RedBlackTree.empty: const, .front/.back: inout

[SimonN]

In RedBlackTree, add all possible qualifiers const pure nothrow @safe @nogc to empty() and length(). The class is final, thus I assume that this agrees with any user code.

Add the single qualifier inout to front() and back().

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[Geod24]

The class is also templated so some changes are superfluous. The const and inout additions are good though.

[SimonN]

Thanks! Right, my user code will need only const and inout.

Tonight, I'll add a unittest to this PR that calls into empty and length from a const pure nothrow @safe @nogc context, and remove all qualifiers from empty and length that are auto-inferred -- which I think will be all four of pure nothrow @safe @nogc.

[SimonN]

Now, I annotate only with const and inout. I test explicitly that empty() and length() still qualify as const pure nothrow @safe @nogc.

Already before this PR, length() had the single necessary qualifier const. Therefore this PR modifies only empty(), front(), and back().

[SimonN]

The unittest for all attributes is now part of the existing constness unittest.

I test all three of empty, length, and opBinaryRight"in" for const pure nothrow @safe @nogc and separately for immutable.

Reviewers: Requiring all qualifiers to be inferred for opBinaryRight!"in" is potentially up for debate. It's a search in a sorted tree for at most a single element, though, and thus not expected to allocate.

[SimonN]

Thanks for the quick review and merge!