add limit for unclosed delimiters in lexer diagnostic

Author: chenyukang

compiler/rustc_parse/src/lexer/tokentrees.rs

@@ -72,16 +72,30 @@ impl<'psess, 'src> TokenTreesReader<'psess, 'src> {
     fn eof_err(&mut self) -> PErr<'psess> {
         let msg = "this file contains an unclosed delimiter";
         let mut err = self.string_reader.dcx().struct_span_err(self.token.span, msg);
-        for &(_, sp) in &self.diag_info.open_braces {
-            err.span_label(sp, "unclosed delimiter");
+
+        let unclosed_delimiter_show_limits = 5;
+        for &(_, span) in &self.diag_info.open_braces[..unclosed_delimiter_show_limits] {
+            err.span_label(span, "unclosed delimiter");
             self.diag_info.unmatched_delims.push(UnmatchedDelim {
                 found_delim: None,
                 found_span: self.token.span,
-                unclosed_span: Some(sp),
+                unclosed_span: Some(span),
                 candidate_span: None,
             });
         }
 
+        if let Some((_, span)) = self.diag_info.open_braces.get(unclosed_delimiter_show_limits)
+            && self.diag_info.open_braces.len() >= unclosed_delimiter_show_limits + 2
+        {
+            err.span_label(
+                *span,
+                format!(
+                    "another {} unclosed delimiters begin from here",
+                    self.diag_info.open_braces.len() - unclosed_delimiter_show_limits
+                ),
+            );
+        }
+
         if let Some((delim, _)) = self.diag_info.open_braces.last() {
             report_suspicious_mismatch_block(
                 &mut err,

tests/ui/parser/mismatched-delimiter-corner-case-issue-127868.stderr

@@ -14,24 +14,9 @@ error: this file contains an unclosed delimiter
 LL | fn main() {
    |           - unclosed delimiter
 LL |     let a = [[[[[[[[[[[[[[[[[[[[1, {, (, [,;
-   |             --------------------   - this delimiter might not be properly closed...
-   |             ||||||||||||||||||||
-   |             |||||||||||||||||||unclosed delimiter
-   |             ||||||||||||||||||unclosed delimiter
-   |             |||||||||||||||||unclosed delimiter
-   |             ||||||||||||||||unclosed delimiter
-   |             |||||||||||||||unclosed delimiter
-   |             ||||||||||||||unclosed delimiter
-   |             |||||||||||||unclosed delimiter
-   |             ||||||||||||unclosed delimiter
-   |             |||||||||||unclosed delimiter
-   |             ||||||||||unclosed delimiter
-   |             |||||||||unclosed delimiter
-   |             ||||||||unclosed delimiter
-   |             |||||||unclosed delimiter
-   |             ||||||unclosed delimiter
-   |             |||||unclosed delimiter
-   |             ||||unclosed delimiter
+   |             -----                  - this delimiter might not be properly closed...
+   |             |||||
+   |             ||||another 16 unclosed delimiters begin from here
    |             |||unclosed delimiter
    |             ||unclosed delimiter
    |             |unclosed delimiter