perf(turbo-rcstr): Fix the fast codepath for RcStr::into_owned()

[bgw]

If we uniquely own the only reference to an RcStr, into_owned should just unwrap the Arc and return the inner value instead of cloning it.

It seems this optimization was accidentally broken at some point. Arc::try_unwrap(arc.clone()) is always going to return the non-unique Err value because the arc.clone() will always set the refcount >= 2.

This makes RcStr::into_owned significantly faster for large strings that we own a unique Arc for.

Includes a unit test and a benchmark to prevent regressing.

RcStr::map/unique       time:   [17.311 ns 17.539 ns 17.797 ns]
                        change: [-97.932% -97.855% -97.778%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 10 outliers among 100 measurements (10.00%)
  4 (4.00%) high mild
  6 (6.00%) high severe

RcStr::map/cloned       time:   [1.0424 µs 1.0529 µs 1.0655 µs]
                        change: [-1.2209% +1.1551% +3.8229%] (p = 0.39 > 0.05)
                        No change in performance detected.
Found 22 outliers among 100 measurements (22.00%)
  9 (9.00%) low severe
  7 (7.00%) low mild
  2 (2.00%) high mild
  4 (4.00%) high severe

Closes PACK-3858

[bgw]

• perf(turbo-rcstr): Fix the fast codepath for RcStr::into_owned() #75504 👈 (View in Graphite)
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[ijjk]

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General

	vercel/next.js canary	vercel/next.js bgw/avoid-clone-in-rcstr-into-owned	Change
buildDuration	20.2s	17.2s	N/A
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build cache Overall increase ⚠️

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Commit: bcd8de7

[ijjk]

Tests Passed

[bgw]

Merge activity

• Feb 3, 2:47 PM EST: A user merged this pull request with Graphite.