Conditionally allow lowering RTN (..) in paths

compiler/rustc_ast_lowering/src/asm.rs

@@ -20,8 +20,8 @@ use super::errors::{
 };
 use super::LoweringContext;
 use crate::{
-    fluent_generated as fluent, ImplTraitContext, ImplTraitPosition, ParamMode,
+    fluent_generated as fluent, AllowReturnTypeNotation, ImplTraitContext, ImplTraitPosition,
-    ResolverAstLoweringExt,
+    ParamMode, ResolverAstLoweringExt,
 };
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
@@ -201,6 +201,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 &sym.qself,
                                 &sym.path,
                                 ParamMode::Optional,
+                                AllowReturnTypeNotation::No,
                                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                                 None,
                             );

compiler/rustc_ast_lowering/src/delegation.rs

@@ -52,7 +52,7 @@ use rustc_target::spec::abi;
 use {rustc_ast as ast, rustc_hir as hir};
 
 use super::{GenericArgsMode, ImplTraitContext, LoweringContext, ParamMode};
-use crate::{ImplTraitPosition, ResolverAstLoweringExt};
+use crate::{AllowReturnTypeNotation, ImplTraitPosition, ResolverAstLoweringExt};
 
 pub(crate) struct DelegationResults<'hir> {
     pub body_id: hir::BodyId,
@@ -340,6 +340,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 &delegation.qself,
                 &delegation.path,
                 ParamMode::Optional,
+                AllowReturnTypeNotation::No,
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 None,
             );

compiler/rustc_ast_lowering/src/expr.rs

@@ -23,7 +23,7 @@ use super::{
     GenericArgsMode, ImplTraitContext, LoweringContext, ParamMode, ResolverAstLoweringExt,
 };
 use crate::errors::YieldInClosure;
-use crate::{fluent_generated, FnDeclKind, ImplTraitPosition};
+use crate::{fluent_generated, AllowReturnTypeNotation, FnDeclKind, ImplTraitPosition};
 
 impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
@@ -281,6 +281,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
+                        AllowReturnTypeNotation::No,
                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         None,
                     );
@@ -328,6 +329,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             &se.qself,
                             &se.path,
                             ParamMode::Optional,
+                            AllowReturnTypeNotation::No,
                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                             None,
                         )),
@@ -1291,6 +1293,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
+                        AllowReturnTypeNotation::No,
                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         None,
                     );
@@ -1311,6 +1314,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         qself,
                         path,
                         ParamMode::Optional,
+                        AllowReturnTypeNotation::No,
                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         None,
                     );
@@ -1336,6 +1340,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &se.qself,
                     &se.path,
                     ParamMode::Optional,
+                    AllowReturnTypeNotation::No,
                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     None,
                 );

compiler/rustc_ast_lowering/src/lib.rs

@@ -485,8 +485,16 @@ enum ParamMode {
     Optional,
 }
 
+#[derive(Copy, Clone, Debug)]
+enum AllowReturnTypeNotation {
+    Yes,
+    No,
+}
+
+#[derive(Copy, Clone, Debug)]
 enum GenericArgsMode {
     ParenSugar,
+    ReturnTypeNotation,
     Err,
 }
 
@@ -1226,7 +1234,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
 
         let id = self.lower_node_id(t.id);
-        let qpath = self.lower_qpath(t.id, qself, path, param_mode, itctx, None);
+        let qpath = self.lower_qpath(
+            t.id,
+            qself,
+            path,
+            param_mode,
+            // We deny these after the fact in HIR->middle type lowering.
+            AllowReturnTypeNotation::Yes,
+            itctx,
+            None,
+        );
         self.ty_path(id, t.span, qpath)
     }
 
@@ -2203,6 +2220,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             &None,
             &p.path,
             ParamMode::Explicit,
+            AllowReturnTypeNotation::No,
             itctx,
             Some(modifiers),
         ) {
@@ -2341,6 +2359,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     &None,
                     path,
                     ParamMode::Optional,
+                    AllowReturnTypeNotation::No,
                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                     None,
                 );
@@ -2419,6 +2438,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 qself,
                 path,
                 ParamMode::Optional,
+                AllowReturnTypeNotation::No,
                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                 None,
             );

compiler/rustc_ast_lowering/src/pat.rs

@@ -11,7 +11,7 @@ use super::errors::{
     ArbitraryExpressionInPattern, ExtraDoubleDot, MisplacedDoubleDot, SubTupleBinding,
 };
 use super::{ImplTraitContext, LoweringContext, ParamMode, ResolverAstLoweringExt};
-use crate::ImplTraitPosition;
+use crate::{AllowReturnTypeNotation, ImplTraitPosition};
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     pub(crate) fn lower_pat(&mut self, pattern: &Pat) -> &'hir hir::Pat<'hir> {
@@ -38,6 +38,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
+                            AllowReturnTypeNotation::No,
                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                             None,
                         );
@@ -55,6 +56,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
+                            AllowReturnTypeNotation::No,
                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                             None,
                         );
@@ -66,6 +68,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
+                            AllowReturnTypeNotation::No,
                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                             None,
                         );

compiler/rustc_ast_lowering/src/path.rs

@@ -5,6 +5,7 @@ use rustc_hir::def::{DefKind, PartialRes, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::GenericArg;
 use rustc_middle::span_bug;
+use rustc_session::parse::add_feature_diagnostics;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{BytePos, DesugaringKind, Span, Symbol, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
@@ -15,8 +16,8 @@ use super::errors::{
     GenericTypeWithParentheses, UseAngleBrackets,
 };
 use super::{
-    GenericArgsCtor, GenericArgsMode, ImplTraitContext, LifetimeRes, LoweringContext, ParamMode,
+    AllowReturnTypeNotation, GenericArgsCtor, GenericArgsMode, ImplTraitContext, LifetimeRes,
-    ResolverAstLoweringExt,
+    LoweringContext, ParamMode, ResolverAstLoweringExt,
 };
 use crate::ImplTraitPosition;
 
@@ -28,6 +29,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         qself: &Option<ptr::P<QSelf>>,
         p: &Path,
         param_mode: ParamMode,
+        allow_return_type_notation: AllowReturnTypeNotation,
         itctx: ImplTraitContext,
         // modifiers of the impl/bound if this is a trait path
         modifiers: Option<ast::TraitBoundModifiers>,
@@ -103,6 +105,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         {
                             GenericArgsMode::ParenSugar
                         }
+                        Res::Def(DefKind::AssocFn, _) if i + 1 == proj_start => {
+                            match allow_return_type_notation {
+                                AllowReturnTypeNotation::Yes => GenericArgsMode::ReturnTypeNotation,
+                                AllowReturnTypeNotation::No => GenericArgsMode::Err,
+                            }
+                        }
                         // Avoid duplicated errors.
                         Res::Err => GenericArgsMode::ParenSugar,
                         // An error
@@ -164,11 +172,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         //   3. `<<std::vec::Vec<T>>::IntoIter>::Item`
         // * final path is `<<<std::vec::Vec<T>>::IntoIter>::Item>::clone`
         for (i, segment) in p.segments.iter().enumerate().skip(proj_start) {
+            // If this is a type-dependent `T::method(..)`.
+            let generic_args_mode = if i + 1 == p.segments.len()
+                && matches!(allow_return_type_notation, AllowReturnTypeNotation::Yes)
+            {
+                GenericArgsMode::ReturnTypeNotation
+            } else {
+                GenericArgsMode::Err
+            };
+
             let hir_segment = self.arena.alloc(self.lower_path_segment(
                 p.span,
                 segment,
                 param_mode,
-                GenericArgsMode::Err,
+                generic_args_mode,
                 itctx,
                 None,
             ));
@@ -238,6 +255,40 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
                 }
                 GenericArgs::Parenthesized(data) => match generic_args_mode {
+                    GenericArgsMode::ReturnTypeNotation => {
+                        let mut err = if !data.inputs.is_empty() {
+                            self.dcx().create_err(BadReturnTypeNotation::Inputs {
+                                span: data.inputs_span,
+                            })
+                        } else if let FnRetTy::Ty(ty) = &data.output {
+                            self.dcx().create_err(BadReturnTypeNotation::Output {
+                                span: data.inputs_span.shrink_to_hi().to(ty.span),
+                            })
+                        } else {
+                            self.dcx().create_err(BadReturnTypeNotation::NeedsDots {
+                                span: data.inputs_span,
+                            })
+                        };
+                        if !self.tcx.features().return_type_notation
+                            && self.tcx.sess.is_nightly_build()
+                        {
+                            add_feature_diagnostics(
+                                &mut err,
+                                &self.tcx.sess,
+                                sym::return_type_notation,
+                            );
+                        }
+                        err.emit();
+                        (
+                            GenericArgsCtor {
+                                args: Default::default(),
+                                constraints: &[],
+                                parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
+                                span: path_span,
+                            },
+                            false,
+                        )
+                    }
                     GenericArgsMode::ParenSugar => self.lower_parenthesized_parameter_data(
                         data,
                         itctx,
@@ -279,7 +330,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     }
                 },
                 GenericArgs::ParenthesizedElided(span) => {
+                    match generic_args_mode {
+                        GenericArgsMode::ReturnTypeNotation => {
+                            // Ok
+                        }
+                        GenericArgsMode::ParenSugar | GenericArgsMode::Err => {
                             self.dcx().emit_err(BadReturnTypeNotation::Position { span: *span });
+                        }
+                    }
                     (
                         GenericArgsCtor {
                             args: Default::default(),

compiler/rustc_hir_analysis/messages.ftl

@@ -48,6 +48,8 @@ hir_analysis_auto_deref_reached_recursion_limit = reached the recursion limit wh
 
 hir_analysis_bad_precise_capture = expected {$kind} parameter in `use<...>` precise captures list, found {$found}
 
+hir_analysis_bad_return_type_notation_position = return type notation not allowed in this position yet
+
 hir_analysis_cannot_capture_late_bound_const =
     cannot capture late-bound const parameter in {$what}
     .label = parameter defined here

compiler/rustc_hir_analysis/src/errors.rs

@@ -1693,3 +1693,10 @@ pub(crate) struct CmseCallGeneric {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_bad_return_type_notation_position)]
+pub(crate) struct BadReturnTypeNotation {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -53,7 +53,7 @@ use rustc_trait_selection::traits::{self, ObligationCtxt};
 use tracing::{debug, debug_span, instrument};
 
 use crate::bounds::Bounds;
-use crate::errors::{AmbiguousLifetimeBound, WildPatTy};
+use crate::errors::{AmbiguousLifetimeBound, BadReturnTypeNotation, WildPatTy};
 use crate::hir_ty_lowering::errors::{prohibit_assoc_item_constraint, GenericsArgsErrExtend};
 use crate::hir_ty_lowering::generics::{check_generic_arg_count, lower_generic_args};
 use crate::middle::resolve_bound_vars as rbv;
@@ -1910,6 +1910,19 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     ty::BoundConstness::NotConst,
                 )
             }
+            // Deny any qpath types that were successfully lowered in AST lowering.
+            Res::Def(DefKind::AssocFn, _)
+                if let [.., _trait_segment, item_segment] = &path.segments[..]
+                    && item_segment.args.is_some_and(|args| {
+                        matches!(
+                            args.parenthesized,
+                            hir::GenericArgsParentheses::ReturnTypeNotation
+                        )
+                    }) =>
+            {
+                let guar = self.dcx().emit_err(BadReturnTypeNotation { span: path.span });
+                Ty::new_error(tcx, guar)
+            }
             Res::PrimTy(prim_ty) => {
                 assert_eq!(opt_self_ty, None);
                 let _ = self.prohibit_generic_args(

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

@@ -10,17 +10,14 @@ trait Tr {
 fn foo<T: Tr>()
 where
     T::method(..): Send,
-    //~^ ERROR return type notation not allowed in this position yet
+    //~^ ERROR expected type, found function
-    //~| 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
-    //~| ERROR expected type, found function
 }
 
 fn main() {}

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

@@ -1,9 +1,3 @@
-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
    |
@@ -14,29 +8,11 @@ LL | #![feature(return_type_notation)]
    = note: `#[warn(incomplete_features)]` on by default
 
 error: return type notation not allowed in this position yet
-  --> $DIR/bare-path.rs:19:23
+  --> $DIR/bare-path.rs:17:23
    |
 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
    |
@@ -49,8 +25,14 @@ note: the associated function is defined here
 LL |     fn method() -> impl Sized;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+error: return type notation not allowed in this position yet
+  --> $DIR/bare-path.rs:14:5
+   |
+LL |     <T as Tr>::method(..): Send,
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
 error: expected type, found function
-  --> $DIR/bare-path.rs:21:15
+  --> $DIR/bare-path.rs:19:15
    |
 LL |     let _: T::method(..);
    |               ^^^^^^ unexpected function
@@ -61,6 +43,5 @@ note: the associated function is defined here
 LL |     fn method() -> impl Sized;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 7 previous errors; 1 warning emitted
+error: aborting due to 4 previous errors; 1 warning emitted
 
-For more information about this error, try `rustc --explain E0575`.

tests/ui/suggestions/let-binding-init-expr-as-ty.rs

@@ -1,7 +1,7 @@
 pub fn foo(num: i32) -> i32 {
     let foo: i32::from_be(num);
     //~^ ERROR expected type, found local variable `num`
-    //~| ERROR parenthesized type parameters may only be used with a `Fn` trait
+    //~| ERROR argument types not allowed with return type notation
     //~| ERROR ambiguous associated type
     foo
 }

tests/ui/suggestions/let-binding-init-expr-as-ty.stderr

@@ -6,16 +6,15 @@ LL |     let foo: i32::from_be(num);
    |            |
    |            help: use `=` if you meant to assign
 
-error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
+error: argument types not allowed with return type notation
-  --> $DIR/let-binding-init-expr-as-ty.rs:2:19
+  --> $DIR/let-binding-init-expr-as-ty.rs:2:26
    |
 LL |     let foo: i32::from_be(num);
-   |                   ^^^^^^^^^^^^ only `Fn` traits may use parentheses
+   |                          ^^^^^ help: remove the input types: `()`
    |
-help: use angle brackets instead
+   = 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
-LL |     let foo: i32::from_be<num>;
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   |                          ~   ~
 
 error[E0223]: ambiguous associated type
   --> $DIR/let-binding-init-expr-as-ty.rs:2:14
@@ -30,5 +29,5 @@ LL |     let foo: <i32 as Example>::from_be;
 
 error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0214, E0223, E0573.
+Some errors have detailed explanations: E0223, E0573.
-For more information about an error, try `rustc --explain E0214`.
+For more information about an error, try `rustc --explain E0223`.