Trait rustc_data_structures::unify::EqUnifyValue
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A convenient helper for unification values which must be equal or
else an error occurs. For example, if you are unifying types in a
simple functional language, this may be appropriate, since (e.g.)
you can’t unify a type variable bound to int with one bound to
float (but you can unify two type variables both bound to
int).
Any type which implements EqUnifyValue automatially implements
UnifyValue; if the two values are equal, merging is permitted.
Otherwise, the error (v1, v2) is returned, where v1 and v2
are the two unequal values.