Rollup merge of #127350 - veera-sivarajan:bugfix-126311, r=lcnr

Parser: Suggest Placing the Return Type After Function Parameters

Fixes #126311

This PR suggests placing the return type after the function parameters when it's misplaced after a `where` clause.

This also tangentially improves diagnostics for cases like [this](https://github.com/veera-sivarajan/rust/blob/86d6f1312a77997ef994240e716288d61a343a6d/tests/ui/parser/issues/misplaced-return-type-without-where-issue-126311.rs#L1C1-L1C28) and adds doc comments for `parser::AllowPlus`.

1 files changed, 15 insertions, 7 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index da0701efddb..109d36fe689 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -1503,6 +1503,20 @@ pub(crate) struct FnPtrWithGenerics {
 }
 
 #[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    parse_misplaced_return_type,
+    style = "verbose",
+    applicability = "maybe-incorrect"
+)]
+pub(crate) struct MisplacedReturnType {
+    #[suggestion_part(code = " {snippet}")]
+    pub fn_params_end: Span,
+    pub snippet: String,
+    #[suggestion_part(code = "")]
+    pub ret_ty_span: Span,
+}
+
+#[derive(Subdiagnostic)]
 #[multipart_suggestion(parse_suggestion, applicability = "maybe-incorrect")]
 pub(crate) struct FnPtrWithGenericsSugg {
     #[suggestion_part(code = "{snippet}")]
@@ -1516,7 +1530,6 @@ pub(crate) struct FnPtrWithGenericsSugg {
 
 pub(crate) struct FnTraitMissingParen {
     pub span: Span,
-    pub machine_applicable: bool,
 }
 
 impl Subdiagnostic for FnTraitMissingParen {
@@ -1526,16 +1539,11 @@ impl Subdiagnostic for FnTraitMissingParen {
         _: &F,
     ) {
         diag.span_label(self.span, crate::fluent_generated::parse_fn_trait_missing_paren);
-        let applicability = if self.machine_applicable {
-            Applicability::MachineApplicable
-        } else {
-            Applicability::MaybeIncorrect
-        };
         diag.span_suggestion_short(
             self.span.shrink_to_hi(),
             crate::fluent_generated::parse_add_paren,
             "()",
-            applicability,
+            Applicability::MachineApplicable,
         );
     }
 }