Auto merge of #81635 - michaelwoerister:structured_def_path_hash, r=pnkfelix

Let a portion of DefPathHash uniquely identify the DefPath's crate.

This allows to directly map from a `DefPathHash` to the crate it originates from, without constructing side tables to do that mapping -- something that is useful for incremental compilation where we deal with `DefPathHash` instead of `DefId` a lot.

It also allows to reliably and cheaply check for `DefPathHash` collisions which allows the compiler to gracefully abort compilation instead of running into a subsequent ICE at some random place in the code.

The following new piece of documentation describes the most interesting aspects of the changes:

```rust
/// A `DefPathHash` is a fixed-size representation of a `DefPath` that is
/// stable across crate and compilation session boundaries. It consists of two
/// separate 64-bit hashes. The first uniquely identifies the crate this
/// `DefPathHash` originates from (see [StableCrateId]), and the second
/// uniquely identifies the corresponding `DefPath` within that crate. Together
/// they form a unique identifier within an entire crate graph.
///
/// There is a very small chance of hash collisions, which would mean that two
/// different `DefPath`s map to the same `DefPathHash`. Proceeding compilation
/// with such a hash collision would very probably lead to an ICE and, in the
/// worst case, to a silent mis-compilation. The compiler therefore actively
/// and exhaustively checks for such hash collisions and aborts compilation if
/// it finds one.
///
/// `DefPathHash` uses 64-bit hashes for both the crate-id part and the
/// crate-internal part, even though it is likely that there are many more
/// `LocalDefId`s in a single crate than there are individual crates in a crate
/// graph. Since we use the same number of bits in both cases, the collision
/// probability for the crate-local part will be quite a bit higher (though
/// still very small).
///
/// This imbalance is not by accident: A hash collision in the
/// crate-local part of a `DefPathHash` will be detected and reported while
/// compiling the crate in question. Such a collision does not depend on
/// outside factors and can be easily fixed by the crate maintainer (e.g. by
/// renaming the item in question or by bumping the crate version in a harmless
/// way).
///
/// A collision between crate-id hashes on the other hand is harder to fix
/// because it depends on the set of crates in the entire crate graph of a
/// compilation session. Again, using the same crate with a different version
/// number would fix the issue with a high probability -- but that might be
/// easier said then done if the crates in questions are dependencies of
/// third-party crates.
///
/// That being said, given a high quality hash function, the collision
/// probabilities in question are very small. For example, for a big crate like
/// `rustc_middle` (with ~50000 `LocalDefId`s as of the time of writing) there
/// is a probability of roughly 1 in 14,750,000,000 of a crate-internal
/// collision occurring. For a big crate graph with 1000 crates in it, there is
/// a probability of 1 in 36,890,000,000,000 of a `StableCrateId` collision.
```

Given the probabilities involved I hope that no one will ever actually see the error messages. Nonetheless, I'd be glad about some feedback on how to improve them. Should we create a GH issue describing the problem and possible solutions to point to? Or a page in the rustc book?

r? `@pnkfelix` (feel free to re-assign)

2 files changed, 0 insertions, 36 deletions

diff --git a/compiler/rustc_ast/src/crate_disambiguator.rs b/compiler/rustc_ast/src/crate_disambiguator.rs
deleted file mode 100644
index bd7d8516714..00000000000
--- a/compiler/rustc_ast/src/crate_disambiguator.rs
+++ /dev/null
@@ -1,35 +0,0 @@
-// This is here because `rustc_session` wants to refer to it,
-// and so does `rustc_hir`, but `rustc_hir` shouldn't refer to `rustc_session`.
-
-use rustc_data_structures::fingerprint::Fingerprint;
-use rustc_data_structures::{base_n, impl_stable_hash_via_hash};
-
-use std::fmt;
-
-/// Hash value constructed out of all the `-C metadata` arguments passed to the
-/// compiler. Together with the crate-name forms a unique global identifier for
-/// the crate.
-#[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Clone, Copy, Encodable, Decodable)]
-pub struct CrateDisambiguator(Fingerprint);
-
-impl CrateDisambiguator {
-    pub fn to_fingerprint(self) -> Fingerprint {
-        self.0
-    }
-}
-
-impl fmt::Display for CrateDisambiguator {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
-        let (a, b) = self.0.as_value();
-        let as_u128 = a as u128 | ((b as u128) << 64);
-        f.write_str(&base_n::encode(as_u128, base_n::CASE_INSENSITIVE))
-    }
-}
-
-impl From<Fingerprint> for CrateDisambiguator {
-    fn from(fingerprint: Fingerprint) -> CrateDisambiguator {
-        CrateDisambiguator(fingerprint)
-    }
-}
-
-impl_stable_hash_via_hash!(CrateDisambiguator);
diff --git a/compiler/rustc_ast/src/lib.rs b/compiler/rustc_ast/src/lib.rs
index 4eaef85043c..03ec4b8a44d 100644
--- a/compiler/rustc_ast/src/lib.rs
+++ b/compiler/rustc_ast/src/lib.rs
@@ -42,7 +42,6 @@ pub mod util {
 pub mod ast;
 pub mod ast_like;
 pub mod attr;
-pub mod crate_disambiguator;
 pub mod entry;
 pub mod expand;
 pub mod mut_visit;