Rollup merge of #153210 - Delta17920:fix-query-dep-graph, r=jdonszelmann,JonathanBrouwer

Fix ICE on empty file with -Zquery-dep-graph

added a guard in `DepGraph::register_dep_node_debug_str` so it early-returns
when `self.data` is still `None`. this happens for the first few dep-nodes
in an empty crate with `-Z query-dep-graph`, which caused the previous
`unwrap()` panic.

fixes #153199

Author: JonathanBrouwer

compiler/rustc_middle/src/dep_graph/graph.rs

@@ -826,7 +826,13 @@ impl DepGraph {
     where
         F: FnOnce() -> String,
     {
-        let dep_node_debug = &self.data.as_ref().unwrap().dep_node_debug;
+        // Early queries (e.g., `-Z query-dep-graph` on empty crates) can reach here
+        // before the graph is initialized. Return early to prevent an ICE.
+        let data = match &self.data {
+            Some(d) => d,
+            None => return,
+        };
+        let dep_node_debug = &data.dep_node_debug;
 
         if dep_node_debug.borrow().contains_key(&dep_node) {
             return;

tests/ui/dep-graph/query-dep-graph-empty.rs

@@ -0,0 +1,7 @@
+//@ build-pass
+//@ compile-flags: -Zquery-dep-graph --crate-type lib
+//@ edition: 2021
+
+// This file is intentionally left empty to reproduce issue #153199.
+// rustc used to ICE when generating a dependency graph for an empty file
+// because early queries would panic when unwrapping an uninitialized graph.