Improve readability in a few sorts

[ljedrz]

Use sort_by_key where possible.

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

@bors r+ rollup

[bors]

📌 Commit f653bf4 has been approved by petrochenkov

[leonardo-m]

That improves readability, but keep sort_by_key allocates some new memory, and this sometimes makes the code slower than using a cmp without allocations. So in theory it's better to benchmark before/after this change.

[scottmcm]

@leonardo-m Are you sure about that? The recent sort_by_cached_key pre-computes the keys into new memory, but ordinary sort_by_key seems to just be the same as the cmp closure:

rust/src/liballoc/slice.rs

Line 279 in fefe816

merge_sort(self, |a, b| f(a).lt(&f(b)));

[leonardo-m]

OK, right