Improve error message

compiler/rustc_parse/messages.ftl

@@ -53,6 +53,9 @@ parse_bare_cr = {$double_quotes ->
 
 parse_bare_cr_in_raw_string = bare CR not allowed in raw string
 
+parse_binder_before_modifiers = `for<...>` binder should be placed before trait bound modifiers
+    .label = place the `for<...>` binder before any modifiers
+
 parse_bounds_not_allowed_on_trait_aliases = bounds are not allowed on trait aliases
 
 parse_box_not_pat = expected pattern, found {$descr}

compiler/rustc_parse/src/errors.rs

@@ -3041,3 +3041,12 @@ pub struct UnsafeAttrOutsideUnsafeSuggestion {
     #[suggestion_part(code = ")")]
     pub right: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_binder_before_modifiers)]
+pub struct BinderBeforeModifiers {
+    #[primary_span]
+    pub binder_span: Span,
+    #[label]
+    pub modifiers_span: Span,
+}

compiler/rustc_parse/src/parser/ty.rs

@@ -989,7 +989,10 @@ impl<'a> Parser<'a> {
         leading_token: &Token,
     ) -> PResult<'a, GenericBound> {
         let (mut lifetime_defs, binder_span) = self.parse_late_bound_lifetime_defs()?;
+
+        let modifiers_lo = self.token.span;
         let modifiers = self.parse_trait_bound_modifiers()?;
+        let modifiers_span = modifiers_lo.to(self.prev_token.span);
 
         // Recover erroneous lifetime bound with modifiers or binder.
         // e.g. `T: for<'a> 'a` or `T: ~const 'a`.
@@ -998,6 +1001,11 @@ impl<'a> Parser<'a> {
             return self.parse_generic_lt_bound(lo, has_parens);
         }
 
+        if let (more_lifetime_defs, Some(binder_span)) = self.parse_late_bound_lifetime_defs()? {
+            lifetime_defs.extend(more_lifetime_defs);
+            self.dcx().emit_err(errors::BinderBeforeModifiers { binder_span, modifiers_span });
+        }
+
         let mut path = if self.token.is_keyword(kw::Fn)
             && self.look_ahead(1, |tok| tok.kind == TokenKind::OpenDelim(Delimiter::Parenthesis))
             && let Some(path) = self.recover_path_from_fn()