N.B. After fixing #10430, `ArcStr` became the fastest implementation
(and the gains were significantly reduced, down to 1-2%). See:
https://github.com/astral-sh/uv/pull/10453#issuecomment-2583344414.
## Summary
I tried out a variety of small string crates, but `Arc<str>`
outperformed them, giving a ~10% speed-up:
```console
❯ hyperfine "../arcstr lock" "../flexstr lock" "uv lock" "../arc lock" "../compact_str lock" --prepare "rm -f uv.lock" --min-runs 50 --warmup 20
Benchmark 1: ../arcstr lock
Time (mean ± σ): 304.6 ms ± 2.3 ms [User: 302.9 ms, System: 117.8 ms]
Range (min … max): 299.0 ms … 311.3 ms 50 runs
Benchmark 2: ../flexstr lock
Time (mean ± σ): 319.2 ms ± 1.7 ms [User: 317.7 ms, System: 118.2 ms]
Range (min … max): 316.8 ms … 323.3 ms 50 runs
Benchmark 3: uv lock
Time (mean ± σ): 330.6 ms ± 1.5 ms [User: 328.1 ms, System: 139.3 ms]
Range (min … max): 326.6 ms … 334.2 ms 50 runs
Benchmark 4: ../arc lock
Time (mean ± σ): 303.0 ms ± 1.2 ms [User: 301.6 ms, System: 118.4 ms]
Range (min … max): 300.3 ms … 305.3 ms 50 runs
Benchmark 5: ../compact_str lock
Time (mean ± σ): 320.4 ms ± 2.0 ms [User: 318.7 ms, System: 120.8 ms]
Range (min … max): 317.3 ms … 326.7 ms 50 runs
Summary
../arc lock ran
1.01 ± 0.01 times faster than ../arcstr lock
1.05 ± 0.01 times faster than ../flexstr lock
1.06 ± 0.01 times faster than ../compact_str lock
1.09 ± 0.01 times faster than uv lock
```
## Summary
This appears to be a consistent 1% performance improvement and should
also reduce memory quite a bit. We've also decided to use these for
markers, so it's nice to use the same optimization here.
```
❯ hyperfine "./uv pip compile --universal scripts/requirements/airflow.in" "./arcstr pip compile --universal scripts/requirements/airflow.in" --min-runs 50 --warmup 20
Benchmark 1: ./uv pip compile --universal scripts/requirements/airflow.in
Time (mean ± σ): 136.3 ms ± 4.0 ms [User: 139.1 ms, System: 241.9 ms]
Range (min … max): 131.5 ms … 149.5 ms 50 runs
Benchmark 2: ./arcstr pip compile --universal scripts/requirements/airflow.in
Time (mean ± σ): 134.9 ms ± 3.2 ms [User: 137.6 ms, System: 239.0 ms]
Range (min … max): 130.1 ms … 151.8 ms 50 runs
Summary
./arcstr pip compile --universal scripts/requirements/airflow.in ran
1.01 ± 0.04 times faster than ./uv pip compile --universal scripts/requirements/airflow.in
```
This should be essentially the exact same behaviour, but backon is a
total API redesign, so things had to be expressed slightly differently.
Overall I think the code is more readable, which is nice.
Fixes#10001
## Summary
This follows Ruff's design exactly: you can provide a version specifier
(like `>=0.5`), and we'll enforce it at runtime.
Closes https://github.com/astral-sh/uv/issues/8605.
uv gives priorities to packages by package name, not by virtual package
(`PubGrubPackage`). pubgrub otoh when prioritizing order the virtual
packages. When the order of virtual packages changes, uv changes its
resolutions and error messages. This means uv was depending on
implementation details of pubgrub's prioritization caching.
This broke with https://github.com/pubgrub-rs/pubgrub/pull/299, which
added a tiebreaker term that made pubgrub's sorting deterministic given
a deterministic ordering of allocating the packages (which happens the
first time pubgrub sees a package).
The new custom tiebreaker decreases the difference to upstream pubgrub.
