save_analysis: fix panic in write_sub_paths_truncated

src/test/ui/save-analysis/issue-73020.rs

@@ -0,0 +1,5 @@
+// compile-flags: -Zsave-analysis
+use {self}; //~ ERROR E0431
+
+fn main () {
+}

src/test/ui/save-analysis/issue-73020.stderr

@@ -0,0 +1,9 @@
+error[E0431]: `self` import can only appear in an import list with a non-empty prefix
+  --> $DIR/issue-73020.rs:2:6
+   |
+LL | use {self};
+   |      ^^^^ can only appear in an import list with a non-empty prefix
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0431`.