
                     output,
-                    transform,
+                    coro,
                     span,
                     ImplTraitContext::ReturnPositionOpaqueTy {
                         origin: hir::OpaqueTyOrigin::FnReturn(fn_def_id),
@@ -1958,7 +1952,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_coroutine_fn_output_type_to_future_bound(
         &mut self,
         output: &FnRetTy,
-        transform: FnReturnTransformation,
+        coro: CoroutineKind,
         span: Span,
         nested_impl_trait_context: ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
@@ -1974,11 +1968,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         };
 
         // "<Output|Item = T>"
-        let (symbol, lang_item) = match transform {
-            FnReturnTransformation::Async(..) => (hir::FN_OUTPUT_NAME, hir::LangItem::Future),
-            FnReturnTransformation::Iterator(..) => {
-                (hir::ITERATOR_ITEM_NAME, hir::LangItem::Iterator)
-            }
+        let (symbol, lang_item) = match coro {
+            CoroutineKind::Async { .. } => (hir::FN_OUTPUT_NAME, hir::LangItem::Future),
+            CoroutineKind::Gen { .. } => (hir::ITERATOR_ITEM_NAME, hir::LangItem::Iterator),
+            CoroutineKind::None => panic!("attemping to lower output type of non-coroutine fn"),
         };
 
         let future_args = self.arena.alloc(hir::GenericArgs {
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 45b5e63bd1b..ce680afdc3e 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1271,7 +1271,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         // Functions cannot both be `const async`
         if let Some(&FnHeader {
             constness: Const::Yes(cspan),
-            asyncness: Async::Yes { span: aspan, .. },
+            coro_kind: CoroutineKind::Async { span: aspan, .. },
             ..
         }) = fk.header()
         {
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index cbdcc683b56..1e69674d30a 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -1490,9 +1490,15 @@ impl<'a> State<'a> {
         }
     }
 
-    fn print_asyncness(&mut self, asyncness: ast::Async) {
-        if asyncness.is_async() {
-            self.word_nbsp("async");
+    fn print_coro_kind(&mut self, coro_kind: ast::CoroutineKind) {
+        match coro_kind {
+            ast::CoroutineKind::None => {}
+            ast::CoroutineKind::Gen { .. } => {
+                self.word_nbsp("gen");
+            }
+            ast::CoroutineKind::Async { .. } => {
+                self.word_nbsp("async");
+            }
         }
     }
 
@@ -1685,7 +1691,7 @@ impl<'a> State<'a> {
 
     fn print_fn_header_info(&mut self, header: ast::FnHeader) {
         self.print_constness(header.constness);
-        self.print_asyncness(header.asyncness);
+        self.print_coro_kind(header.coro_kind);
         self.print_unsafety(header.unsafety);
 
         match header.ext {
diff --git a/compiler/rustc_ast_pretty/src/pprust/state/expr.rs b/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
index 45a55e20ca7..d8641e745d0 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state/expr.rs
+++ b/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);
+                self.print_coro_kind(*coro_kind);
                 self.print_capture_clause(*capture_clause);
 
                 self.print_fn_params_and_ret(fn_decl, true);
diff --git a/compiler/rustc_builtin_macros/src/test.rs b/compiler/rustc_builtin_macros/src/test.rs
index bf1e1ebf5dd..8f4234b4138 100644
--- a/compiler/rustc_builtin_macros/src/test.rs
+++ b/compiler/rustc_builtin_macros/src/test.rs
@@ -541,7 +541,7 @@ 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 ast::CoroutineKind::Async { span, .. } = f.sig.header.coro_kind {
         return Err(sd.emit_err(errors::TestBadFn { span: i.span, cause: span, kind: "async" }));
     }
 
diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/build.rs
index 9a8d0d691f0..49b52d265d6 100644
--- a/compiler/rustc_expand/src/build.rs
+++ b/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: ast::CoroutineKind::None,
                 movability: ast::Movability::Movable,
                 fn_decl,
                 body,
diff --git a/compiler/rustc_lint/src/early.rs b/compiler/rustc_lint/src/early.rs
index 4cccaeeca84..41fbf1f3e2c 100644
--- a/compiler/rustc_lint/src/early.rs
+++ b/compiler/rustc_lint/src/early.rs
@@ -162,7 +162,9 @@ 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 ast::CoroutineKind::Async { closure_id, .. }
+            | ast::CoroutineKind::Gen { closure_id, .. } = sig.header.coro_kind
+            {
                 self.check_id(closure_id);
             }
         }
@@ -223,7 +225,9 @@ 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:
+                    ast::CoroutineKind::Async { closure_id, .. }
+                    | ast::CoroutineKind::Gen { closure_id, .. },
                 ..
             }) => self.check_id(closure_id),
             _ => {}
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index f8444881d1a..8a09d4e0549 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -21,7 +21,9 @@ 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, CoroutineKind, 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 +2239,7 @@ impl<'a> Parser<'a> {
         let asyncness = if self.token.uninterpolated_span().at_least_rust_2018() {
             self.parse_asyncness(Case::Sensitive)
         } else {
-            Async::No
+            CoroutineKind::None
         };
 
         let capture_clause = self.parse_capture_clause()?;
@@ -2261,7 +2263,7 @@ impl<'a> Parser<'a> {
             }
         };
 
-        if let Async::Yes { span, .. } = asyncness {
+        if let CoroutineKind::Async { span, .. } = asyncness {
             // Feature-gate `async ||` closures.
             self.sess.gated_spans.gate(sym::async_closure, span);
         }
@@ -2284,7 +2286,7 @@ impl<'a> Parser<'a> {
                 binder,
                 capture_clause,
                 constness,
-                asyncness,
+                coro_kind: asyncness,
                 movability,
                 fn_decl,
                 body,
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index c6d1ea882e9..2bee4d5d5c6 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/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 CoroutineKind::Async { span, .. } = asyncness {
             if span.is_rust_2015() {
                 self.sess.emit_err(errors::AsyncFnIn2015 {
                     span,
@@ -2410,12 +2410,14 @@ impl<'a> Parser<'a> {
             }
         }
 
-        if let Gen::Yes { span, .. } = genness {
+        if let CoroutineKind::Gen { span, .. } = genness {
             self.sess.gated_spans.gate(sym::gen_blocks, span);
         }
 
-        if let (Async::Yes { span: async_span, .. }, Gen::Yes { span: gen_span, .. }) =
-            (asyncness, genness)
+        if let (
+            CoroutineKind::Async { span: async_span, .. },
+            CoroutineKind::Gen { span: gen_span, .. },
+        ) = (asyncness, genness)
         {
             self.sess.emit_err(errors::AsyncGenFn { span: async_span.to(gen_span) });
         }
@@ -2450,9 +2452,12 @@ impl<'a> Parser<'a> {
                         }
                     } else if self.check_keyword(kw::Async) {
                         match asyncness {
-                            Async::Yes { span, .. } => Some(WrongKw::Duplicated(span)),
-                            Async::No => {
-                                recover_asyncness = Async::Yes {
+                            CoroutineKind::Async { span, .. } => Some(WrongKw::Duplicated(span)),
+                            CoroutineKind::Gen { .. } => {
+                                panic!("not sure how to recover here")
+                            }
+                            CoroutineKind::None => {
+                                recover_asyncness = CoroutineKind::Async {
                                     span: self.token.span,
                                     closure_id: DUMMY_NODE_ID,
                                     return_impl_trait_id: DUMMY_NODE_ID,
@@ -2537,6 +2542,8 @@ impl<'a> Parser<'a> {
                         }
                     }
 
+                    // FIXME(eholk): 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))
@@ -2548,8 +2555,7 @@ impl<'a> Parser<'a> {
                         return Ok(FnHeader {
                             constness: recover_constness,
                             unsafety: recover_unsafety,
-                            asyncness: recover_asyncness,
-                            genness, // FIXME(eholk): add keyword recovery logic here too.
+                            coro_kind: recover_asyncness,
                             ext,
                         });
                     }
@@ -2559,7 +2565,13 @@ impl<'a> Parser<'a> {
             }
         }
 
-        Ok(FnHeader { constness, unsafety, asyncness, ext, genness })
+        let coro_kind = match asyncness {
+            CoroutineKind::Async { .. } => asyncness,
+            CoroutineKind::Gen { .. } => unreachable!("asycness cannot be Gen"),
+            CoroutineKind::None => genness,
+        };
+
+        Ok(FnHeader { constness, unsafety, coro_kind, ext })
     }
 
     /// Parses the parameter list and result type of a function declaration.
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index c680a950584..a9da3043117 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/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) -> 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 }
+            CoroutineKind::Async {
+                span,
+                closure_id: DUMMY_NODE_ID,
+                return_impl_trait_id: DUMMY_NODE_ID,
+            }
         } else {
-            Async::No
+            CoroutineKind::None
         }
     }
 
     /// Parses genness: `gen` or nothing.
-    fn parse_genness(&mut self, case: Case) -> Gen {
+    fn parse_genness(&mut self, case: Case) -> 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 }
+            CoroutineKind::Gen {
+                span,
+                closure_id: DUMMY_NODE_ID,
+                return_impl_trait_id: DUMMY_NODE_ID,
+            }
         } else {
-            Gen::No
+            CoroutineKind::None
         }
     }
 
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index cf0dd39b562..a8e147a05b0 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/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, genness: _ } =
+        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)?;
@@ -609,7 +609,7 @@ 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 ast::CoroutineKind::Async { span, .. } = coro_kind {
             self.sess.emit_err(FnPointerCannotBeAsync { span: whole_span, qualifier: span });
         }
         // FIXME(eholk): emit a similar error for `gen fn()`
diff --git a/compiler/rustc_resolve/src/def_collector.rs b/compiler/rustc_resolve/src/def_collector.rs
index 306492eaa96..c43ec99f42a 100644
--- a/compiler/rustc_resolve/src/def_collector.rs
+++ b/compiler/rustc_resolve/src/def_collector.rs
@@ -157,8 +157,8 @@ 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 {
             // FIXME(eholk): handle `async gen fn`
-            if let (Async::Yes { closure_id, .. }, _) | (_, Gen::Yes { closure_id, .. }) =
-                (sig.header.asyncness, sig.header.genness)
+            if let CoroutineKind::Async { closure_id, .. } | CoroutineKind::Gen { closure_id, .. } =
+                sig.header.coro_kind
             {
                 self.visit_generics(generics);
 
@@ -284,11 +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 {
-                    Async::Yes { closure_id, .. } => {
+                match closure.coro_kind {
+                    CoroutineKind::Async { closure_id, .. }
+                    | CoroutineKind::Gen { closure_id, .. } => {
                         self.create_def(closure_id, kw::Empty, DefKind::Closure, expr.span)
                     }
-                    Async::No => closure_def,
+                    CoroutineKind::None => closure_def,
                 }
             }
             ExprKind::Gen(_, _, _) => {
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index f20d5829030..2f7d1835113 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -916,7 +916,7 @@ 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((async_node_id, _)) = sig.header.coro_kind.opt_return_id() {
                             this.record_lifetime_params_for_impl_trait(async_node_id);
                         }
                     },
@@ -940,7 +940,7 @@ 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 async_node_id = sig.header.coro_kind.opt_return_id();
 
                         this.with_lifetime_rib(
                             LifetimeRibKind::AnonymousCreateParameter {
@@ -4289,7 +4289,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             // 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.
             ExprKind::Closure(box ast::Closure {
-                asyncness: Async::Yes { .. },
+                coro_kind: CoroutineKind::Async { .. },
                 ref fn_decl,
                 ref body,
                 ..
diff --git a/src/tools/clippy/clippy_lints/src/doc/mod.rs b/src/tools/clippy/clippy_lints/src/doc/mod.rs
index ba452775015..ec4abc21f48 100644
--- a/src/tools/clippy/clippy_lints/src/doc/mod.rs
+++ b/src/tools/clippy/clippy_lints/src/doc/mod.rs
@@ -655,6 +655,205 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
     headers
 }
 
+<<<<<<< HEAD:src/tools/clippy/clippy_lints/src/doc/mod.rs
+=======
+fn check_link_quotes(cx: &LateContext<'_>, trimmed_text: &str, range: Range<usize>, fragments: Fragments<'_>) {
+    if trimmed_text.starts_with('\'')
+        && trimmed_text.ends_with('\'')
+        && let Some(span) = fragments.span(cx, range)
+    {
+        span_lint(
+            cx,
+            DOC_LINK_WITH_QUOTES,
+            span,
+            "possible intra-doc link using quotes instead of backticks",
+        );
+    }
+}
+
+fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, range: Range<usize>, fragments: Fragments<'_>) {
+    fn has_needless_main(code: String, edition: Edition) -> bool {
+        rustc_driver::catch_fatal_errors(|| {
+            rustc_span::create_session_globals_then(edition, || {
+                let filename = FileName::anon_source_code(&code);
+
+                let fallback_bundle =
+                    rustc_errors::fallback_fluent_bundle(rustc_driver::DEFAULT_LOCALE_RESOURCES.to_vec(), false);
+                let emitter = EmitterWriter::new(Box::new(io::sink()), fallback_bundle);
+                let handler = Handler::with_emitter(Box::new(emitter)).disable_warnings();
+                #[expect(clippy::arc_with_non_send_sync)] // `Lrc` is expected by with_span_handler
+                let sm = Lrc::new(SourceMap::new(FilePathMapping::empty()));
+                let sess = ParseSess::with_span_handler(handler, sm);
+
+                let mut parser = match maybe_new_parser_from_source_str(&sess, filename, code) {
+                    Ok(p) => p,
+                    Err(errs) => {
+                        drop(errs);
+                        return false;
+                    },
+                };
+
+                let mut relevant_main_found = false;
+                loop {
+                    match parser.parse_item(ForceCollect::No) {
+                        Ok(Some(item)) => match &item.kind {
+                            ItemKind::Fn(box Fn {
+                                sig, body: Some(block), ..
+                            }) if item.ident.name == sym::main => {
+                                let is_async = sig.header.coro_kind.is_async();
+                                let returns_nothing = match &sig.decl.output {
+                                    FnRetTy::Default(..) => true,
+                                    FnRetTy::Ty(ty) if ty.kind.is_unit() => true,
+                                    FnRetTy::Ty(_) => false,
+                                };
+
+                                if returns_nothing && !is_async && !block.stmts.is_empty() {
+                                    // This main function should be linted, but only if there are no other functions
+                                    relevant_main_found = true;
+                                } else {
+                                    // This main function should not be linted, we're done
+                                    return false;
+                                }
+                            },
+                            // Tests with one of these items are ignored
+                            ItemKind::Static(..)
+                            | ItemKind::Const(..)
+                            | ItemKind::ExternCrate(..)
+                            | ItemKind::ForeignMod(..)
+                            // Another function was found; this case is ignored
+                            | ItemKind::Fn(..) => return false,
+                            _ => {},
+                        },
+                        Ok(None) => break,
+                        Err(e) => {
+                            e.cancel();
+                            return false;
+                        },
+                    }
+                }
+
+                relevant_main_found
+            })
+        })
+        .ok()
+        .unwrap_or_default()
+    }
+
+    let trailing_whitespace = text.len() - text.trim_end().len();
+
+    // Because of the global session, we need to create a new session in a different thread with
+    // the edition we need.
+    let text = text.to_owned();
+    if thread::spawn(move || has_needless_main(text, edition))
+        .join()
+        .expect("thread::spawn failed")
+        && let Some(span) = fragments.span(cx, range.start..range.end - trailing_whitespace)
+    {
+        span_lint(cx, NEEDLESS_DOCTEST_MAIN, span, "needless `fn main` in doctest");
+    }
+}
+
+fn check_text(cx: &LateContext<'_>, valid_idents: &FxHashSet<String>, text: &str, span: Span) {
+    for word in text.split(|c: char| c.is_whitespace() || c == '\'') {
+        // Trim punctuation as in `some comment (see foo::bar).`
+        //                                                   ^^
+        // Or even as in `_foo bar_` which is emphasized. Also preserve `::` as a prefix/suffix.
+        let mut word = word.trim_matches(|c: char| !c.is_alphanumeric() && c != ':');
+
+        // Remove leading or trailing single `:` which may be part of a sentence.
+        if word.starts_with(':') && !word.starts_with("::") {
+            word = word.trim_start_matches(':');
+        }
+        if word.ends_with(':') && !word.ends_with("::") {
+            word = word.trim_end_matches(':');
+        }
+
+        if valid_idents.contains(word) || word.chars().all(|c| c == ':') {
+            continue;
+        }
+
+        // Adjust for the current word
+        let offset = word.as_ptr() as usize - text.as_ptr() as usize;
+        let span = Span::new(
+            span.lo() + BytePos::from_usize(offset),
+            span.lo() + BytePos::from_usize(offset + word.len()),
+            span.ctxt(),
+            span.parent(),
+        );
+
+        check_word(cx, word, span);
+    }
+}
+
+fn check_word(cx: &LateContext<'_>, word: &str, span: Span) {
+    /// Checks if a string is upper-camel-case, i.e., starts with an uppercase and
+    /// contains at least two uppercase letters (`Clippy` is ok) and one lower-case
+    /// letter (`NASA` is ok).
+    /// Plurals are also excluded (`IDs` is ok).
+    fn is_camel_case(s: &str) -> bool {
+        if s.starts_with(|c: char| c.is_ascii_digit() | c.is_ascii_lowercase()) {
+            return false;
+        }
+
+        let s = s.strip_suffix('s').unwrap_or(s);
+
+        s.chars().all(char::is_alphanumeric)
+            && s.chars().filter(|&c| c.is_uppercase()).take(2).count() > 1
+            && s.chars().filter(|&c| c.is_lowercase()).take(1).count() > 0
+    }
+
+    fn has_underscore(s: &str) -> bool {
+        s != "_" && !s.contains("\\_") && s.contains('_')
+    }
+
+    fn has_hyphen(s: &str) -> bool {
+        s != "-" && s.contains('-')
+    }
+
+    if let Ok(url) = Url::parse(word) {
+        // try to get around the fact that `foo::bar` parses as a valid URL
+        if !url.cannot_be_a_base() {
+            span_lint(
+                cx,
+                DOC_MARKDOWN,
+                span,
+                "you should put bare URLs between `<`/`>` or make a proper Markdown link",
+            );
+
+            return;
+        }
+    }
+
+    // We assume that mixed-case words are not meant to be put inside backticks. (Issue #2343)
+    if has_underscore(word) && has_hyphen(word) {
+        return;
+    }
+
+    if has_underscore(word) || word.contains("::") || is_camel_case(word) {
+        let mut applicability = Applicability::MachineApplicable;
+
+        span_lint_and_then(
+            cx,
+            DOC_MARKDOWN,
+            span,
+            "item in documentation is missing backticks",
+            |diag| {
+                let snippet = snippet_with_applicability(cx, span, "..", &mut applicability);
+                diag.span_suggestion_with_style(
+                    span,
+                    "try",
+                    format!("`{snippet}`"),
+                    applicability,
+                    // always show the suggestion in a separate line, since the
+                    // inline presentation adds another pair of backticks
+                    SuggestionStyle::ShowAlways,
+                );
+            },
+        );
+    }
+}
+
+>>>>>>> d116f1718f1 (Merge Async and Gen into CoroutineKind):src/tools/clippy/clippy_lints/src/doc.rs
 struct FindPanicUnwrap<'a, 'tcx> {
     cx: &'a LateContext<'tcx>,
     panic_span: Option<Span>,
diff --git a/src/tools/clippy/clippy_utils/src/ast_utils.rs b/src/tools/clippy/clippy_utils/src/ast_utils.rs
index a2c61e07b70..ac9da383b93 100644
--- a/src/tools/clippy/clippy_utils/src/ast_utils.rs
+++ b/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,
@@ -565,7 +565,7 @@ pub fn eq_fn_sig(l: &FnSig, r: &FnSig) -> bool {
 
 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()
+        && (l.coro_kind.is_async() == r.coro_kind.is_async() || l.coro_kind.is_gen() == r.coro_kind.is_gen())
         && matches!(l.constness, Const::No) == matches!(r.constness, Const::No)
         && eq_ext(&l.ext, &r.ext)
 }
diff --git a/src/tools/rustfmt/src/closures.rs b/src/tools/rustfmt/src/closures.rs
index 8a4089a56f0..23cd6e4c092 100644
--- a/src/tools/rustfmt/src/closures.rs
+++ b/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: &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: &ast::CoroutineKind,
     movability: ast::Movability,
     fn_decl: &ast::FnDecl,
     body: &ast::Expr,
@@ -263,7 +263,8 @@ fn rewrite_closure_fn_decl(
     } else {
         ""
     };
-    let is_async = if asyncness.is_async() { "async " } else { "" };
+    let is_async = if coro_kind.is_async() { "async " } else { "" };
+    let is_gen = if coro_kind.is_gen() { "gen " } else { "" };
     let mover = if matches!(capture, ast::CaptureBy::Value { .. }) {
         "move "
     } else {
@@ -272,7 +273,14 @@ 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()
+                + is_async.len()
+                + is_gen.len()
+                + mover.len(),
+        )?
         .sub_width(4)?;
 
     // 1 = |
@@ -310,7 +318,7 @@ 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}{is_async}{is_gen}{mover}|{list_str}|");
 
     if !ret_str.is_empty() {
         if prefix.contains('\n') {
@@ -339,7 +347,7 @@ pub(crate) fn rewrite_last_closure(
             ref binder,
             constness,
             capture_clause,
-            ref asyncness,
+            ref coro_kind,
             movability,
             ref fn_decl,
             ref body,
@@ -360,7 +368,7 @@ pub(crate) fn rewrite_last_closure(
             binder,
             constness,
             capture_clause,
-            asyncness,
+            coro_kind,
             movability,
             fn_decl,
             body,
diff --git a/src/tools/rustfmt/src/expr.rs b/src/tools/rustfmt/src/expr.rs
index 60e0e007b1d..4515c27be37 100644
--- a/src/tools/rustfmt/src/expr.rs
+++ b/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,
diff --git a/src/tools/rustfmt/src/items.rs b/src/tools/rustfmt/src/items.rs
index edb5a5b629a..0a1f823fe87 100644
--- a/src/tools/rustfmt/src/items.rs
+++ b/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, 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 +343,7 @@ 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));
+        result.push_str(format_coro(&self.coro_kind));
         result.push_str(format_unsafety(self.unsafety));
         result.push_str(&format_extern(
             self.ext,
diff --git a/src/tools/rustfmt/src/utils.rs b/src/tools/rustfmt/src/utils.rs
index fd49030bf1b..18d8f0cdbd7 100644
--- a/src/tools/rustfmt/src/utils.rs
+++ b/src/tools/rustfmt/src/utils.rs
@@ -75,10 +75,11 @@ pub(crate) fn format_visibility(
 }
 
 #[inline]
-pub(crate) fn format_async(is_async: &ast::Async) -> &'static str {
-    match is_async {
-        ast::Async::Yes { .. } => "async ",
-        ast::Async::No => "",
+pub(crate) fn format_coro(coro_kind: &ast::CoroutineKind) -> &'static str {
+    match coro_kind {
+        ast::CoroutineKind::Async { .. } => "async ",
+        ast::CoroutineKind::Gen { .. } => "gen ",
+        ast::CoroutineKind::None => "",
     }
 }
 
diff --git a/tests/ui/coroutine/gen_fn_iter.rs b/tests/ui/coroutine/gen_fn_iter.rs
index 222ab571415..da01bc96ef4 100644
--- a/tests/ui/coroutine/gen_fn_iter.rs
+++ b/tests/ui/coroutine/gen_fn_iter.rs
@@ -3,6 +3,8 @@
 // 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;