RawVec: don't recompute capacity after allocating.

[nnethercote]

Currently it sets the capacity to ptr.len() / mem::size_of::<T>()
after any buffer allocation/reallocation. This would be useful if
allocators ever returned a NonNull<[u8]> with a size larger than
requested. But this never happens, so it's not useful.

Removing this slightly reduces the size of generated LLVM IR, and
slightly speeds up the hot path of RawVec growth.

r? @ghost

[nnethercote]

@bors try @rust-timer queue

I expect this will be tiny wins at best, it's more about the code cleanliness.

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 8217138 with merge bb1b81b9cddfac8bb2c85068496511df09d999bc...

[bors]

☀️ Try build successful - checks-actions
Build commit: bb1b81b9cddfac8bb2c85068496511df09d999bc (bb1b81b9cddfac8bb2c85068496511df09d999bc)

[rust-timer]

Queued bb1b81b9cddfac8bb2c85068496511df09d999bc with parent e983092, future comparison URL.

[rust-timer]

Finished benchmarking commit (bb1b81b9cddfac8bb2c85068496511df09d999bc): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[nnethercote]

If you deselect "Filter out very small changes" in the perf results you see lots of tiny wins (< 0.3%), as expected.

[joshtriplett]

@bors r+

[bors]

📌 Commit 8217138 has been approved by joshtriplett

[bors]

⌛ Testing commit 8217138 with merge e6f1f04...

[bors]

☀️ Test successful - checks-actions
Approved by: joshtriplett
Pushing e6f1f04 to master...

[rust-timer]

Finished benchmarking commit (e6f1f04): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression