Emit an error when RTN is used in an incorrect place

We miss one place: associated type bindings aka. `impl Trait<Type(..): Send>`, but we also miss it for Fn-style parenthesizes error so I left it out for now.

crates/hir-ty/src/lower/path.rs

@@ -607,8 +607,14 @@ impl<'a, 'b> PathLoweringContext<'a, 'b> {
     ) -> Substitution {
         let prohibit_parens = match def {
             GenericDefId::TraitId(trait_) => {
-                let trait_data = self.ctx.db.trait_data(trait_);
-                !trait_data.flags.contains(TraitFlags::RUSTC_PAREN_SUGAR)
+                // RTN is prohibited anyways if we got here.
+                let is_rtn =
+                    self.current_or_prev_segment.args_and_bindings.is_some_and(|generics| {
+                        generics.parenthesized == GenericArgsParentheses::ReturnTypeNotation
+                    });
+                let is_fn_trait =
+                    !self.ctx.db.trait_data(trait_).flags.contains(TraitFlags::RUSTC_PAREN_SUGAR);
+                is_rtn || is_fn_trait
             }
             _ => true,
         };

crates/hir/src/diagnostics.rs

@@ -113,6 +113,7 @@ diagnostics![
     UnusedVariable,
     GenericArgsProhibited,
     ParenthesizedGenericArgsWithoutFnTrait,
+    BadRtn,
 ];
 
 #[derive(Debug)]
@@ -420,6 +421,11 @@ pub struct ParenthesizedGenericArgsWithoutFnTrait {
     pub args: InFile<AstPtr<ast::ParenthesizedArgList>>,
 }
 
+#[derive(Debug)]
+pub struct BadRtn {
+    pub rtn: InFile<AstPtr<ast::ReturnTypeSyntax>>,
+}
+
 impl AnyDiagnostic {
     pub(crate) fn body_validation_diagnostic(
         db: &dyn HirDatabase,
@@ -712,6 +718,12 @@ impl AnyDiagnostic {
         Some(match *diag {
             PathLoweringDiagnostic::GenericArgsProhibited { segment, reason } => {
                 let segment = hir_segment_to_ast_segment(&path.value, segment)?;
+
+                if let Some(rtn) = segment.return_type_syntax() {
+                    // RTN errors are emitted as `GenericArgsProhibited` or `ParenthesizedGenericArgsWithoutFnTrait`.
+                    return Some(BadRtn { rtn: path.with_value(AstPtr::new(&rtn)) }.into());
+                }
+
                 let args = if let Some(generics) = segment.generic_arg_list() {
                     AstPtr::new(&generics).wrap_left()
                 } else {
@@ -722,6 +734,12 @@ impl AnyDiagnostic {
             }
             PathLoweringDiagnostic::ParenthesizedGenericArgsWithoutFnTrait { segment } => {
                 let segment = hir_segment_to_ast_segment(&path.value, segment)?;
+
+                if let Some(rtn) = segment.return_type_syntax() {
+                    // RTN errors are emitted as `GenericArgsProhibited` or `ParenthesizedGenericArgsWithoutFnTrait`.
+                    return Some(BadRtn { rtn: path.with_value(AstPtr::new(&rtn)) }.into());
+                }
+
                 let args = AstPtr::new(&segment.parenthesized_arg_list()?);
                 let args = path.with_value(args);
                 ParenthesizedGenericArgsWithoutFnTrait { args }.into()

crates/ide-diagnostics/src/handlers/bad_rtn.rs

@@ -0,0 +1,52 @@
+use ide_db::Severity;
+
+use crate::{Diagnostic, DiagnosticCode, DiagnosticsContext};
+
+// Diagnostic: bad-rtn
+//
+// This diagnostic is shown when a RTN (Return Type Notation, `Type::method(..): Send`) is written in an improper place.
+pub(crate) fn bad_rtn(ctx: &DiagnosticsContext<'_>, d: &hir::BadRtn) -> Diagnostic {
+    Diagnostic::new_with_syntax_node_ptr(
+        ctx,
+        DiagnosticCode::Ra("bad-rtn", Severity::Error),
+        "return type notation not allowed in this position yet",
+        d.rtn.map(Into::into),
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::check_diagnostics;
+
+    #[test]
+    fn fn_traits_also_emit() {
+        check_diagnostics(
+            r#"
+//- minicore: fn
+fn foo<
+    A: Fn(..),
+      // ^^^^ error: return type notation not allowed in this position yet
+>() {}
+        "#,
+        );
+    }
+
+    #[test]
+    fn bad_rtn() {
+        check_diagnostics(
+            r#"
+mod module {
+    pub struct Type;
+}
+trait Trait {}
+
+fn foo()
+where
+    module(..)::Type: Trait
+       // ^^^^ error: return type notation not allowed in this position yet
+{
+}
+        "#,
+        );
+    }
+}

crates/ide-diagnostics/src/lib.rs

@@ -25,6 +25,7 @@
 
 mod handlers {
     pub(crate) mod await_outside_of_async;
+    pub(crate) mod bad_rtn;
     pub(crate) mod break_outside_of_loop;
     pub(crate) mod expected_function;
     pub(crate) mod generic_args_prohibited;
@@ -493,6 +494,7 @@ pub fn semantic_diagnostics(
             AnyDiagnostic::ParenthesizedGenericArgsWithoutFnTrait(d) => {
                 handlers::parenthesized_generic_args_without_fn_trait::parenthesized_generic_args_without_fn_trait(&ctx, &d)
             }
+            AnyDiagnostic::BadRtn(d) => handlers::bad_rtn::bad_rtn(&ctx, &d),
         };
         res.push(d)
     }