fix quadratic naming of duplicate sidebar links by xonx4l · Pull Request #162976 · rust-lang/rust

xonx4l · 2026-09-18T16:05:27Z

This PR fixes quadratic naming of duplicate sidebar links as discussed in #158174.

get_next_url gives duplicate sidebar links with unique names but it started counting from 1 every time which makes the page quadratic.

So to make it more optimal I changed that to remember the next unique number for each name making it linear per page.

Also added the test at tests/rustdoc-html/deref/sidebar-links-deref-chain.rs

Results

N	Instructions (before)	Instructions (after)	Time (before)	Time (after)
200	5.71 B	3.69 B	1.20 s	1.01 s
400	22.40 B	9.52 B	3.79 s	1.98 s
800	126.83 B	30.82 B	19.68 s	6.04 s
1600	—	—	118.59 s	37.76 s

r? @GuillaumeGomez

GuillaumeGomez · 2026-09-18T18:30:01Z

Implementation looks good to me. Let's confirm the perf gain.

Also I'd like @lolbinarycat to have a look as she worked on this recently (iirc).

@bors try @rust-timer queue

fix quadratic naming of duplicate sidebar links

rust-bors · 2026-09-18T19:31:10Z

☀️ Try build successful (CI)
Build commit: cc18f18 (cc18f1802395cbbddd322ff59434f18580ea7cc7)
Base parent: 420ed2a (420ed2a0c3d7225b1744266fd884d431b4d8cfe0)

lolbinarycat · 2026-09-18T19:58:36Z

@@ -0,0 +1,21 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/158174>.


I feel like this should be a rustc-perf benchmark, not a regular test, since this is a performance improvement, not a bugfix. @GuillaumeGomez what do you think?

View changes since the review

Why not both? =D

But yes, very good idea!

lolbinarycat · 2026-09-18T20:11:39Z

+struct UsedLinks {
+    links: FxHashSet<String>,
+    /// Next number to try for each anchor.
+    next_suffix: FxHashMap<String, usize>,
+}
+
+fn get_next_url(used_links: &mut UsedLinks, url: String) -> String {
+    if used_links.links.insert(url.clone()) {
        return url;
    }
-    let mut add = 1;
-    while !used_links.insert(format!("{url}-{add}")) {
-        add += 1;
+    let add = used_links.next_suffix.entry(url.clone()).or_insert(1);
+    loop {
+        let candidate = format!("{url}-{add}");
+        *add += 1;
+        if used_links.links.insert(candidate.clone()) {
+            return candidate;
+        }
    }


Unless I'm missing an edge case somehow, this should be doable with 1 hashmap and no loop.

Suggested change

struct UsedLinks {

links: FxHashSet<String>,

/// Next number to try for each anchor.

next_suffix: FxHashMap<String, usize>,

}

fn get_next_url(used_links: &mut UsedLinks, url: String) -> String {

if used_links.links.insert(url.clone()) {

return url;

}

let mut add = 1;

while !used_links.insert(format!("{url}-{add}")) {

add += 1;

let add = used_links.next_suffix.entry(url.clone()).or_insert(1);

loop {

let candidate = format!("{url}-{add}");

*add += 1;

if used_links.links.insert(candidate.clone()) {

return candidate;

}

}

type UsedLinks = FxHashMap<String, usize>;

fn get_next_url(used_links: &mut UsedLinks, url: String) -> String {

let count = used_links.entry(url.clone()).or_insert(0);

let res = if count == 0 { url } else { format!("{url}-{count}") };

*count += 1;

res

}

View changes since the review

Yes, a counter is enough and better indeed . Will make the change with a bit of correctness here ( *count == 0) as in the suggested snippet it compares a &mut usize with a number . Which won't compile . Thank you for the review!

rust-timer · 2026-09-18T20:14:40Z

Finished benchmarking commit (cc18f18): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

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

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 3.8%, secondary -3.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.8%	[3.8%, 3.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	3.8%	[3.8%, 3.8%]	1

Cycles

Results (secondary -3.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	71
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	53
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	71

Bootstrap: 498.333s -> 499.277s (0.19%)
Artifact size: 408.93 MiB -> 408.87 MiB (-0.01%)

GuillaumeGomez · 2026-09-18T20:16:59Z

Hum... Do we even have a test to stress test the sidebar? Should we add the new sidebar perf test?

lolbinarycat · 2026-09-18T23:59:40Z

Hum... Do we even have a test to stress test the sidebar?

it doesn't seem like it

Should we add the new sidebar perf test?

yes, that would be good, maybe even add a larger test using the python script in the original issue.

Kobzol · 2026-09-19T09:42:37Z

If the perf. regression can be shown with a simple crate, I think it would be fine to add it as a stress test benchmark to rustc-perf.

xonx4l · 2026-09-19T17:47:16Z

Thanks! I'll open a follow-up rustc-perf PR with a stress benchmark based on the script from #158174 .

GuillaumeGomez · 2026-09-19T20:38:25Z

Please add the new perf test first, like that we can have a more precise view of the impact of this PR (in addition to the numbers you already provided).

Kobzol · 2026-09-23T14:38:07Z

After the next PR is merged by bors, we can do a perf. run here, rustc-perf should now contain the benchmark.

GuillaumeGomez · 2026-09-23T14:39:59Z

Since it contains the perf, can't we do it now?

Kobzol · 2026-09-23T14:49:34Z

Not really, because there would be no parent main commit to compare to. Or rather, the parent would not contain the new benchmark yet.

GuillaumeGomez · 2026-09-23T14:53:15Z

We just need a new merge in-between then? So we can wait for next merge on rust. Or I'm missing something obvious. ^^'

Kobzol · 2026-09-23T14:59:58Z

Yes, that's in fact what I meant by "After the next PR is merged by bors" 😆

GuillaumeGomez · 2026-09-23T15:01:43Z

Ok so we were saying the same thing. Perfect! 🤣

Kobzol · 2026-09-23T18:05:57Z

@bors try @rust-timer queue

fix quadratic naming of duplicate sidebar links

rust-bors · 2026-09-23T19:05:27Z

☀️ Try build successful (CI)
Build commit: 16d9f56 (16d9f566ab8b344e190a48387562ae5e90e6da9a)
Base parent: a92214c (a92214c92df8cc0261f5ab5c08c6376c2c03a958)

rust-timer · 2026-09-23T20:03:43Z

Finished benchmarking commit (16d9f56): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-82.5%	[-82.5%, -82.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -1.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.8%, secondary -33.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	4.2%	[4.2%, 4.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-72.1%	[-72.1%, -72.1%]	1
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 501.272s -> 489.678s (-2.31%)
Artifact size: 406.37 MiB -> 406.39 MiB (0.00%)

GuillaumeGomez · 2026-09-23T20:42:13Z

Wow. Now that's an impressive improvement. Congrats!

@bors r=lolbinarycat,GuillaumeGomez rollup=iffy

rust-bors · 2026-09-23T20:42:16Z

📌 Commit 1875276 has been approved by lolbinarycat,GuillaumeGomez

It is now in the queue for this repository.

@GuillaumeGomez

…r=lolbinarycat,GuillaumeGomez fix quadratic naming of duplicate sidebar links This PR fixes quadratic naming of duplicate sidebar links as discussed in rust-lang#158174. Fixes rust-lang#158174 `get_next_url` gives duplicate sidebar links with unique names but it started counting from 1 every time which makes the page quadratic. So to make it more optimal I changed that to remember the next unique number for each name making it linear per page. Also added the test at `tests/rustdoc-html/deref/sidebar-links-deref-chain.rs` ### Results | N | Instructions (before) | Instructions (after) | Time (before) | Time (after) | |-----:|----------------------:|---------------------:|--------------:|-------------:| | 200 | 5.71 B | 3.69 B | 1.20 s | 1.01 s | | 400 | 22.40 B | 9.52 B | 3.79 s | 1.98 s | | 800 | 126.83 B | 30.82 B | 19.68 s | 6.04 s | | 1600 | — | — | 118.59 s | 37.76 s | r? @GuillaumeGomez

Rollup of 8 pull requests Successful merges: - #162976 (fix quadratic naming of duplicate sidebar links) - #163222 (Enable EII tests for cg_gcc) - #162942 (Remove `StashKey::AssociatedTypeSuggestion`) - #163096 (Don't suggest `std::` rustfix paths in `#![no_std]` crates) - #163110 (Mark `std::os::wasip2` with correct doc-cfgs, mark as unstable) - #163185 (properly decrement available_depth on cycles and provisional cache hits) - #163214 (revert r14 register names for arm) - #163226 (miri subtree update)

…uwer Rollup of 14 pull requests Successful merges: - #162976 (fix quadratic naming of duplicate sidebar links) - #161275 (Refactor `core::cmp::{smallest, largest}` & add `mir-opt` test) - #163143 (cg_llvm: Use fewer FFI calls to check the target CPU's features) - #163188 (Adjust for Arm64EC name mangling when checking for exported symbols) - #163211 (`rustc_builtin_macros` cleanup, part 6) - #161386 (Don't merge distinct impl candidates) - #162942 (Remove `StashKey::AssociatedTypeSuggestion`) - #163096 (Don't suggest `std::` rustfix paths in `#![no_std]` crates) - #163110 (Mark `std::os::wasip2` with correct doc-cfgs, mark as unstable) - #163185 (properly decrement available_depth on cycles and provisional cache hits) - #163214 (revert r14 register names for arm) - #163226 (miri subtree update) - #163228 (Add regression test for trait predicate with escaping bounds) - #163234 (`rustc_dump_symbol_name`: add demangling information as a note instead)

@GuillaumeGomez

Rollup merge of #162976 - xonx4l:rustdoc-sidebar-quadratic, r=lolbinarycat,GuillaumeGomez fix quadratic naming of duplicate sidebar links This PR fixes quadratic naming of duplicate sidebar links as discussed in #158174. Fixes #158174 `get_next_url` gives duplicate sidebar links with unique names but it started counting from 1 every time which makes the page quadratic. So to make it more optimal I changed that to remember the next unique number for each name making it linear per page. Also added the test at `tests/rustdoc-html/deref/sidebar-links-deref-chain.rs` ### Results | N | Instructions (before) | Instructions (after) | Time (before) | Time (after) | |-----:|----------------------:|---------------------:|--------------:|-------------:| | 200 | 5.71 B | 3.69 B | 1.20 s | 1.01 s | | 400 | 22.40 B | 9.52 B | 3.79 s | 1.98 s | | 800 | 126.83 B | 30.82 B | 19.68 s | 6.04 s | | 1600 | — | — | 118.59 s | 37.76 s | r? @GuillaumeGomez

rust-timer · 2026-09-24T19:57:45Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (303bef3): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-82.5%	[-82.5%, -82.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

Results (secondary -34.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-72.0%	[-72.0%, -72.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: missing data
Artifact size: 406.30 MiB -> 406.32 MiB (0.01%)

…uwer Rollup of 14 pull requests Successful merges: - rust-lang/rust#162976 (fix quadratic naming of duplicate sidebar links) - rust-lang/rust#161275 (Refactor `core::cmp::{smallest, largest}` & add `mir-opt` test) - rust-lang/rust#163143 (cg_llvm: Use fewer FFI calls to check the target CPU's features) - rust-lang/rust#163188 (Adjust for Arm64EC name mangling when checking for exported symbols) - rust-lang/rust#163211 (`rustc_builtin_macros` cleanup, part 6) - rust-lang/rust#161386 (Don't merge distinct impl candidates) - rust-lang/rust#162942 (Remove `StashKey::AssociatedTypeSuggestion`) - rust-lang/rust#163096 (Don't suggest `std::` rustfix paths in `#![no_std]` crates) - rust-lang/rust#163110 (Mark `std::os::wasip2` with correct doc-cfgs, mark as unstable) - rust-lang/rust#163185 (properly decrement available_depth on cycles and provisional cache hits) - rust-lang/rust#163214 (revert r14 register names for arm) - rust-lang/rust#163226 (miri subtree update) - rust-lang/rust#163228 (Add regression test for trait predicate with escaping bounds) - rust-lang/rust#163234 (`rustc_dump_symbol_name`: add demangling information as a note instead)

fix quadratic naming of duplicate sidebar links

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rustbot assigned GuillaumeGomez Sep 18, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 18, 2026

Auto merge of #162976 - xonx4l:rustdoc-sidebar-quadratic, r=<try>

cc18f18

fix quadratic naming of duplicate sidebar links

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lolbinarycat reviewed Sep 18, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 18, 2026

xonx4l mentioned this pull request Sep 21, 2026

Add deref-sidebar-stress benchmark. rust-lang/rustc-perf#2587

Merged

simplify get_next_url to a single counter map

1875276

Dnreikronos mentioned this pull request Sep 23, 2026

trait_solver: Preserve concrete candidates in ambiguous ORs #162442

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rust-bors Bot pushed a commit that referenced this pull request Sep 23, 2026

Auto merge of #162976 - xonx4l:rustdoc-sidebar-quadratic, r=<try>

16d9f56

fix quadratic naming of duplicate sidebar links

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 23, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 23, 2026

jhpratt mentioned this pull request Sep 23, 2026

Rollup of 8 pull requests #163236

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JonathanBrouwer mentioned this pull request Sep 24, 2026

Rollup of 14 pull requests #163244

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rust-bors Bot merged commit b255ecd into rust-lang:main Sep 24, 2026
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rustbot added this to the 1.100.0 milestone Sep 24, 2026

		@@ -0,0 +1,21 @@
		// Regression test for <https://github.com/rust-lang/rust/issues/158174>.

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rust-timer commented Sep 18, 2026

Overall result: no relevant changes - no action needed

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Overall result: ✅ improvements - no action needed

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Overall result: ✅ improvements - no action needed

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