Struct rustc_ap_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.

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) -> Result<Self, <__D as Decoder>::Error>

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