Avoid more NonNull-raw-NonNull roundtrips in Vec
r? the8472
The standard library in general has a lot of these round-trips from niched types to their raw innards and back. Such round-trips have overhead in debug builds since https://github.com/rust-lang/rust/pull/120594. I removed some such round-trips in that initial PR and I've been meaning to come back and hunt down more such examples (this is the last item on https://github.com/rust-lang/rust/issues/120848).
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