Enforce that ? and for<...> are not combined

compiler/rustc_parse/src/errors.rs

@@ -3050,3 +3050,13 @@ pub struct BinderBeforeModifiers {
     #[label]
     pub modifiers_span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_binder_and_polarity)]
+pub struct BinderAndPolarity {
+    #[primary_span]
+    pub polarity_span: Span,
+    #[label]
+    pub binder_span: Span,
+    pub polarity: &'static str,
+}

compiler/rustc_parse/src/parser/ty.rs

@@ -994,6 +994,19 @@ impl<'a> Parser<'a> {
         let modifiers = self.parse_trait_bound_modifiers()?;
         let modifiers_span = modifiers_lo.to(self.prev_token.span);
 
+        if let Some(binder_span) = binder_span {
+            match modifiers.polarity {
+                BoundPolarity::Negative(polarity_span) | BoundPolarity::Maybe(polarity_span) => {
+                    self.dcx().emit_err(errors::BinderAndPolarity {
+                        binder_span,
+                        polarity_span,
+                        polarity: modifiers.polarity.as_str(),
+                    });
+                }
+                BoundPolarity::Positive => {}
+            }
+        }
+
         // Recover erroneous lifetime bound with modifiers or binder.
         // e.g. `T: for<'a> 'a` or `T: ~const 'a`.
         if self.token.is_lifetime() {