HashStable for rustc_feature::Features: stop hashing compile-time constant

[RalfJung]

It seems like back in 542bc75 this was added as "hash the boolean value of each lang feature", but then in 1487bd6 this got split into first hashing a sequence of bools (representing all the features) and then hashing all the feature names... but the list of feature names is a compile-time constant, so it seems entirely unnecessary to hash them?

Cc @Mark-Simulacrum who wrote the second of the commits mentioned above.
Cc @nnethercote

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[nnethercote]

LGTM, though I will wait for Mark to weigh in before approving, just in case.

[Mark-Simulacrum]

I think my reasoning was that the [bool] list might be the same if we reorder features in the list, but ideally would produce a different hash. However thinking more on it the compiler version etc. should also get hashed in if we care about that so it should be fine (as you point out).

To some extent it looks like we've regressed on those goals anyway, we're hashing far more than bools these days:

$ rg 'fn enabled_(lang|lib)_features'
compiler/rustc_feature/src/unstable.rs
57:    pub fn enabled_lang_features(&self) -> &Vec<(Symbol, Span, Option<Symbol>)> {
61:    pub fn enabled_lib_features(&self) -> &Vec<(Symbol, Span)> {

But, still, this seems like a good change to make regardless.

@bors r=nnethercote,Mark-Simulacrum

[bors]

📌 Commit 0304809 has been approved by nnethercote,Mark-Simulacrum

It is now in the queue for this repository.