Add testcase for issue 105209

tests/ui/parser/issues/issue-105209.rs

@@ -0,0 +1,3 @@
+// compile-flags: -Zunpretty=ast-tree
+#![c={#![c[)x   //~ ERROR mismatched closing delimiter
+                //~ ERROR this file contains an unclosed delimiter

tests/ui/parser/issues/issue-105209.stderr

@@ -0,0 +1,22 @@
+error: mismatched closing delimiter: `)`
+  --> $DIR/issue-105209.rs:2:11
+   |
+LL | #![c={#![c[)x
+   |           ^^ mismatched closing delimiter
+   |           |
+   |           unclosed delimiter
+
+error: this file contains an unclosed delimiter
+  --> $DIR/issue-105209.rs:3:68
+   |
+LL | #![c={#![c[)x
+   |   -  -  -  - missing open `(` for this delimiter
+   |   |  |  |
+   |   |  |  unclosed delimiter
+   |   |  unclosed delimiter
+   |   unclosed delimiter
+LL |
+   |                                                                    ^
+
+error: aborting due to 2 previous errors
+