Query the fingerprint style during key reconstruction

Keys can be reconstructed from fingerprints that are not DefPathHash, but then
we cannot extract a DefId from them.

3 files changed, 30 insertions, 9 deletions

diff --git a/compiler/rustc_query_system/src/dep_graph/dep_node.rs b/compiler/rustc_query_system/src/dep_graph/dep_node.rs
index dd500015374..8602219a7f3 100644
--- a/compiler/rustc_query_system/src/dep_graph/dep_node.rs
+++ b/compiler/rustc_query_system/src/dep_graph/dep_node.rs
@@ -42,7 +42,7 @@
 //!   `DefId` it was computed from. In other cases, too much information gets
 //!   lost during fingerprint computation.
 
-use super::{DepContext, DepKind};
+use super::{DepContext, DepKind, FingerprintStyle};
 use crate::ich::StableHashingContext;
 
 use rustc_data_structures::fingerprint::{Fingerprint, PackedFingerprint};
@@ -75,7 +75,7 @@ impl<K: DepKind> DepNode<K> {
 
         #[cfg(debug_assertions)]
         {
-            if !kind.can_reconstruct_query_key()
+            if !kind.fingerprint_style().reconstructible()
                 && (tcx.sess().opts.debugging_opts.incremental_info
                     || tcx.sess().opts.debugging_opts.query_dep_graph)
             {
@@ -94,7 +94,7 @@ impl<K: DepKind> fmt::Debug for DepNode<K> {
 }
 
 pub trait DepNodeParams<Ctxt: DepContext>: fmt::Debug + Sized {
-    fn can_reconstruct_query_key() -> bool;
+    fn fingerprint_style() -> FingerprintStyle;
 
     /// This method turns the parameters of a DepNodeConstructor into an opaque
     /// Fingerprint to be used in DepNode.
@@ -111,7 +111,7 @@ pub trait DepNodeParams<Ctxt: DepContext>: fmt::Debug + Sized {
     /// This method tries to recover the query key from the given `DepNode`,
     /// something which is needed when forcing `DepNode`s during red-green
     /// evaluation. The query system will only call this method if
-    /// `can_reconstruct_query_key()` is `true`.
+    /// `fingerprint_style()` is not `FingerprintStyle::Opaque`.
     /// It is always valid to return `None` here, in which case incremental
     /// compilation will treat the query as having changed instead of forcing it.
     fn recover(tcx: Ctxt, dep_node: &DepNode<Ctxt::DepKind>) -> Option<Self>;
@@ -122,8 +122,8 @@ where
     T: for<'a> HashStable<StableHashingContext<'a>> + fmt::Debug,
 {
     #[inline]
-    default fn can_reconstruct_query_key() -> bool {
-        false
+    default fn fingerprint_style() -> FingerprintStyle {
+        FingerprintStyle::Opaque
     }
 
     default fn to_fingerprint(&self, tcx: Ctxt) -> Fingerprint {
diff --git a/compiler/rustc_query_system/src/dep_graph/mod.rs b/compiler/rustc_query_system/src/dep_graph/mod.rs
index 2afef210254..dcda5728334 100644
--- a/compiler/rustc_query_system/src/dep_graph/mod.rs
+++ b/compiler/rustc_query_system/src/dep_graph/mod.rs
@@ -50,6 +50,27 @@ impl<T: DepContext> HasDepContext for T {
     }
 }
 
+/// Describes the contents of the fingerprint generated by a given query.
+#[derive(PartialEq, Eq, Copy, Clone)]
+pub enum FingerprintStyle {
+    /// The fingerprint is actually a DefPathHash.
+    DefPathHash,
+    /// Query key was `()` or equivalent, so fingerprint is just zero.
+    Unit,
+    /// Some opaque hash.
+    Opaque,
+}
+
+impl FingerprintStyle {
+    #[inline]
+    pub fn reconstructible(self) -> bool {
+        match self {
+            FingerprintStyle::DefPathHash | FingerprintStyle::Unit => true,
+            FingerprintStyle::Opaque => false,
+        }
+    }
+}
+
 /// Describe the different families of dependency nodes.
 pub trait DepKind: Copy + fmt::Debug + Eq + Hash + Send + Encodable<FileEncoder> + 'static {
     const NULL: Self;
@@ -73,5 +94,5 @@ pub trait DepKind: Copy + fmt::Debug + Eq + Hash + Send + Encodable<FileEncoder>
     where
         OP: for<'a> FnOnce(Option<&'a Lock<TaskDeps<Self>>>);
 
-    fn can_reconstruct_query_key(&self) -> bool;
+    fn fingerprint_style(&self) -> FingerprintStyle;
 }
diff --git a/compiler/rustc_query_system/src/query/plumbing.rs b/compiler/rustc_query_system/src/query/plumbing.rs
index 3534c324295..75184cd6f51 100644
--- a/compiler/rustc_query_system/src/query/plumbing.rs
+++ b/compiler/rustc_query_system/src/query/plumbing.rs
@@ -540,7 +540,7 @@ where
         // We always expect to find a cached result for things that
         // can be forced from `DepNode`.
         debug_assert!(
-            !dep_node.kind.can_reconstruct_query_key() || result.is_some(),
+            !dep_node.kind.fingerprint_style().reconstructible() || result.is_some(),
             "missing on-disk cache entry for {:?}",
             dep_node
         );
@@ -778,7 +778,7 @@ where
         return false;
     }
 
-    if !<Q::Key as DepNodeParams<CTX::DepContext>>::can_reconstruct_query_key() {
+    if !<Q::Key as DepNodeParams<CTX::DepContext>>::fingerprint_style().reconstructible() {
         return false;
     }