perf(allocator/vec): remove SetLenOnDrop

[overlookmotel]

Simplify Vec::truncate and Vec::extend_with methods.

Unlike standard library, our Vec<'a, T> requires that T is not Drop. Therefore we don't need to worry about dropping elements.

There are likely many more places where we can make similar simplifications (optimizations). I've only done these 2 now in order to get rid of SetLenOnDrop. I wanted to get rid of it because it requires direct access to self.buf.len, which I'd like to make private in a later PR.

[overlookmotel]

• perf(allocator/vec): reduce pointer maths in Vec::extend_desugared #11087
• perf(allocator/vec): Vec::extend_desugared set len only at end #11082
• refactor(allocator/vec): access len and cap fields via getters/setters #11081
• refactor(allocator/vec): re-order methods #11080
• perf(allocator/vec): remove SetLenOnDrop #11079 👈 (View in Graphite)
• refactor(allocator/vec): limit scope of #[expect(clippy::cast_possible_truncation)] #11074
• perf(allocator/vec): remove alloc_guard from RawVec::from_raw_parts_in + clarify safety docs #11073
• docs(allocator/vec): correct safety comments for RawVec::append_elements #11072
• refactor(allocator/vec): clarify comment #11071
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11079 will not alter performance

_{Comparing 05-16-perf_allocator_vec_remove_setlenondrop_ (a6057c7) with main (f0b76e8)}

Summary

✅ 36 untouched benchmarks

[Dunqing]

Looks good! Your comments let me know better what SetLenOnDrop is going on.

[overlookmotel]

Merge activity

• May 17, 6:15 AM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 17, 10:52 AM EDT: overlookmotel added this pull request to the Graphite merge queue.
• May 17, 10:59 AM EDT: Merged by the Graphite merge queue.

[overlookmotel]

I'll fix the Miri fail. It appears not to be due to Miri flagging UB, but due to lint errors which have been added on nightly.

[overlookmotel]

Miri fail was unrelated. Fixed in #11092.