Pluralize error messages.

This commit pluralizes error messages when more than a single trailing
`>` character is present.

Author: davidtwco

src/libsyntax/parse/parser.rs

@@ -2869,11 +2869,18 @@ impl<'a> Parser<'a> {
             self.eat_to_tokens(&[&token::OpenDelim(token::Paren)]);
             let span = lo.until(self.span);
 
+            // We needn't check `encountered_gt` to determine if we should pluralize "bracket".
+            // `encountered_gt` can only represent a single `>` character, if `number_of_shr >= 1`
+            // then there is either `>>` or `>>>` - in either case a plural is warranted.
+            let plural = number_of_shr >= 1;
             self.diagnostic()
-                .struct_span_err(span, "unmatched angle bracket")
+                .struct_span_err(
+                    span,
+                    &format!("unmatched angle bracket{}", if plural { "s" } else { "" }),
+                )
                 .span_suggestion_with_applicability(
                     span,
-                    "remove extra angle bracket",
+                    &format!("remove extra angle bracket{}", if plural { "s" } else { "" }),
                     String::new(),
                     Applicability::MachineApplicable,
                 )

src/test/ui/issues/issue-54521.stderr

@@ -1,20 +1,20 @@
-error: unmatched angle bracket
+error: unmatched angle brackets
   --> $DIR/issue-54521.rs:11:60
    |
 LL |     let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>>>>>();
-   |                                                            ^^^^ help: remove extra angle bracket
+   |                                                            ^^^^ help: remove extra angle brackets
 
-error: unmatched angle bracket
+error: unmatched angle brackets
   --> $DIR/issue-54521.rs:14:60
    |
 LL |     let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>>>>();
-   |                                                            ^^^ help: remove extra angle bracket
+   |                                                            ^^^ help: remove extra angle brackets
 
-error: unmatched angle bracket
+error: unmatched angle brackets
   --> $DIR/issue-54521.rs:17:60
    |
 LL |     let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>>>();
-   |                                                            ^^ help: remove extra angle bracket
+   |                                                            ^^ help: remove extra angle brackets
 
 error: unmatched angle bracket
   --> $DIR/issue-54521.rs:20:60