Struct rustc_span::def_id::DefPathHash

pub struct DefPathHash(pub Fingerprint);

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

Implementations

impl DefPathHash

pub fn stable_crate_id(&self) -> StableCrateId

Returns the StableCrateId identifying the crate this DefPathHash originates from.

pub fn local_hash(&self) -> u64

Returns the crate-local part of the DefPathHash.

Used for tests.

pub fn new(stable_crate_id: StableCrateId, local_hash: u64) -> DefPathHash

Builds a new DefPathHash with the given StableCrateId and local_hash, where local_hash must be unique within its crate.

Trait Implementations

impl Borrow<Fingerprint> for DefPathHash

fn borrow(&self) -> &Fingerprint

Immutably borrows from an owned value.

impl Clone for DefPathHash

fn clone(&self) -> DefPathHash

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for DefPathHash

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<__D: Decoder> Decodable<__D> for DefPathHash

fn decode(__decoder: &mut __D) -> Self

impl<__E: Encoder> Encodable<__E> for DefPathHash

fn encode(&self, __encoder: &mut __E) -> Result<(), <__E as Encoder>::Error>

impl Hash for DefPathHash

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given Hasher.

impl<__CTX> HashStable<__CTX> for DefPathHash where
    __CTX: HashStableContext, 

fn hash_stable(&self, __hcx: &mut __CTX, __hasher: &mut StableHasher)

impl Ord for DefPathHash

fn cmp(&self, other: &DefPathHash) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

Restrict a value to a certain interval.

impl PartialEq<DefPathHash> for DefPathHash

fn eq(&self, other: &DefPathHash) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &DefPathHash) -> bool

This method tests for !=.

impl PartialOrd<DefPathHash> for DefPathHash

fn partial_cmp(&self, other: &DefPathHash) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for DefPathHash

impl Eq for DefPathHash

impl StructuralEq for DefPathHash

impl StructuralPartialEq for DefPathHash

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.

Size: 16 bytes