Implement values_mut on {BTree, Hash}Map

[frewsxcv]

#32551

[rust-highfive]

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[gereeter]

Is there any reason that you chose to use Map here and not in BTreeMap's ValuesMut? I don't think that there are variance issues (since &mut forces V to be invariant anyway), but the inconsistency seems a little weird.

[gereeter]

Actually, now that I think about it some more, using Map makes K incorrectly invariant. This probably doesn't matter in practice, but I'd still be more comfortable implementing that mapping by hand and getting the correct covariance.

[frewsxcv]

I chose Map here for HashMap::ValuesMut because HashMap::Values uses a Map.

[gereeter]

Ah, that makes sense. I think the existing iterators are actually incorrect with respect to variance then - see #30642.

[frewsxcv]

Maybe I'll wait for #32635 merge before moving forward on this.

[frewsxcv]

#32635 merged and this PR is now rebased on top of those changes.

[alexcrichton]

Looks the travis error is legit (an error in one of the tests)

[frewsxcv]

@alexcrichton Which error are you looking at? If possible, could you paste in here?

[frewsxcv]

Nevermind. Found it. Fixing.

[alexcrichton]

Looks like the features in the doctests may still be off?

(I'd recommend running make check)

[frewsxcv]

Seems to pass the tests now.

[alexcrichton]

@bors: r+ 2084f2e

[bors]

⌛ Testing commit 2084f2e with merge 080edd9...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-64-x-armhf, auto-linux-64-x-armsf, auto-linux-64-x-freebsd, auto-linux-64-x-netbsd, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild