Nicholas Nethercote
fdfba6737d
`Interned` does pointer equality/hashing, which is valid in two cases. - The values are guaranteed to be unique (e.g. via interning, or construction). This is how `rustc_middle` uses `Interned`. - The type has "identity" and different values should be considered distinct even if they are identical. This is how `rustc_resolve` uses `Interned`. PR 137202 tried to clarify things by adding a `T: Hash` constraint to `Interned<'a, T>`. This constraint isn't actually used, because `Interned` is hashed based on pointer value, not contents. But it was intended to communicate the idea that a type stored in `Interned` is actually interned, which is likely to be done with hashing. Panicking impls of `Hash` were added for the relevant `rustc_resolve` types to work around the fact that it doesn't use hashing-based interning. In my opinion PR 137202 didn't improve things. The `T: Hash` constraint is only aimed at the interning case, and even for that case it's not quite right because you could use a `BTreeMap` to intern instead of a `HashMap`. This commit does several things. - Removes the `T: Hash` constraint and the `Hash` impls for `rustc_resolve` types added in PR 137202. - Improves the comments on `Interned` to cover the non-interning cases. - Removes the `PartialOrd`/`Ord` impls on `Interned` because (a) they're not used, and (b) their meaning is unclear for the "identity" case. - Improves the documentation in `rustc_resolve` to explain how `Interned` usage is valid there.
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