## DefPathHash

# Struct DefPathHash

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    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 `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 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 `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.

## Tuple Fields§

§`0: Fingerprint`

## Implementations§

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### impl DefPathHash

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#### pub fn stable_crate_id(&self) -> StableCrateId

Returns the StableCrateId identifying the crate this DefPathHash originates from.

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#### pub fn local_hash(&self) -> Hash64

Returns the crate-local part of the DefPathHash.

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#### pub fn new(stable_crate_id: StableCrateId, local_hash: Hash64) -> DefPathHash

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

## Trait Implementations§

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### impl Clone for DefPathHash

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#### fn clone(&self) -> DefPathHash

Returns a duplicate of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for DefPathHash

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

Formats the value using the given formatter. Read more
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### impl<__D: SpanDecoder> Decodable<__D> for DefPathHash

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

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### impl Default for DefPathHash

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#### fn default() -> Self

Returns the “default value” for a type. Read more
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### impl<__E: SpanEncoder> Encodable<__E> for DefPathHash

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#### fn encode(&self, __encoder: &mut __E)

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### impl Hash for DefPathHash

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

Feeds this value into the given `Hasher`. Read more
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#### fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
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### impl<__CTX> HashStable<__CTX> for DefPathHash
where __CTX: HashStableContext,

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

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### impl Ord for DefPathHash

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

This method returns an `Ordering` between `self` and `other`. Read more
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#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
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#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
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#### fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized,

Restrict a value to a certain interval. Read more
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### impl PartialEq for DefPathHash

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

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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### impl PartialOrd for DefPathHash

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

This method returns an ordering between `self` and `other` values if one exists. Read more
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#### fn lt(&self, other: &Rhs) -> bool

Tests less than (for `self` and `other`) and is used by the `<` operator. Read more
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#### fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
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#### fn gt(&self, other: &Rhs) -> bool

Tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
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#### fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
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### impl StableOrd for DefPathHash

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#### const CAN_USE_UNSTABLE_SORT: bool = true

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#### const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ()

Marker to ensure that implementors have carefully considered whether their `Ord` implementation obeys this trait’s contract.
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### impl<CTX: HashStableContext> ToStableHashKey<CTX> for DefPathHash

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#### type KeyType = DefPathHash

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#### fn to_stable_hash_key(&self, _: &CTX) -> DefPathHash

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### impl Copy for DefPathHash

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### impl Eq for DefPathHash

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### impl StructuralPartialEq for DefPathHash

## Auto Trait Implementations§

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### impl DynSend for DefPathHash

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### impl DynSync for DefPathHash

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### impl Freeze for DefPathHash

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### impl RefUnwindSafe for DefPathHash

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### impl Send for DefPathHash

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### impl Sync for DefPathHash

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### impl Unpin for DefPathHash

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### impl UnwindSafe for DefPathHash

## Blanket Implementations§

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### impl<T> Aligned for T

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#### const ALIGN: Alignment

Alignment of `Self`.
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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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where 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> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<Q, K> Comparable<K> for Q
where Q: Ord + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn compare(&self, key: &K) -> Ordering

Compare self to `key` and return their ordering.
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Compare self to `key` and return `true` if they are equal.
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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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### impl<T> Instrument for T

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#### fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided `Span`, returning an `Instrumented` wrapper. Read more
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#### fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current `Span`, returning an `Instrumented` wrapper. Read more
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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> IntoEither for T

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#### fn into_either(self, into_left: bool) -> Either<Self, Self>

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left` is `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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#### fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
where F: FnOnce(&Self) -> bool,

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left(&self)` returns `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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### impl<T> Pointable for T

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#### const ALIGN: usize

The alignment of pointer.
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#### type Init = T

The type for initializers.
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#### unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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#### unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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Mutably dereferences the given pointer. Read more
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#### unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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### impl<T> Same for T

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#### type Output = T

Should always be `Self`
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### impl<T> StableCompare for T
where T: StableOrd,

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#### const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT

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#### fn stable_cmp(&self, other: &T) -> Ordering

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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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where U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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Performs the conversion.
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#### fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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Attaches the current default `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more

## 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