save_analysis: fix panic in write_sub_paths_truncated

src/librustc_save_analysis/dump_visitor.rs

@@ -210,9 +210,11 @@ impl<'l, 'tcx> DumpVisitor<'l, 'tcx> {
     // As write_sub_paths, but does not process the last ident in the path (assuming it
     // will be processed elsewhere). See note on write_sub_paths about global.
     fn write_sub_paths_truncated(&mut self, path: &'tcx hir::Path<'tcx>) {
-        for seg in &path.segments[..path.segments.len() - 1] {
-            if let Some(data) = self.save_ctxt.get_path_segment_data(seg) {
-                self.dumper.dump_ref(data);
+        if path.segments.len() > 0 {
+            for seg in &path.segments[..path.segments.len() - 1] {
+                if let Some(data) = self.save_ctxt.get_path_segment_data(seg) {
+                    self.dumper.dump_ref(data);
+                }
             }
         }
     }

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`.