pub struct DefPathHash(pub Fingerprint);
Expand description

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 DefPaths map to the same DefPathHash. Proceeding compilation with such a hash collision would very probably lead to an ICE, and in the worst case lead 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 LocalDefIds 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 LocalDefIds 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.

Tuple Fields§

§0: Fingerprint

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impl<T> Any for Twhere T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for Twhere T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for Twhere T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> CallHasher for Twhere T: Hash + ?Sized,

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default fn get_hash<H, B>(value: &H, build_hasher: &B) -> u64where H: Hash + ?Sized, B: BuildHasher,

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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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fn into(self) -> U

Calls U::from(self).

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type Owned = T

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type Error = Infallible

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type Error = <U as TryFrom<T>>::Error

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Performs the conversion.