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.

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;
 }