Auto merge of #21401 - kballard:optimize-shrink-to-fit, r=nikomatsakis

Don't reallocate when capacity is already equal to length

`Vec::shrink_to_fit()` may be called on vectors that are already the
correct length. Calling out to `reallocate()` in this case is a bad idea
because there is no guarantee that `reallocate()` won't allocate a new
buffer anyway, and based on performance seen in external benchmarks, it
seems likely that it is in fact reallocating a new buffer.

Before:

    test string::tests::bench_exact_size_shrink_to_fit         ... bench:        45 ns/iter (+/- 2)

After:

    test string::tests::bench_exact_size_shrink_to_fit         ... bench:        26 ns/iter (+/- 1)

0 files changed, 0 insertions, 0 deletions