Trait rustc_data_structures::stable_hasher::StableOrd

source · 
pub unsafe trait StableOrd: Ord { }
Expand description

Trait for marking a type as having a sort order that is stable across compilation session boundaries. More formally:

Ord::cmp(a1, b1) == Ord:cmp(a2, b2)
   where a2 = decode(encode(a1, context1), context2)
         b2 = decode(encode(b1, context1), context2)

i.e. the result of Ord::cmp is not influenced by encoding the values in one session and then decoding them in another session.

This is trivially true for types where encoding and decoding don’t change the bytes of the values that are used during comparison and comparison only depends on these bytes (as opposed to some non-local state). Examples are u32, String, Path, etc.

But it is not true for:

  • *const T and *mut T because the values of these pointers will change between sessions.
  • DefIndex, CrateNum, LocalDefId, because their concrete values depend on state that might be different between compilation sessions.

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