don't include trailing open parens in labels for reference patterns

(cherry picked from commit a064e78)

Author: dianne

compiler/rustc_hir_typeck/src/pat.rs

@@ -2639,7 +2639,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // If the subpattern's span is is from an expansion, the emitted label will not be trimmed.
         let source_map = self.tcx.sess.source_map();
         let cutoff_span = source_map
-            .span_extend_prev_while(cutoff_span, char::is_whitespace)
+            .span_extend_prev_while(cutoff_span, |c| c.is_whitespace() || c == '(')
             .unwrap_or(cutoff_span);
         // Ensure we use the syntax context and thus edition of `subpat.span`; this will be a hard
         // error if the subpattern is of edition >= 2024.

tests/ui/pattern/rfc-3627-match-ergonomics-2024/migration_lint.stderr

@@ -284,7 +284,7 @@ error: reference patterns may only be written when the default binding mode is `
   --> $DIR/migration_lint.rs:150:10
    |
 LL |     let [&(_)] = &[&0];
-   |         -^^---
+   |         -^----
    |         ||
    |         |this reference pattern
    |         default binding mode is `ref`