BTreeMap: disentangle Drop implementation from IntoIter

[ssomers]

No longer require every BTreeMap to dig up its last leaf edge before dying. This speeds up the clone_ benchmarks by 25% for normal keys and values (far less for huge values).

r? @Mark-Simulacrum

[ssomers]

Last commit conforms to #81073, dismisses two ManualDrop, introduces finalize.

[ssomers]

I wasn't happy with the rather primitive code in finalize that belongs in navigate.rs, so ended up slightly rearranging the deallocating code. Now the benchmarks are more spectacular, though not for huge values:

benchcmp old new --threshold 10
 name                                           old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::clone_fat_100                      45,646       72,143             26,497   58.05%   x 0.63
 btree::map::clone_fat_100_and_clear            45,567       81,445             35,878   78.74%   x 0.56
 btree::map::clone_fat_100_and_drain_half       106,640      124,207            17,567   16.47%   x 0.86
 btree::map::clone_fat_val_100                  23,640       38,426             14,786   62.55%   x 0.62
 btree::map::clone_fat_val_100_and_clear        23,687       41,178             17,491   73.84%   x 0.58
 btree::map::clone_fat_val_100_and_drain_half   48,727       57,688              8,961   18.39%   x 0.84
 btree::map::clone_fat_val_100_and_into_iter    34,402       41,846              7,444   21.64%   x 0.82
 btree::map::clone_fat_val_100_and_remove_half  52,027       61,435              9,408   18.08%   x 0.85
 btree::map::clone_slim_100                     2,184        1,696                -488  -22.34%   x 1.29
 btree::map::clone_slim_100_and_clear           5,088        1,670              -3,418  -67.18%   x 3.05
 btree::map::clone_slim_100_and_drain_all       6,518        3,560              -2,958  -45.38%   x 1.83
 btree::map::clone_slim_100_and_drain_half      5,963        2,722              -3,241  -54.35%   x 2.19
 btree::map::clone_slim_100_and_into_iter       5,100        1,655              -3,445  -67.55%   x 3.08
 btree::map::clone_slim_100_and_pop_all         6,765        3,803              -2,962  -43.78%   x 1.78
 btree::map::clone_slim_100_and_remove_all      7,826        4,944              -2,882  -36.83%   x 1.58
 btree::map::clone_slim_100_and_remove_half     6,131        3,031              -3,100  -50.56%   x 2.02
 btree::map::clone_slim_10k                     262,765      201,860           -60,905  -23.18%   x 1.30
 btree::map::clone_slim_10k_and_clear           262,713      204,480           -58,233  -22.17%   x 1.28
 btree::map::clone_slim_10k_and_into_iter       261,446      206,657           -54,789  -20.96%   x 1.27
 btree::map::clone_slim_10k_and_remove_half     574,250      512,200           -62,050  -10.81%   x 1.12
 btree::map::find_seq_100                       15           12                     -3  -20.00%   x 1.25
 btree::map::first_and_last_100                 47           42                     -5  -10.64%   x 1.12
 btree::set::clone_100                          1,883        1,352                -531  -28.20%   x 1.39
 btree::set::clone_100_and_clear                1,920        1,384                -536  -27.92%   x 1.39
 btree::set::clone_100_and_into_iter            1,928        1,410                -518  -26.87%   x 1.37
 btree::set::clone_100_and_remove_half          2,685        2,248                -437  -16.28%   x 1.19
 btree::set::clone_10k                          225,550      157,252           -68,298  -30.28%   x 1.43
 btree::set::clone_10k_and_clear                224,802      165,220           -59,582  -26.50%   x 1.36
 btree::set::clone_10k_and_into_iter            224,367      167,968           -56,399  -25.14%   x 1.34
 btree::set::clone_10k_and_remove_half          501,320      447,430           -53,890  -10.75%   x 1.12
 btree::set::difference_random_100_vs_10k       2,261        2,830                 569   25.17%   x 0.80
 btree::set::difference_staggered_100_vs_10k    2,804        2,150                -654  -23.32%   x 1.30
 btree::set::intersection_random_100_vs_10k     2,136        2,650                 514   24.06%   x 0.81
 btree::set::intersection_random_10k_vs_100     2,163        2,576                 413   19.09%   x 0.84
 btree::set::intersection_staggered_100_vs_10k  2,597        1,980                -617  -23.76%   x 1.31
 btree::set::is_subset_100_vs_10k               1,619        1,182                -437  -26.99%   x 1.37

[bors]

☔ The latest upstream changes (presumably #81853) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

⌛ Trying commit 4230d999ee1d76ab09380e172dc418f9f925be6b with merge c3d1be590b1ba9ceb83fe3737f5cf2e767f72377...

[bors]

☀️ Try build successful - checks-actions
Build commit: c3d1be590b1ba9ceb83fe3737f5cf2e767f72377 (c3d1be590b1ba9ceb83fe3737f5cf2e767f72377)

[rust-timer]

Queued c3d1be590b1ba9ceb83fe3737f5cf2e767f72377 with parent 0fc6756, future comparison URL.

[rust-timer]

Finished benchmarking try commit (c3d1be590b1ba9ceb83fe3737f5cf2e767f72377): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

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

[ssomers]

Rebased and tweaked a few comments.

[Mark-Simulacrum]

@bors r+

Performance seems largely "noise" I guess, hard to say precisely. Change seems good from sort of "first principles" and I like that it removes the macros, at least partially; that seems like a win.

[bors]

📌 Commit 3045b75 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 3045b75 with merge a118ee2...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing a118ee2 to master...