Auto merge of #91742 - cjgillot:force-backtrace, r=estebank

Print a backtrace when query forcing fails.

The aim of this PR is to help debugging incremental compilation bugs where query forcing panics.
For instance: https://github.com/rust-lang/rust/issues/90682 https://github.com/rust-lang/rust/issues/90697 https://github.com/rust-lang/rust/issues/90715 https://github.com/rust-lang/rust/issues/90739 https://github.com/rust-lang/rust/issues/91401

These bugs happen when the dep-graph attempts to force a dep-node whose fingerprint does not correspond to an actual DefPathHash. PR https://github.com/rust-lang/rust/pull/91741 attempts to hide this bug.

I still don't know how to reproduce these bugs, so I sadly could not test this debugging device.

compiler/rustc_query_system/src/dep_graph/graph.rs

@@ -6,6 +6,7 @@ use rustc_data_structures::sharded::{self, Sharded};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{AtomicU32, AtomicU64, Lock, Lrc, Ordering};
+use rustc_data_structures::OnDrop;
 use rustc_index::vec::IndexVec;
 use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
 use smallvec::{smallvec, SmallVec};
@@ -671,17 +672,24 @@ impl<K: DepKind> DepGraph<K> {
         let prev_index = data.previous.node_to_index_opt(dep_node)?;
 
         match data.colors.get(prev_index) {
-            Some(DepNodeColor::Green(dep_node_index)) => Some((prev_index, dep_node_index)),
+            Some(DepNodeColor::Green(dep_node_index)) => return Some((prev_index, dep_node_index)),
-            Some(DepNodeColor::Red) => None,
+            Some(DepNodeColor::Red) => return None,
-            None => {
+            None => {}
-                // This DepNode and the corresponding query invocation existed
-                // in the previous compilation session too, so we can try to
-                // mark it as green by recursively marking all of its
-                // dependencies green.
-                self.try_mark_previous_green(qcx, data, prev_index, &dep_node)
-                    .map(|dep_node_index| (prev_index, dep_node_index))
-            }
         }
+
+        let backtrace = backtrace_printer(qcx.dep_context().sess(), data, prev_index);
+
+        // This DepNode and the corresponding query invocation existed
+        // in the previous compilation session too, so we can try to
+        // mark it as green by recursively marking all of its
+        // dependencies green.
+        let ret = self
+            .try_mark_previous_green(qcx, data, prev_index, &dep_node)
+            .map(|dep_node_index| (prev_index, dep_node_index));
+
+        // We succeeded, no backtrace.
+        backtrace.disable();
+        return ret;
     }
 
     #[instrument(skip(self, qcx, data, parent_dep_node_index), level = "debug")]
@@ -794,7 +802,10 @@ impl<K: DepKind> DepGraph<K> {
         let prev_deps = data.previous.edge_targets_from(prev_dep_node_index);
 
         for &dep_dep_node_index in prev_deps {
-            self.try_mark_parent_green(qcx, data, dep_dep_node_index, dep_node)?
+            let backtrace = backtrace_printer(qcx.dep_context().sess(), data, dep_dep_node_index);
+            let success = self.try_mark_parent_green(qcx, data, dep_dep_node_index, dep_node);
+            backtrace.disable();
+            success?;
         }
 
         // If we got here without hitting a `return` that means that all
@@ -1364,3 +1375,26 @@ impl DepNodeColorMap {
         )
     }
 }
+
+fn backtrace_printer<'a, K: DepKind>(
+    sess: &'a rustc_session::Session,
+    graph: &'a DepGraphData<K>,
+    node: SerializedDepNodeIndex,
+) -> OnDrop<impl Fn() + 'a> {
+    OnDrop(
+        #[inline(never)]
+        #[cold]
+        move || {
+            let node = graph.previous.index_to_node(node);
+            // Do not try to rely on DepNode's Debug implementation, since it may panic.
+            let diag = rustc_errors::Diagnostic::new(
+                rustc_errors::Level::FailureNote,
+                &format!(
+                    "encountered while trying to mark dependency green: {:?}({})",
+                    node.kind, node.hash
+                ),
+            );
+            sess.diagnostic().force_print_diagnostic(diag);
+        },
+    )
+}