Rollup merge of #110203 - compiler-errors:rtn-dots, r=eholk

Remove `..` from return type notation

`@nikomatsakis` and I decided that using `..` in the return-type notation syntax is probably overkill.

r? `@eholk` since you reviewed the last one

Since this is piggybacking now totally off of a pre-existing syntax (parenthesized generics), let me know if you need any explanation of the logic here, since it's a bit more complicated now.

15 files changed, 57 insertions, 63 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index fb9d71b52a8..df1a716755b 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -167,9 +167,6 @@ pub enum GenericArgs {
     AngleBracketed(AngleBracketedArgs),
     /// The `(A, B)` and `C` in `Foo(A, B) -> C`.
     Parenthesized(ParenthesizedArgs),
-    /// Associated return type bounds, like `T: Trait<method(..): Send>`
-    /// which applies the `Send` bound to the return-type of `method`.
-    ReturnTypeNotation(Span),
 }
 
 impl GenericArgs {
@@ -181,7 +178,6 @@ impl GenericArgs {
         match self {
             AngleBracketed(data) => data.span,
             Parenthesized(data) => data.span,
-            ReturnTypeNotation(span) => *span,
         }
     }
 }
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 2424073ae53..694d31d8f1f 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -561,7 +561,6 @@ pub fn noop_visit_generic_args<T: MutVisitor>(generic_args: &mut GenericArgs, vi
     match generic_args {
         GenericArgs::AngleBracketed(data) => vis.visit_angle_bracketed_parameter_data(data),
         GenericArgs::Parenthesized(data) => vis.visit_parenthesized_parameter_data(data),
-        GenericArgs::ReturnTypeNotation(_span) => {}
     }
 }
 
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index 3b08467fde2..ac9b321b71c 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -482,7 +482,6 @@ where
             walk_list!(visitor, visit_ty, &data.inputs);
             walk_fn_ret_ty(visitor, &data.output);
         }
-        GenericArgs::ReturnTypeNotation(_span) => {}
     }
 }
 
diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
index 59fdeacc7bf..3e9f9b43623 100644
--- a/compiler/rustc_ast_lowering/src/errors.rs
+++ b/compiler/rustc_ast_lowering/src/errors.rs
@@ -353,13 +353,7 @@ pub enum BadReturnTypeNotation {
     #[diag(ast_lowering_bad_return_type_notation_inputs)]
     Inputs {
         #[primary_span]
-        #[suggestion(code = "(..)", applicability = "maybe-incorrect")]
-        span: Span,
-    },
-    #[diag(ast_lowering_bad_return_type_notation_needs_dots)]
-    NeedsDots {
-        #[primary_span]
-        #[suggestion(code = "(..)", applicability = "maybe-incorrect")]
+        #[suggestion(code = "()", applicability = "maybe-incorrect")]
         span: Span,
     },
     #[diag(ast_lowering_bad_return_type_notation_output)]
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index ca659db4dbe..f7ae96b7c4a 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -987,15 +987,22 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 GenericArgs::AngleBracketed(data) => {
                     self.lower_angle_bracketed_parameter_data(data, ParamMode::Explicit, itctx).0
                 }
-                &GenericArgs::ReturnTypeNotation(span) => GenericArgsCtor {
-                    args: Default::default(),
-                    bindings: &[],
-                    parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
-                    span,
-                },
                 GenericArgs::Parenthesized(data) => {
-                    if let Some(start_char) = constraint.ident.as_str().chars().next()
-                        && start_char.is_ascii_lowercase()
+                    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(),
+                            bindings: &[],
+                            parenthesized,
+                            span: data.inputs_span,
+                        }
+                    } else if let Some(first_char) = constraint.ident.as_str().chars().next()
+                        && first_char.is_ascii_lowercase()
                     {
                         let mut err = if !data.inputs.is_empty() {
                             self.tcx.sess.create_err(errors::BadReturnTypeNotation::Inputs {
@@ -1006,9 +1013,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 span: data.inputs_span.shrink_to_hi().to(ty.span),
                             })
                         } else {
-                            self.tcx.sess.create_err(errors::BadReturnTypeNotation::NeedsDots {
-                                span: data.inputs_span,
-                            })
+                            unreachable!("inputs are empty and return type is not provided")
                         };
                         if !self.tcx.features().return_type_notation
                             && self.tcx.sess.is_nightly_build()
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index 1c47a969696..8eb84c036a0 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -13,7 +13,6 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{BytePos, Span, DUMMY_SP};
 
 use smallvec::{smallvec, SmallVec};
-use thin_vec::ThinVec;
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     #[instrument(level = "trace", skip(self))]
@@ -219,18 +218,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         )
                     }
                 },
-                &GenericArgs::ReturnTypeNotation(span) => {
-                    self.tcx.sess.emit_err(GenericTypeWithParentheses { span, sub: None });
-                    (
-                        self.lower_angle_bracketed_parameter_data(
-                            &AngleBracketedArgs { span, args: ThinVec::default() },
-                            param_mode,
-                            itctx,
-                        )
-                        .0,
-                        false,
-                    )
-                }
             }
         } else {
             (
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index e7cdfeca6f9..c79626ccd76 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1080,7 +1080,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     self.with_impl_trait(None, |this| this.visit_ty(ty));
                 }
             }
-            GenericArgs::ReturnTypeNotation(_span) => {}
         }
     }
 
@@ -1391,7 +1390,6 @@ fn deny_equality_constraints(
                                     match &mut assoc_path.segments[len].args {
                                         Some(args) => match args.deref_mut() {
                                             GenericArgs::Parenthesized(_) => continue,
-                                            GenericArgs::ReturnTypeNotation(_span) => continue,
                                             GenericArgs::AngleBracketed(args) => {
                                                 args.args.push(arg);
                                             }
diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 007d64f681f..66d82a26c37 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -485,20 +485,23 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
         if let AssocConstraintKind::Bound { .. } = constraint.kind {
-            if let Some(args) = constraint.gen_args.as_ref()
-                && matches!(
-                    args,
-                    ast::GenericArgs::ReturnTypeNotation(..)
-                )
+            if let Some(ast::GenericArgs::Parenthesized(args)) = constraint.gen_args.as_ref()
+                && args.inputs.is_empty()
+                && matches!(args.output, ast::FnRetTy::Default(..))
             {
-                // RTN is gated below with a `gate_all`.
+                gate_feature_post!(
+                    &self,
+                    return_type_notation,
+                    constraint.span,
+                    "return type notation is experimental"
+                );
             } else {
                 gate_feature_post!(
                     &self,
                     associated_type_bounds,
                     constraint.span,
                     "associated type bounds are unstable"
-                )
+                );
             }
         }
         visit::walk_assoc_constraint(self, constraint)
@@ -589,7 +592,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(yeet_expr, "`do yeet` expression is experimental");
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
-    gate_all!(return_type_notation, "return type notation is experimental");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).
@@ -605,6 +607,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
 
     gate_all!(trait_alias, "trait aliases are experimental");
     gate_all!(associated_type_bounds, "associated type bounds are unstable");
+    gate_all!(return_type_notation, "return type notation is experimental");
     gate_all!(decl_macro, "`macro` is experimental");
     gate_all!(box_patterns, "box pattern syntax is experimental");
     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index 1f6838a0278..849336c8669 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -936,10 +936,6 @@ impl<'a> PrintState<'a> for State<'a> {
                 self.word(")");
                 self.print_fn_ret_ty(&data.output);
             }
-
-            ast::GenericArgs::ReturnTypeNotation(_span) => {
-                self.word("(..)");
-            }
         }
     }
 }
diff --git a/compiler/rustc_parse/messages.ftl b/compiler/rustc_parse/messages.ftl
index e21bbd0217b..f11d0ed0f01 100644
--- a/compiler/rustc_parse/messages.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -738,3 +738,7 @@ parse_box_syntax_removed = `box_syntax` has been removed
 parse_bad_return_type_notation_output =
     return type not allowed with return type notation
     .suggestion = remove the return type
+
+parse_bad_return_type_notation_dotdot =
+    return type notation uses `()` instead of `(..)` for elided arguments
+    .suggestion = remove the `..`
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index aead216b61c..069217165fa 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -2324,3 +2324,11 @@ pub(crate) struct BadReturnTypeNotationOutput {
     #[suggestion(code = "", applicability = "maybe-incorrect")]
     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,
+}
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index f1c9f0109f8..c25c23d849f 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -290,16 +290,17 @@ impl<'a> Parser<'a> {
                     })?;
                     let span = lo.to(self.prev_token.span);
                     AngleBracketedArgs { args, span }.into()
-                } else if self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
+                } else if self.may_recover()
+                    && self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
                     // FIXME(return_type_notation): Could also recover `...` here.
                     && self.look_ahead(1, |tok| tok.kind == token::DotDot)
                 {
-                    let lo = self.token.span;
                     self.bump();
+                    self.sess
+                        .emit_err(errors::BadReturnTypeNotationDotDot { span: self.token.span });
                     self.bump();
                     self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
                     let span = lo.to(self.prev_token.span);
-                    self.sess.gated_spans.gate(sym::return_type_notation, span);
 
                     if self.eat_noexpect(&token::RArrow) {
                         let lo = self.prev_token.span;
@@ -308,7 +309,13 @@ impl<'a> Parser<'a> {
                             .emit_err(errors::BadReturnTypeNotationOutput { span: lo.to(ty.span) });
                     }
 
-                    P(GenericArgs::ReturnTypeNotation(span))
+                    ParenthesizedArgs {
+                        span,
+                        inputs: ThinVec::new(),
+                        inputs_span: span,
+                        output: ast::FnRetTy::Default(self.prev_token.span.shrink_to_hi()),
+                    }
+                    .into()
                 } else {
                     // `(T, U) -> R`
                     let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
@@ -566,13 +573,13 @@ impl<'a> Parser<'a> {
                     };
 
                     let span = lo.to(self.prev_token.span);
-
                     // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
                     if let AssocConstraintKind::Bound { .. } = kind {
-                        if gen_args.as_ref().map_or(false, |args| {
-                            matches!(args, GenericArgs::ReturnTypeNotation(..))
-                        }) {
-                            // This is already gated in `parse_path_segment`
+                        if let Some(ast::GenericArgs::Parenthesized(args)) = &gen_args
+                            && args.inputs.is_empty()
+                            && matches!(args.output, ast::FnRetTy::Default(..))
+                        {
+                            self.sess.gated_spans.gate(sym::return_type_notation, span);
                         } else {
                             self.sess.gated_spans.gate(sym::associated_type_bounds, span);
                         }
diff --git a/compiler/rustc_passes/src/hir_stats.rs b/compiler/rustc_passes/src/hir_stats.rs
index ce44f709f3b..47e032758f2 100644
--- a/compiler/rustc_passes/src/hir_stats.rs
+++ b/compiler/rustc_passes/src/hir_stats.rs
@@ -666,7 +666,7 @@ impl<'v> ast_visit::Visitor<'v> for StatCollector<'v> {
     fn visit_generic_args(&mut self, g: &'v ast::GenericArgs) {
         record_variants!(
             (self, g, g, Id::None, ast, GenericArgs, GenericArgs),
-            [AngleBracketed, Parenthesized, ReturnTypeNotation]
+            [AngleBracketed, Parenthesized]
         );
         ast_visit::walk_generic_args(self, g)
     }
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 47d688e48fe..90a2fa89cd2 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -1116,7 +1116,6 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                         }
                     }
                 }
-                GenericArgs::ReturnTypeNotation(_span) => {}
             }
         }
     }
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 274155b19aa..572b0c4cf64 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -312,7 +312,6 @@ impl<'a> From<&'a ast::PathSegment> for Segment {
                     (args.span, found_lifetimes)
                 }
                 GenericArgs::Parenthesized(args) => (args.span, true),
-                GenericArgs::ReturnTypeNotation(span) => (*span, false),
             }
         } else {
             (DUMMY_SP, false)