Rollup merge of #127092 - compiler-errors:rtn-dots-redux, r=estebank

Change return-type-notation to use `(..)`

Aligns the syntax with the current wording of [RFC 3654](https://github.com/rust-lang/rfcs/pull/3654). Also implements rustfmt support (along with making a match exhaustive).

Tracking:
* https://github.com/rust-lang/rust/issues/109417

compiler/rustc_ast/src/ast.rs

@@ -176,6 +176,8 @@ pub enum GenericArgs {
     AngleBracketed(AngleBracketedArgs),
     /// The `(A, B)` and `C` in `Foo(A, B) -> C`.
     Parenthesized(ParenthesizedArgs),
+    /// `(..)` in return type notation
+    ParenthesizedElided(Span),
 }
 
 impl GenericArgs {
@@ -187,6 +189,7 @@ pub fn span(&self) -> Span {
         match self {
             AngleBracketed(data) => data.span,
             Parenthesized(data) => data.span,
+            ParenthesizedElided(span) => *span,
         }
     }
 }
@@ -2051,7 +2054,7 @@ pub fn name(&self) -> Symbol {
 /// * the `A: Bound` in `Trait<A: Bound>`
 /// * the `RetTy` in `Trait(ArgTy, ArgTy) -> RetTy`
 /// * the `C = { Ct }` in `Trait<C = { Ct }>` (feature `associated_const_equality`)
-/// * the `f(): Bound` in `Trait<f(): Bound>` (feature `return_type_notation`)
+/// * the `f(..): Bound` in `Trait<f(..): Bound>` (feature `return_type_notation`)
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct AssocItemConstraint {
     pub id: NodeId,

compiler/rustc_ast/src/mut_visit.rs

@@ -582,6 +582,7 @@ fn noop_visit_generic_args<T: MutVisitor>(generic_args: &mut GenericArgs, vis: &
     match generic_args {
         GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
         GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
+        GenericArgs::ParenthesizedElided(span) => vis.visit_span(span),
     }
 }
 

compiler/rustc_ast/src/util/classify.rs

@@ -311,6 +311,6 @@ fn path_return_type(path: &ast::Path) -> Option<&ast::Ty> {
             ast::FnRetTy::Default(_) => None,
             ast::FnRetTy::Ty(ret) => Some(ret),
         },
-        ast::GenericArgs::AngleBracketed(_) => None,
+        ast::GenericArgs::AngleBracketed(_) | ast::GenericArgs::ParenthesizedElided(_) => None,
     }
 }

compiler/rustc_ast/src/visit.rs

@@ -609,6 +609,7 @@ pub fn walk_generic_args<'a, V>(visitor: &mut V, generic_args: &'a GenericArgs)
             walk_list!(visitor, visit_ty, inputs);
             try_visit!(visitor.visit_fn_ret_ty(output));
         }
+        GenericArgs::ParenthesizedElided(_span) => {}
     }
     V::Result::output()
 }

compiler/rustc_ast_lowering/messages.ftl

@@ -36,10 +36,15 @@ ast_lowering_bad_return_type_notation_inputs =
     argument types not allowed with return type notation
     .suggestion = remove the input types
 
+ast_lowering_bad_return_type_notation_needs_dots = return type notation arguments must be elided with `..`
+    .suggestion = add `..`
+
 ast_lowering_bad_return_type_notation_output =
     return type not allowed with return type notation
     .suggestion = remove the return type
 
+ast_lowering_bad_return_type_notation_position = return type notation not allowed in this position yet
+
 ast_lowering_base_expression_double_dot =
     base expression required after `..`
     .suggestion = add a base expression here

compiler/rustc_ast_lowering/src/errors.rs

@@ -393,6 +393,17 @@ pub enum BadReturnTypeNotation {
         #[suggestion(code = "", applicability = "maybe-incorrect")]
         span: Span,
     },
+    #[diag(ast_lowering_bad_return_type_notation_needs_dots)]
+    NeedsDots {
+        #[primary_span]
+        #[suggestion(code = "(..)", applicability = "maybe-incorrect")]
+        span: Span,
+    },
+    #[diag(ast_lowering_bad_return_type_notation_position)]
+    Position {
+        #[primary_span]
+        span: Span,
+    },
 }
 
 #[derive(Diagnostic)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -985,20 +985,7 @@ fn lower_assoc_item_constraint(
                     self.lower_angle_bracketed_parameter_data(data, ParamMode::Explicit, itctx).0
                 }
                 GenericArgs::Parenthesized(data) => {
-                    if data.inputs.is_empty() && matches!(data.output, FnRetTy::Default(..)) {
-                        let parenthesized = if self.tcx.features().return_type_notation {
-                            hir::GenericArgsParentheses::ReturnTypeNotation
-                        } else {
-                            self.emit_bad_parenthesized_trait_in_assoc_ty(data);
-                            hir::GenericArgsParentheses::No
-                        };
-                        GenericArgsCtor {
-                            args: Default::default(),
-                            constraints: &[],
-                            parenthesized,
-                            span: data.inputs_span,
-                        }
-                    } else if let Some(first_char) = constraint.ident.as_str().chars().next()
+                    if let Some(first_char) = constraint.ident.as_str().chars().next()
                         && first_char.is_ascii_lowercase()
                     {
                         let mut err = if !data.inputs.is_empty() {
@@ -1010,7 +997,9 @@ fn lower_assoc_item_constraint(
                                 span: data.inputs_span.shrink_to_hi().to(ty.span),
                             })
                         } else {
-                            unreachable!("inputs are empty and return type is not provided")
+                            self.dcx().create_err(errors::BadReturnTypeNotation::NeedsDots {
+                                span: data.inputs_span,
+                            })
                         };
                         if !self.tcx.features().return_type_notation
                             && self.tcx.sess.is_nightly_build()
@@ -1040,6 +1029,12 @@ fn lower_assoc_item_constraint(
                         .0
                     }
                 }
+                GenericArgs::ParenthesizedElided(span) => GenericArgsCtor {
+                    args: Default::default(),
+                    constraints: &[],
+                    parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
+                    span: *span,
+                },
             };
             gen_args_ctor.into_generic_args(self)
         } else {

compiler/rustc_ast_lowering/src/path.rs

@@ -1,7 +1,8 @@
 use crate::ImplTraitPosition;
 
 use super::errors::{
-    AsyncBoundNotOnTrait, AsyncBoundOnlyForFnTraits, GenericTypeWithParentheses, UseAngleBrackets,
+    AsyncBoundNotOnTrait, AsyncBoundOnlyForFnTraits, BadReturnTypeNotation,
+    GenericTypeWithParentheses, UseAngleBrackets,
 };
 use super::ResolverAstLoweringExt;
 use super::{GenericArgsCtor, LifetimeRes, ParenthesizedGenericArgs};
@@ -271,6 +272,18 @@ pub(crate) fn lower_path_segment(
                         )
                     }
                 },
+                GenericArgs::ParenthesizedElided(span) => {
+                    self.dcx().emit_err(BadReturnTypeNotation::Position { span: *span });
+                    (
+                        GenericArgsCtor {
+                            args: Default::default(),
+                            constraints: &[],
+                            parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
+                            span: *span,
+                        },
+                        false,
+                    )
+                }
             }
         } else {
             (

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1312,6 +1312,7 @@ fn visit_generic_args(&mut self, generic_args: &'a GenericArgs) {
                     self.with_impl_trait(None, |this| this.visit_ty(ty));
                 }
             }
+            GenericArgs::ParenthesizedElided(_span) => {}
         }
     }
 
@@ -1468,7 +1469,7 @@ fn visit_param_bound(&mut self, bound: &'a GenericBound, ctxt: BoundKind) {
                                 span: args.span,
                             });
                         }
-                        None => {}
+                        Some(ast::GenericArgs::ParenthesizedElided(_)) | None => {}
                     }
                 }
             }
@@ -1716,7 +1717,9 @@ fn deny_equality_constraints(
                 // Add `<Bar = RhsTy>` to `Foo`.
                 match &mut assoc_path.segments[len].args {
                     Some(args) => match args.deref_mut() {
-                        GenericArgs::Parenthesized(_) => continue,
+                        GenericArgs::Parenthesized(_) | GenericArgs::ParenthesizedElided(..) => {
+                            continue;
+                        }
                         GenericArgs::AngleBracketed(args) => {
                             args.args.push(arg);
                         }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -1,6 +1,6 @@
 use rustc_ast as ast;
 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
-use rustc_ast::{attr, AssocItemConstraint, AssocItemConstraintKind, NodeId};
+use rustc_ast::{attr, NodeId};
 use rustc_ast::{token, PatKind};
 use rustc_feature::{AttributeGate, BuiltinAttribute, Features, GateIssue, BUILTIN_ATTRIBUTE_MAP};
 use rustc_session::parse::{feature_err, feature_err_issue, feature_warn};
@@ -445,23 +445,6 @@ fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
         visit::walk_fn(self, fn_kind)
     }
 
-    fn visit_assoc_item_constraint(&mut self, constraint: &'a AssocItemConstraint) {
-        if let AssocItemConstraintKind::Bound { .. } = constraint.kind
-            && let Some(ast::GenericArgs::Parenthesized(args)) = constraint.gen_args.as_ref()
-            && args.inputs.is_empty()
-            && let ast::FnRetTy::Default(..) = args.output
-        {
-            gate!(
-                &self,
-                return_type_notation,
-                constraint.span,
-                "return type notation is experimental"
-            );
-        }
-
-        visit::walk_assoc_item_constraint(self, constraint)
-    }
-
     fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
         let is_fn = match &i.kind {
             ast::AssocItemKind::Fn(_) => true,
@@ -566,6 +549,7 @@ macro_rules! gate_all {
         unsafe_extern_blocks,
         "`unsafe extern {}` blocks and `safe` keyword are experimental"
     );
+    gate_all!(return_type_notation, "return type notation is experimental");
 
     if !visitor.features.never_patterns {
         if let Some(spans) = spans.get(&sym::never_patterns) {
@@ -611,10 +595,6 @@ macro_rules! gate_all_legacy_dont_use {
 
     gate_all_legacy_dont_use!(box_patterns, "box pattern syntax is experimental");
     gate_all_legacy_dont_use!(trait_alias, "trait aliases are experimental");
-    // Despite being a new feature, `where T: Trait<Assoc(): Sized>`, which is RTN syntax now,
-    // used to be gated under associated_type_bounds, which are right above, so RTN needs to
-    // be too.
-    gate_all_legacy_dont_use!(return_type_notation, "return type notation is experimental");
     gate_all_legacy_dont_use!(decl_macro, "`macro` is experimental");
     gate_all_legacy_dont_use!(try_blocks, "`try` blocks are unstable");
     gate_all_legacy_dont_use!(auto_traits, "`auto` traits are unstable");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1060,6 +1060,11 @@ fn print_generic_args(&mut self, args: &ast::GenericArgs, colons_before_params:
                 self.word(")");
                 self.print_fn_ret_ty(&data.output);
             }
+            ast::GenericArgs::ParenthesizedElided(_) => {
+                self.word("(");
+                self.word("..");
+                self.word(")");
+            }
         }
     }
 }

compiler/rustc_hir/src/hir.rs

@@ -2414,7 +2414,7 @@ pub enum ImplItemKind<'hir> {
 /// * the `A: Bound` in `Trait<A: Bound>`
 /// * the `RetTy` in `Trait(ArgTy, ArgTy) -> RetTy`
 /// * the `C = { Ct }` in `Trait<C = { Ct }>` (feature `associated_const_equality`)
-/// * the `f(): Bound` in `Trait<f(): Bound>` (feature `return_type_notation`)
+/// * the `f(..): Bound` in `Trait<f(..): Bound>` (feature `return_type_notation`)
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub struct AssocItemConstraint<'hir> {
     pub hir_id: HirId,

compiler/rustc_parse/messages.ftl

@@ -45,10 +45,6 @@ parse_bad_assoc_type_bounds = bounds on associated types do not belong here
 parse_bad_item_kind = {$descr} is not supported in {$ctx}
     .help = consider moving the {$descr} out to a nearby module scope
 
-parse_bad_return_type_notation_dotdot =
-    return type notation uses `()` instead of `(..)` for elided arguments
-    .suggestion = remove the `..`
-
 parse_bad_return_type_notation_output =
     return type not allowed with return type notation
     .suggestion = remove the return type

compiler/rustc_parse/src/errors.rs

@@ -2567,14 +2567,6 @@ pub(crate) struct BadReturnTypeNotationOutput {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(parse_bad_return_type_notation_dotdot)]
-pub(crate) struct BadReturnTypeNotationDotDot {
-    #[primary_span]
-    #[suggestion(code = "", applicability = "maybe-incorrect")]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(parse_bad_assoc_type_bounds)]
 pub(crate) struct BadAssocTypeBounds {

compiler/rustc_parse/src/parser/path.rs

@@ -353,18 +353,17 @@ pub(super) fn parse_path_segment(
                     })?;
                     let span = lo.to(self.prev_token.span);
                     AngleBracketedArgs { args, span }.into()
-                } else if self.may_recover()
-                    && self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
+                } else if self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
                     // FIXME(return_type_notation): Could also recover `...` here.
                     && self.look_ahead(1, |tok| tok.kind == token::DotDot)
                 {
-                    self.bump();
-                    self.dcx()
-                        .emit_err(errors::BadReturnTypeNotationDotDot { span: self.token.span });
-                    self.bump();
+                    self.bump(); // (
+                    self.bump(); // ..
                     self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
                     let span = lo.to(self.prev_token.span);
 
+                    self.psess.gated_spans.gate(sym::return_type_notation, span);
+
                     if self.eat_noexpect(&token::RArrow) {
                         let lo = self.prev_token.span;
                         let ty = self.parse_ty()?;
@@ -372,13 +371,7 @@ pub(super) fn parse_path_segment(
                             .emit_err(errors::BadReturnTypeNotationOutput { span: lo.to(ty.span) });
                     }
 
-                    ParenthesizedArgs {
-                        span,
-                        inputs: ThinVec::new(),
-                        inputs_span: span,
-                        output: ast::FnRetTy::Default(self.prev_token.span.shrink_to_hi()),
-                    }
-                    .into()
+                    P(ast::GenericArgs::ParenthesizedElided(span))
                 } else {
                     // `(T, U) -> R`
 
@@ -733,14 +726,6 @@ fn parse_angle_arg(
 
                     let span = lo.to(self.prev_token.span);
 
-                    if let AssocItemConstraintKind::Bound { .. } = kind
-                        && let Some(ast::GenericArgs::Parenthesized(args)) = &gen_args
-                        && args.inputs.is_empty()
-                        && let ast::FnRetTy::Default(..) = args.output
-                    {
-                        self.psess.gated_spans.gate(sym::return_type_notation, span);
-                    }
-
                     let constraint =
                         AssocItemConstraint { id: ast::DUMMY_NODE_ID, ident, gen_args, kind, span };
                     Ok(Some(AngleBracketedArg::Constraint(constraint)))

compiler/rustc_passes/src/hir_stats.rs

@@ -695,7 +695,7 @@ fn visit_path_segment(&mut self, path_segment: &'v ast::PathSegment) {
     fn visit_generic_args(&mut self, g: &'v ast::GenericArgs) {
         record_variants!(
             (self, g, g, Id::None, ast, GenericArgs, GenericArgs),
-            [AngleBracketed, Parenthesized]
+            [AngleBracketed, Parenthesized, ParenthesizedElided]
         );
         ast_visit::walk_generic_args(self, g)
     }

compiler/rustc_resolve/src/late.rs

@@ -1221,6 +1221,7 @@ fn visit_path_segment(&mut self, path_segment: &'ast PathSegment) {
                         }
                     }
                 }
+                GenericArgs::ParenthesizedElided(_) => {}
             }
         }
     }

compiler/rustc_resolve/src/lib.rs

@@ -350,6 +350,7 @@ fn from(seg: &'a ast::PathSegment) -> Segment {
                     (args.span, found_lifetimes)
                 }
                 GenericArgs::Parenthesized(args) => (args.span, true),
+                GenericArgs::ParenthesizedElided(span) => (*span, true),
             }
         } else {
             (DUMMY_SP, false)

src/librustdoc/clean/types.rs

@@ -2557,7 +2557,7 @@ pub(crate) struct ProcMacro {
 /// * the `A: Bound` in `Trait<A: Bound>`
 /// * the `RetTy` in `Trait(ArgTy, ArgTy) -> RetTy`
 /// * the `C = { Ct }` in `Trait<C = { Ct }>` (feature `associated_const_equality`)
-/// * the `f(): Bound` in `Trait<f(): Bound>` (feature `return_type_notation`)
+/// * the `f(..): Bound` in `Trait<f(..): Bound>` (feature `return_type_notation`)
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub(crate) struct AssocItemConstraint {
     pub(crate) assoc: PathSegment,

src/tools/rustfmt/src/types.rs

@@ -484,21 +484,25 @@ fn rewrite_generic_args(
     span: Span,
 ) -> Option<String> {
     match gen_args {
-        ast::GenericArgs::AngleBracketed(ref data) if !data.args.is_empty() => {
-            let args = data
-                .args
-                .iter()
-                .map(|x| match x {
-                    ast::AngleBracketedArg::Arg(generic_arg) => {
-                        SegmentParam::from_generic_arg(generic_arg)
-                    }
-                    ast::AngleBracketedArg::Constraint(constraint) => {
-                        SegmentParam::Binding(constraint)
-                    }
-                })
-                .collect::<Vec<_>>();
+        ast::GenericArgs::AngleBracketed(ref data) => {
+            if data.args.is_empty() {
+                Some("".to_owned())
+            } else {
+                let args = data
+                    .args
+                    .iter()
+                    .map(|x| match x {
+                        ast::AngleBracketedArg::Arg(generic_arg) => {
+                            SegmentParam::from_generic_arg(generic_arg)
+                        }
+                        ast::AngleBracketedArg::Constraint(constraint) => {
+                            SegmentParam::Binding(constraint)
+                        }
+                    })
+                    .collect::<Vec<_>>();
 
-            overflow::rewrite_with_angle_brackets(context, "", args.iter(), shape, span)
+                overflow::rewrite_with_angle_brackets(context, "", args.iter(), shape, span)
+            }
         }
         ast::GenericArgs::Parenthesized(ref data) => format_function_type(
             data.inputs.iter().map(|x| &**x),
@@ -508,7 +512,7 @@ fn rewrite_generic_args(
             context,
             shape,
         ),
-        _ => Some("".to_owned()),
+        ast::GenericArgs::ParenthesizedElided(..) => Some("(..)".to_owned()),
     }
 }
 

src/tools/rustfmt/tests/target/return-type-notation.rs

@@ -0,0 +1,10 @@
+fn rtn()
+where
+    T: Trait<method(..): Send + 'static>,
+    T::method(..): Send + 'static,
+{
+}
+
+fn test() {
+    let x: T::method(..);
+}

tests/crashes/125249.rs

@@ -2,7 +2,7 @@
 #![feature(return_position_impl_trait_in_trait, return_type_notation)]
 
 trait IntFactory {
-    fn stream(&self) -> impl IntFactory<stream(): IntFactory<stream(): Send> + Send>;
+    fn stream(&self) -> impl IntFactory<stream(..): IntFactory<stream(..): Send> + Send>;
 }
 
 pub fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.rs

@@ -13,7 +13,7 @@ fn foo<T: Trait<method(i32): Send>>() {}
 fn bar<T: Trait<method() -> (): Send>>() {}
 //~^ ERROR return type not allowed with return type notation
 
-fn baz<T: Trait<method(..): Send>>() {}
-//~^ ERROR return type notation uses `()` instead of `(..)` for elided arguments
+fn baz<T: Trait<method(): Send>>() {}
+//~^ ERROR return type notation arguments must be elided with `..`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bad-inputs-and-output.stderr

@@ -1,9 +1,3 @@
-error: return type notation uses `()` instead of `(..)` for elided arguments
-  --> $DIR/bad-inputs-and-output.rs:16:24
-   |
-LL | fn baz<T: Trait<method(..): Send>>() {}
-   |                        ^^ help: remove the `..`
-
 warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
   --> $DIR/bad-inputs-and-output.rs:3:12
    |
@@ -25,5 +19,11 @@ error: return type not allowed with return type notation
 LL | fn bar<T: Trait<method() -> (): Send>>() {}
    |                         ^^^^^^ help: remove the return type
 
+error: return type notation arguments must be elided with `..`
+  --> $DIR/bad-inputs-and-output.rs:16:23
+   |
+LL | fn baz<T: Trait<method(): Send>>() {}
+   |                       ^^ help: add `..`: `(..)`
+
 error: aborting due to 3 previous errors; 1 warning emitted
 

tests/ui/associated-type-bounds/return-type-notation/bare-path.rs

@@ -0,0 +1,26 @@
+#![feature(return_type_notation)]
+//~^ WARN the feature `return_type_notation` is incomplete
+
+trait Tr {
+    const CONST: usize;
+
+    fn method() -> impl Sized;
+}
+
+fn foo<T: Tr>()
+where
+    T::method(..): Send,
+    //~^ ERROR return type notation not allowed in this position yet
+    //~| ERROR expected type, found function
+    <T as Tr>::method(..): Send,
+    //~^ ERROR return type notation not allowed in this position yet
+    //~| ERROR expected associated type, found associated function `Tr::method`
+{
+    let _ = T::CONST::(..);
+    //~^ ERROR return type notation not allowed in this position yet
+    let _: T::method(..);
+    //~^ ERROR return type notation not allowed in this position yet
+    //~| ERROR expected type, found function
+}
+
+fn main() {}

tests/ui/associated-type-bounds/return-type-notation/bare-path.stderr

@@ -0,0 +1,66 @@
+error[E0575]: expected associated type, found associated function `Tr::method`
+  --> $DIR/bare-path.rs:15:5
+   |
+LL |     <T as Tr>::method(..): Send,
+   |     ^^^^^^^^^^^^^^^^^^^^^ not a associated type
+
+warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/bare-path.rs:1:12
+   |
+LL | #![feature(return_type_notation)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:19:23
+   |
+LL |     let _ = T::CONST::(..);
+   |                       ^^^^
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:21:21
+   |
+LL |     let _: T::method(..);
+   |                     ^^^^
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:12:14
+   |
+LL |     T::method(..): Send,
+   |              ^^^^
+
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:15:22
+   |
+LL |     <T as Tr>::method(..): Send,
+   |                      ^^^^
+
+error: expected type, found function
+  --> $DIR/bare-path.rs:12:8
+   |
+LL |     T::method(..): Send,
+   |        ^^^^^^ unexpected function
+   |
+note: the associated function is defined here
+  --> $DIR/bare-path.rs:7:5
+   |
+LL |     fn method() -> impl Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: expected type, found function
+  --> $DIR/bare-path.rs:21:15
+   |
+LL |     let _: T::method(..);
+   |               ^^^^^^ unexpected function
+   |
+note: the associated function is defined here
+  --> $DIR/bare-path.rs:7:5
+   |
+LL |     fn method() -> impl Sized;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 7 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0575`.

tests/ui/associated-type-bounds/return-type-notation/basic.rs

@@ -17,7 +17,7 @@ async fn foo<T: Foo>() -> Result<(), ()> {
 fn is_send(_: impl Send) {}
 
 fn test<
-    #[cfg(with)] T: Foo<method(): Send>,
+    #[cfg(with)] T: Foo<method(..): Send>,
     #[cfg(without)] T: Foo,
 >() {
     is_send(foo::<T>());

tests/ui/associated-type-bounds/return-type-notation/equality.rs

@@ -9,7 +9,7 @@ trait Trait {
     async fn method() {}
 }
 
-fn test<T: Trait<method() = Box<dyn Future<Output = ()>>>>() {}
+fn test<T: Trait<method(..) = Box<dyn Future<Output = ()>>>>() {}
 //~^ ERROR return type notation is not allowed to use type equality
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/equality.stderr

@@ -10,8 +10,8 @@ LL | #![feature(return_type_notation)]
 error: return type notation is not allowed to use type equality
   --> $DIR/equality.rs:12:18
    |
-LL | fn test<T: Trait<method() = Box<dyn Future<Output = ()>>>>() {}
-   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | fn test<T: Trait<method(..) = Box<dyn Future<Output = ()>>>>() {}
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error; 1 warning emitted
 

tests/ui/associated-type-bounds/return-type-notation/issue-120208-higher-ranked-const.rs

@@ -9,7 +9,7 @@ trait HealthCheck {
 
 async fn do_health_check_par<HC>(hc: HC)
 where
-    HC: HealthCheck<check(): Send> + Send + 'static,
+    HC: HealthCheck<check(..): Send> + Send + 'static,
     //~^ ERROR return type notation is not allowed for functions that have const parameters
 {
 }

tests/ui/associated-type-bounds/return-type-notation/issue-120208-higher-ranked-const.stderr

@@ -13,8 +13,8 @@ error: return type notation is not allowed for functions that have const paramet
 LL |     async fn check<const N: usize>() -> bool;
    |                    -------------- const parameter declared here
 ...
-LL |     HC: HealthCheck<check(): Send> + Send + 'static,
-   |                     ^^^^^^^^^^^^^
+LL |     HC: HealthCheck<check(..): Send> + Send + 'static,
+   |                     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error; 1 warning emitted
 

tests/ui/associated-type-bounds/return-type-notation/missing.rs

@@ -7,7 +7,7 @@ trait Trait {
     async fn method() {}
 }
 
-fn bar<T: Trait<methid(): Send>>() {}
+fn bar<T: Trait<methid(..): Send>>() {}
 //~^ ERROR associated function `methid` not found for `Trait`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/missing.stderr

@@ -10,7 +10,7 @@ LL | #![feature(return_type_notation)]
 error[E0220]: associated function `methid` not found for `Trait`
   --> $DIR/missing.rs:10:17
    |
-LL | fn bar<T: Trait<methid(): Send>>() {}
+LL | fn bar<T: Trait<methid(..): Send>>() {}
    |                 ^^^^^^ help: there is an associated function with a similar name: `method`
 
 error: aborting due to 1 previous error; 1 warning emitted

tests/ui/associated-type-bounds/return-type-notation/non-rpitit.rs

@@ -5,7 +5,7 @@ trait Trait {
     fn method() {}
 }
 
-fn test<T: Trait<method(): Send>>() {}
+fn test<T: Trait<method(..): Send>>() {}
 //~^ ERROR  return type notation used on function that is not `async` and does not return `impl Trait`
 
 fn main() {}

tests/ui/associated-type-bounds/return-type-notation/non-rpitit.stderr

@@ -13,8 +13,8 @@ error: return type notation used on function that is not `async` and does not re
 LL |     fn method() {}
    |     ----------- this function must be `async` or return `impl Trait`
 ...
-LL | fn test<T: Trait<method(): Send>>() {}
-   |                  ^^^^^^^^^^^^^^
+LL | fn test<T: Trait<method(..): Send>>() {}
+   |                  ^^^^^^^^^^^^^^^^
    |
    = note: function returns `()`, which is not compatible with associated type return bounds
 

tests/ui/async-await/return-type-notation/issue-110963-early.rs

@@ -9,7 +9,7 @@ trait HealthCheck {
 
 async fn do_health_check_par<HC>(hc: HC)
 where
-    HC: HealthCheck<check(): Send> + Send + 'static,
+    HC: HealthCheck<check(..): Send> + Send + 'static,
 {
     spawn(async move {
         let mut hc = hc;

tests/ui/async-await/return-type-notation/issue-110963-late.rs

@@ -10,7 +10,7 @@ trait HealthCheck {
 
 async fn do_health_check_par<HC>(hc: HC)
 where
-    HC: HealthCheck<check(): Send> + Send + 'static,
+    HC: HealthCheck<check(..): Send> + Send + 'static,
 {
     spawn(async move {
         let mut hc = hc;

tests/ui/async-await/return-type-notation/normalizing-self-auto-trait-issue-109924.rs

@@ -16,7 +16,7 @@ impl Foo for Bar {
     async fn bar(&self) {}
 }
 
-fn build<T>(_: T) where T: Foo<bar(): Send> {}
+fn build<T>(_: T) where T: Foo<bar(..): Send> {}
 
 fn main() {
     build(Bar);

tests/ui/async-await/return-type-notation/rtn-implied-in-supertrait.rs

@@ -16,7 +16,7 @@ trait Foo {
     async fn bar(&self) -> i32;
 }
 
-trait SendFoo: Foo<bar(): Send> + Send {}
+trait SendFoo: Foo<bar(..): Send> + Send {}
 
 fn foobar(foo: impl SendFoo) -> JoinHandle<i32> {
     spawn(async move {

tests/ui/async-await/return-type-notation/rtn-in-impl-signature.rs

@@ -7,7 +7,7 @@ trait Super1<'a> {
     fn bar<'b>() -> bool;
 }
 
-impl Super1<'_, bar(): Send> for () {}
+impl Super1<'_, bar(..): Send> for () {}
 //~^ ERROR associated item constraints are not allowed here
 //~| ERROR not all trait items implemented
 

tests/ui/async-await/return-type-notation/rtn-in-impl-signature.stderr

@@ -10,13 +10,13 @@ LL | #![feature(return_type_notation)]
 error[E0229]: associated item constraints are not allowed here
   --> $DIR/rtn-in-impl-signature.rs:10:17
    |
-LL | impl Super1<'_, bar(): Send> for () {}
-   |                 ^^^^^^^^^^^ associated item constraint not allowed here
+LL | impl Super1<'_, bar(..): Send> for () {}
+   |                 ^^^^^^^^^^^^^ associated item constraint not allowed here
    |
 help: consider removing this associated item constraint
    |
-LL | impl Super1<'_, bar(): Send> for () {}
-   |               ~~~~~~~~~~~~~
+LL | impl Super1<'_, bar(..): Send> for () {}
+   |               ~~~~~~~~~~~~~~~
 
 error[E0046]: not all trait items implemented, missing: `bar`
   --> $DIR/rtn-in-impl-signature.rs:10:1
@@ -24,8 +24,8 @@ error[E0046]: not all trait items implemented, missing: `bar`
 LL |     fn bar<'b>() -> bool;
    |     --------------------- `bar` from trait
 ...
-LL | impl Super1<'_, bar(): Send> for () {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `bar` in implementation
+LL | impl Super1<'_, bar(..): Send> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `bar` in implementation
 
 error: aborting due to 2 previous errors; 1 warning emitted
 

tests/ui/async-await/return-type-notation/super-method-bound-ambig.rs

@@ -22,7 +22,7 @@ impl Foo for () {}
 
 fn test<T>()
 where
-    T: Foo<test(): Send>,
+    T: Foo<test(..): Send>,
     //~^ ERROR ambiguous associated function `test` in bounds of `Foo`
 {
 }

tests/ui/async-await/return-type-notation/super-method-bound-ambig.stderr

@@ -16,8 +16,8 @@ LL |     async fn test();
 LL |     async fn test();
    |     ---------------- ambiguous `test` from `Super2`
 ...
-LL |     T: Foo<test(): Send>,
-   |            ^^^^^^^^^^^^ ambiguous associated function `test`
+LL |     T: Foo<test(..): Send>,
+   |            ^^^^^^^^^^^^^^ ambiguous associated function `test`
 
 error: aborting due to 1 previous error; 1 warning emitted
 

tests/ui/async-await/return-type-notation/super-method-bound.rs

@@ -16,7 +16,7 @@ impl Foo for () {}
 
 fn test<T>()
 where
-    T: Foo<test(): Send>,
+    T: Foo<test(..): Send>,
 {
 }
 

tests/ui/async-await/return-type-notation/supertrait-bound.rs

@@ -6,6 +6,6 @@
 trait IntFactory {
     fn stream(&self) -> impl Iterator<Item = i32>;
 }
-trait SendIntFactory: IntFactory<stream(): Send> + Send {}
+trait SendIntFactory: IntFactory<stream(..): Send> + Send {}
 
 fn main() {}

tests/ui/async-await/return-type-notation/ty-or-ct-params.rs

@@ -11,7 +11,7 @@ async fn baz<const N: usize>() {}
 
 fn test<T>()
 where
-    T: Foo<bar(): Send, baz(): Send>,
+    T: Foo<bar(..): Send, baz(..): Send>,
     //~^ ERROR return type notation is not allowed for functions that have const parameters
     //~| ERROR return type notation is not allowed for functions that have type parameters
 {

tests/ui/async-await/return-type-notation/ty-or-ct-params.stderr

@@ -13,17 +13,17 @@ error: return type notation is not allowed for functions that have type paramete
 LL |     async fn bar<T>() {}
    |                  - type parameter declared here
 ...
-LL |     T: Foo<bar(): Send, baz(): Send>,
-   |            ^^^^^^^^^^^
+LL |     T: Foo<bar(..): Send, baz(..): Send>,
+   |            ^^^^^^^^^^^^^
 
 error: return type notation is not allowed for functions that have const parameters
-  --> $DIR/ty-or-ct-params.rs:14:25
+  --> $DIR/ty-or-ct-params.rs:14:27
    |
 LL |     async fn baz<const N: usize>() {}
    |                  -------------- const parameter declared here
 ...
-LL |     T: Foo<bar(): Send, baz(): Send>,
-   |                         ^^^^^^^^^^^
+LL |     T: Foo<bar(..): Send, baz(..): Send>,
+   |                           ^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors; 1 warning emitted
 

tests/ui/borrowck/alias-liveness/rtn-static.rs

@@ -7,7 +7,7 @@ trait Foo {
     fn borrow(&mut self) -> impl Sized + '_;
 }
 
-fn live_past_borrow<T: Foo<borrow(): 'static>>(mut t: T) {
+fn live_past_borrow<T: Foo<borrow(..): 'static>>(mut t: T) {
     let x = t.borrow();
     drop(t);
     drop(x);
@@ -15,7 +15,7 @@ fn live_past_borrow<T: Foo<borrow(): 'static>>(mut t: T) {
 
 // Test that the `'_` item bound in `borrow` does not cause us to
 // overlook the `'static` RTN bound.
-fn overlapping_mut<T: Foo<borrow(): 'static>>(mut t: T) {
+fn overlapping_mut<T: Foo<borrow(..): 'static>>(mut t: T) {
     let x = t.borrow();
     let x = t.borrow();
 }

tests/ui/feature-gates/feature-gate-return_type_notation.cfg.stderr

@@ -1,33 +1,13 @@
 error[E0658]: return type notation is experimental
-  --> $DIR/feature-gate-return_type_notation.rs:14:17
+  --> $DIR/feature-gate-return_type_notation.rs:10:18
    |
-LL | fn foo<T: Trait<m(): Send>>() {}
-   |                 ^^^^^^^^^
+LL | fn foo<T: Trait<m(..): Send>>() {}
+   |                  ^^^^
    |
    = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
    = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: parenthesized generic arguments cannot be used in associated type constraints
-  --> $DIR/feature-gate-return_type_notation.rs:14:17
-   |
-LL | fn foo<T: Trait<m(): Send>>() {}
-   |                 ^--
-   |                  |
-   |                  help: remove these parentheses
-
-error: expected type, found function
-  --> $DIR/feature-gate-return_type_notation.rs:14:17
-   |
-LL | fn foo<T: Trait<m(): Send>>() {}
-   |                 ^ unexpected function
-   |
-note: the associated function is defined here
-  --> $DIR/feature-gate-return_type_notation.rs:10:5
-   |
-LL |     async fn m();
-   |     ^^^^^^^^^^^^^
-
-error: aborting due to 3 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-return_type_notation.no.stderr

@@ -1,14 +1,13 @@
-warning: return type notation is experimental
-  --> $DIR/feature-gate-return_type_notation.rs:14:17
+error[E0658]: return type notation is experimental
+  --> $DIR/feature-gate-return_type_notation.rs:10:18
    |
-LL | fn foo<T: Trait<m(): Send>>() {}
-   |                 ^^^^^^^^^
+LL | fn foo<T: Trait<m(..): Send>>() {}
+   |                  ^^^^
    |
    = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
    = help: add `#![feature(return_type_notation)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = warning: unstable syntax can change at any point in the future, causing a hard error!
-   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
 
-warning: 1 warning emitted
+error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/feature-gate-return_type_notation.rs

@@ -1,21 +1,13 @@
 //@ edition: 2021
 //@ revisions: cfg no
 
-//@ [no] check-pass
-// Since we're not adding new syntax, `cfg`'d out RTN must pass.
-
-
 trait Trait {
     #[allow(async_fn_in_trait)]
     async fn m();
 }
 
 #[cfg(cfg)]
-fn foo<T: Trait<m(): Send>>() {}
-//[cfg]~^ ERROR return type notation is experimental
-//[cfg]~| ERROR parenthesized generic arguments cannot be used in associated type constraints
-//[cfg]~| ERROR expected type, found function
-//[no]~^^^^ WARN return type notation is experimental
-//[no]~| WARN unstable syntax can change at any point in the future, causing a hard error!
+fn foo<T: Trait<m(..): Send>>() {}
+//~^ ERROR return type notation is experimental
 
 fn main() {}