diagnostics: correct generic bounds with doubled colon

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -2369,6 +2369,34 @@ impl<'a> Parser<'a> {
         Err(err)
     }
 
+    crate fn maybe_recover_bounds_doubled_colon(&mut self, ty: &Ty) -> PResult<'a, ()> {
+        let TyKind::Path(qself, path) = &ty.kind else { return Ok(()) };
+        let qself_position = qself.as_ref().map(|qself| qself.position);
+        for (i, segments) in path.segments.windows(2).enumerate() {
+            if qself_position.map(|pos| i < pos).unwrap_or(false) {
+                continue;
+            }
+            if let [a, b] = segments {
+                let (a_span, b_span) = (a.span(), b.span());
+                let between_span = a_span.shrink_to_hi().to(b_span.shrink_to_lo());
+                if self.span_to_snippet(between_span).as_ref().map(|a| &a[..]) == Ok(":: ") {
+                    let mut err = self.struct_span_err(
+                        path.span.shrink_to_hi(),
+                        "expected `:` followed by trait or lifetime",
+                    );
+                    err.span_suggestion(
+                        between_span,
+                        "use single colon",
+                        ": ".to_owned(),
+                        Applicability::MachineApplicable,
+                    );
+                    return Err(err);
+                }
+            }
+        }
+        Ok(())
+    }
+
     /// Parse and throw away a parenthesized comma separated
     /// sequence of patterns until `)` is reached.
     fn skip_pat_list(&mut self) -> PResult<'a, ()> {

compiler/rustc_parse/src/parser/generics.rs

@@ -312,6 +312,7 @@ impl<'a> Parser<'a> {
                 id: ast::DUMMY_NODE_ID,
             }))
         } else {
+            self.maybe_recover_bounds_doubled_colon(&ty)?;
             self.unexpected()
         }
     }

src/test/ui/generics/issue-95208-ignore-qself.fixed

@@ -0,0 +1,11 @@
+// run-rustfix
+
+#[allow(unused)]
+struct Struct<T>(T);
+
+impl<T: Iterator> Struct<T> where <T as std:: iter::Iterator>::Item: std::fmt::Display {
+//~^ ERROR expected `:` followed by trait or lifetime
+//~| HELP use single colon
+}
+
+fn main() {}

src/test/ui/generics/issue-95208-ignore-qself.rs

@@ -0,0 +1,11 @@
+// run-rustfix
+
+#[allow(unused)]
+struct Struct<T>(T);
+
+impl<T: Iterator> Struct<T> where <T as std:: iter::Iterator>::Item:: std::fmt::Display {
+//~^ ERROR expected `:` followed by trait or lifetime
+//~| HELP use single colon
+}
+
+fn main() {}

src/test/ui/generics/issue-95208-ignore-qself.stderr

@@ -0,0 +1,10 @@
+error: expected `:` followed by trait or lifetime
+  --> $DIR/issue-95208-ignore-qself.rs:6:88
+   |
+LL | impl<T: Iterator> Struct<T> where <T as std:: iter::Iterator>::Item:: std::fmt::Display {
+   |                                                                    ---                 ^
+   |                                                                    |
+   |                                                                    help: use single colon: `:`
+
+error: aborting due to previous error
+

src/test/ui/generics/issue-95208.fixed

@@ -0,0 +1,11 @@
+// run-rustfix
+
+#[allow(unused)]
+struct Struct<T>(T);
+
+impl<T> Struct<T> where T: std::fmt::Display {
+//~^ ERROR expected `:` followed by trait or lifetime
+//~| HELP use single colon
+}
+
+fn main() {}

src/test/ui/generics/issue-95208.rs

@@ -0,0 +1,11 @@
+// run-rustfix
+
+#[allow(unused)]
+struct Struct<T>(T);
+
+impl<T> Struct<T> where T:: std::fmt::Display {
+//~^ ERROR expected `:` followed by trait or lifetime
+//~| HELP use single colon
+}
+
+fn main() {}

src/test/ui/generics/issue-95208.stderr

@@ -0,0 +1,10 @@
+error: expected `:` followed by trait or lifetime
+  --> $DIR/issue-95208.rs:6:46
+   |
+LL | impl<T> Struct<T> where T:: std::fmt::Display {
+   |                          ---                 ^
+   |                          |
+   |                          help: use single colon: `:`
+
+error: aborting due to previous error
+