don't point at nonexisting code beyond EOF when warning about delims

compiler/rustc_lint/src/unused.rs

@@ -661,6 +661,10 @@ trait UnusedDelimLint {
         keep_space: (bool, bool),
     ) {
         let primary_span = if let Some((lo, hi)) = spans {
+            if hi.is_empty() {
+                // do not point at delims that do not exist
+                return;
+            }
             MultiSpan::from(vec![lo, hi])
         } else {
             MultiSpan::from(value_span)

tests/ui/malformed/issue-107423-unused-delim-only-one-no-pair.rs

@@ -0,0 +1,7 @@
+// check that we don't generate a span that points beyond EOF
+
+// error-pattern: unclosed delimiter
+// error-pattern: unclosed delimiter
+// error-pattern: unclosed delimiter
+
+fn a(){{{

tests/ui/malformed/issue-107423-unused-delim-only-one-no-pair.stderr

@@ -0,0 +1,32 @@
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-107423-unused-delim-only-one-no-pair.rs:7:11
+   |
+LL | fn a(){{{
+   |       --- ^
+   |       |||
+   |       ||unclosed delimiter
+   |       |unclosed delimiter
+   |       unclosed delimiter
+
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-107423-unused-delim-only-one-no-pair.rs:7:11
+   |
+LL | fn a(){{{
+   |       --- ^
+   |       |||
+   |       ||unclosed delimiter
+   |       |unclosed delimiter
+   |       unclosed delimiter
+
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-107423-unused-delim-only-one-no-pair.rs:7:11
+   |
+LL | fn a(){{{
+   |       --- ^
+   |       |||
+   |       ||unclosed delimiter
+   |       |unclosed delimiter
+   |       unclosed delimiter
+
+error: aborting due to 3 previous errors
+