Improve ICE message for forbidden dep-graph reads.

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -459,7 +459,8 @@ impl<D: Deps> DepGraph<D> {
                     }
                     TaskDepsRef::Ignore => return,
                     TaskDepsRef::Forbid => {
-                        panic!("Illegal read of: {dep_node_index:?}")
+                        // Reading is forbidden in this context. ICE with a useful error message.
+                        panic_on_forbidden_read(data, dep_node_index)
                     }
                 };
                 let task_deps = &mut *task_deps;
@@ -1366,3 +1367,41 @@ pub(crate) fn print_markframe_trace<D: Deps>(graph: &DepGraph<D>, frame: Option<
 
     eprintln!("end of try_mark_green dep node stack");
 }
+
+#[cold]
+#[inline(never)]
+fn panic_on_forbidden_read<D: Deps>(data: &DepGraphData<D>, dep_node_index: DepNodeIndex) -> ! {
+    // We have to do an expensive reverse-lookup of the DepNode that
+    // corresponds to `dep_node_index`, but that's OK since we are about
+    // to ICE anyway.
+    let mut dep_node = None;
+
+    // First try to find the dep node among those that already existed in the
+    // previous session
+    for (prev_index, index) in data.current.prev_index_to_index.lock().iter_enumerated() {
+        if index == &Some(dep_node_index) {
+            dep_node = Some(data.previous.index_to_node(prev_index));
+            break;
+        }
+    }
+
+    if dep_node.is_none() {
+        // Try to find it among the new nodes
+        for shard in data.current.new_node_to_index.lock_shards() {
+            if let Some((node, _)) = shard.iter().find(|(_, index)| **index == dep_node_index) {
+                dep_node = Some(*node);
+                break;
+            }
+        }
+    }
+
+    let dep_node = dep_node.map_or_else(
+        || format!("with index {:?}", dep_node_index),
+        |dep_node| format!("`{:?}`", dep_node),
+    );
+
+    panic!(
+        "Error: trying to record dependency on DepNode {dep_node} in a \
+         context that does not allow it (e.g. during query deserialization)."
+    )
+}