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Document requirements for HashStable implementations better.

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michaelwoerister committed May 17, 2019
1 parent eed1e1e commit a79c06a9ce1986d3c4dbc27b5e51d60d77befafb
Showing with 24 additions and 0 deletions.
  1. +24 −0 src/librustc_data_structures/stable_hasher.rs
@@ -140,6 +140,30 @@ impl<W> Hasher for StableHasher<W> {

/// Something that implements `HashStable<CTX>` can be hashed in a way that is
/// stable across multiple compilation sessions.
///
/// Note that `HashStable` imposes rather more strict requirements than usual
/// hash functions:
///
/// - Stable hashes are sometimes used as identifiers. Therefore they must
/// conform to the corresponding `PartialEq` implementations:
///
/// - `x == y` implies `hash_stable(x) == hash_stable(y)`, and
/// - `x != y` implies `hash_stable(x) != hash_stable(y)`.
///
/// That second condition is usually not required for hash functions
/// (e.g. `Hash`). In practice this means that `hash_stable` must feed any
/// information into the hasher that a `PartialEq` comparision takes into
/// account. See [#49300](https://github.com/rust-lang/rust/issues/49300)
/// for an example where violating this invariant has caused trouble in the
/// past.
///
/// - `hash_stable()` must be independent of the current
/// compilation session. E.g. they must not hash memory addresses or other
/// things that are "randomly" assigned per compilation session.
///
/// - `hash_stable()` must be independent of the host architecture. The
/// `StableHasher` takes care of endianness and `isize`/`usize` platform
/// differences.
pub trait HashStable<CTX> {
fn hash_stable<W: StableHasherResult>(&self,
hcx: &mut CTX,

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