rustdoc-search: optimize unifyFunctionTypes

[notriddle]

Final profile output:
https://notriddle.com/rustdoc-html-demo-5/profile-4/index.html

This PR contains three commits that improve performance of this hot inner loop: reduces the number of allocations, a fast path for the 1-element basic query case, and reconstructing the multi-element query case to use recursion instead of an explicit backtracking array. It also adds new test cases that I found while working on this.

r? @GuillaumeGomez

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[GuillaumeGomez]

That's great, thanks a lot!

@bors r+ rollup

[bors]

📌 Commit a66972d has been approved by GuillaumeGomez

It is now in the queue for this repository.

[notriddle]

@bors r-

One more commit that I came up with while waiting for the review. Sorry for the noise!

[notriddle]

@GuillaumeGomez

Again, sorry about the noise. Here's the last commit, which switches from using a backtracking array to using functional recursion to implement the full algorithm:

• The "fast path" now becomes the "base case" that the recursive function builds upon. This results in a significant speed up for the query T, U, though it still doesn't perform very well.

• It reduces the amount of code by a lot. This full stack of PR's is now, on net, two lines of code added to search.js.

  $ git diff --stat 4d7f952a02d0bca67c98a6b74895b7e3fbe38341 src/librustdoc 
   src/librustdoc/html/static/js/search.js | 302 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----------------------------------------------------------------
   1 file changed, 152 insertions(+), 150 deletions(-)

• Here's the new profiler results: https://notriddle.com/rustdoc-html-demo-5/profile-4/index.html

[GuillaumeGomez]

New changes look good to me. Big question though: would it be possible to include your benchmark in the rust performance tool? I think it's becoming quite an important topic to have good performance when performing a search.

In any case, thanks for this PR!

@bors r+ rollup

[bors]

📌 Commit d82b374 has been approved by GuillaumeGomez

It is now in the queue for this repository.

[bors]

⌛ Testing commit d82b374 with merge 27794f9...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing 27794f9 to master...

[rust-timer]

Finished benchmarking commit (27794f9): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-3.8%, -2.7%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 676.326s -> 676.664s (0.05%)
Artifact size: 313.84 MiB -> 313.81 MiB (-0.01%)