fix ICE in Definitions::create_def

compiler/rustc_hir/src/definitions.rs

@@ -338,7 +338,12 @@ pub fn new(stable_crate_id: StableCrateId) -> Definitions {
 
     /// Adds a definition with a parent definition.
     pub fn create_def(&mut self, parent: LocalDefId, data: DefPathData) -> LocalDefId {
-        debug!("create_def(parent={:?}, data={:?})", parent, data);
+        // We can't use `Debug` implementation for `LocalDefId` here, since it tries to acquire a
+        // reference to `Definitions` and we're already holding a mutable reference.
+        debug!(
+            "create_def(parent={}, data={data:?})",
+            self.def_path(parent).to_string_no_crate_verbose(),
+        );
 
         // The root node must be created with `create_root_def()`.
         assert!(data != DefPathData::CrateRoot);

compiler/rustc_middle/src/ty/context.rs

@@ -1459,11 +1459,11 @@ pub fn def_path_debug_str(self, def_id: DefId) -> String {
         };
 
         format!(
-            "{}[{}]{}",
+            "{}[{:04x}]{}",
             crate_name,
             // Don't print the whole stable crate id. That's just
             // annoying in debug output.
-            &(format!("{:08x}", stable_crate_id.to_u64()))[..4],
+            stable_crate_id.to_u64() >> 8 * 6,
             self.def_path(def_id).to_string_no_crate_verbose()
         )
     }