Add unit test to ensure that both parts of a DefPathHash depend on the defining crate's ID.

compiler/rustc_hir/src/definitions.rs

@@ -58,8 +58,11 @@ impl DefPathTable {
             // being used.
             //
             // See the documentation for DefPathHash for more information.
-            panic!("found DefPathHash collsion between {:?} and {:?}. \
-                    Compilation cannot continue.", def_path1, def_path2);
+            panic!(
+                "found DefPathHash collsion between {:?} and {:?}. \
+                    Compilation cannot continue.",
+                def_path1, def_path2
+            );
         }
 
         // Assert that all DefPathHashes correctly contain the local crate's
@@ -138,7 +141,7 @@ pub struct DefKey {
 }
 
 impl DefKey {
-    fn compute_stable_hash(&self, parent: DefPathHash) -> DefPathHash {
+    pub(crate) fn compute_stable_hash(&self, parent: DefPathHash) -> DefPathHash {
         let mut hasher = StableHasher::new();
 
         parent.hash(&mut hasher);

compiler/rustc_hir/src/lib.rs

@@ -31,6 +31,9 @@ mod stable_hash_impls;
 mod target;
 pub mod weak_lang_items;
 
+#[cfg(test)]
+mod tests;
+
 pub use hir::*;
 pub use hir_id::*;
 pub use lang_items::{LangItem, LanguageItems};

compiler/rustc_hir/src/tests.rs

@@ -0,0 +1,39 @@
+use crate::definitions::{DefKey, DefPathData, DisambiguatedDefPathData};
+use rustc_data_structures::fingerprint::Fingerprint;
+use rustc_span::crate_disambiguator::CrateDisambiguator;
+use rustc_span::def_id::{DefPathHash, StableCrateId};
+
+#[test]
+fn def_path_hash_depends_on_crate_id() {
+    // This test makes sure that *both* halves of a DefPathHash depend on
+    // the crate-id of the defining crate. This is a desirable property
+    // because the crate-id can be more easily changed than the DefPath
+    // of an item, so, in the case of a crate-local DefPathHash collision,
+    // the user can simply "role the dice again" for all DefPathHashes in
+    // the crate by changing the crate disambiguator (e.g. via bumping the
+    // crate's version number).
+
+    let d0 = CrateDisambiguator::from(Fingerprint::new(12, 34));
+    let d1 = CrateDisambiguator::from(Fingerprint::new(56, 78));
+
+    let h0 = mk_test_hash("foo", d0);
+    let h1 = mk_test_hash("foo", d1);
+
+    assert_ne!(h0.stable_crate_id(), h1.stable_crate_id());
+    assert_ne!(h0.local_hash(), h1.local_hash());
+
+    fn mk_test_hash(crate_name: &str, crate_disambiguator: CrateDisambiguator) -> DefPathHash {
+        let stable_crate_id = StableCrateId::new(crate_name, crate_disambiguator);
+        let parent_hash = DefPathHash::new(stable_crate_id, 0);
+
+        let key = DefKey {
+            parent: None,
+            disambiguated_data: DisambiguatedDefPathData {
+                data: DefPathData::CrateRoot,
+                disambiguator: 0,
+            },
+        };
+
+        key.compute_stable_hash(parent_hash)
+    }
+}

compiler/rustc_span/src/def_id.rs

@@ -159,6 +159,12 @@ impl DefPathHash {
         StableCrateId(self.0.as_value().0)
     }
 
+    /// Returns the crate-local part of the [DefPathHash].
+    #[inline]
+    pub fn local_hash(&self) -> u64 {
+        self.0.as_value().1
+    }
+
     /// Builds a new [DefPathHash] with the given [StableCrateId] and
     /// `local_hash`, where `local_hash` must be unique within its crate.
     pub fn new(stable_crate_id: StableCrateId, local_hash: u64) -> DefPathHash {