Rustdoc performance improvements #151368

GuillaumeGomez · 2026-01-19T14:01:29Z

A few things I had sitting around for a while. Let's check what perf says about them

GuillaumeGomez · 2026-01-19T14:02:24Z

Rustdoc performance improvements

src/librustdoc/html/render/write_shared.rs

rust-bors · 2026-01-19T16:21:06Z

☀️ Try build successful (CI)
Build commit: aa5a68b (aa5a68b1f547922fb3ea589e3327c1cafd0e76b1, parent: 158ae9ee50d82b415fc9479575bc33f8985f4f1a)

rust-timer · 2026-01-19T17:12:58Z

Finished benchmarking commit (aa5a68b): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

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

Max RSS (memory usage)

Results (secondary -0.4%)

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)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 1.9%, secondary -2.6%)

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

	mean	range	count
Regressions ❌ (primary)	1.9%	[1.9%, 1.9%]	1
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-5.8%, -2.5%]	4
All ❌✅ (primary)	1.9%	[1.9%, 1.9%]	1

Binary size

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

Bootstrap: 474.261s -> 473.66s (-0.13%)
Artifact size: 383.28 MiB -> 383.28 MiB (-0.00%)

GuillaumeGomez · 2026-01-19T17:23:08Z

But it makes sense to allocate directly what you need and there is still one less string comparison. Well, whatever you prefer @yotamofek. Applied your suggestion as well.

GuillaumeGomez · 2026-01-19T20:01:04Z

Reverted the suggestion.

yotamofek · 2026-01-26T13:50:29Z

src/librustdoc/html/render/write_shared.rs

@@ -488,7 +490,15 @@ impl Hierarchy {
        let files = self.elems.borrow();
        let name = OrderedJson::serialize(self.elem.to_str().expect("invalid osstring conversion"))
            .unwrap();
-        let mut out = Vec::from([name]);
+        let needed_capacity = if !files.is_empty() {


This change seems very verbose, considering it doesn't show on perf. Other two changes are IMHO improvements, regardless of perf.
Maybe discard just this one?

GuillaumeGomez · 2026-01-26T13:52:08Z

Removed the pre-computed vec size.

yotamofek · 2026-01-26T13:54:24Z

src/librustdoc/html/render/write_shared.rs

-                if let Some(parent) = h.parent.upgrade() {
-                    h = parent;
+
+        while let Some(component) = components.next() {


just noticed there's a very small change in behavior here.
previous version would panic if path.components() doesn't yield any items.
not sure if it's worth preserving? (you could add a assert!(components.peek().is_some()) before the loop)

Good catch. Yeah let's go with an assert.

yotamofek · 2026-01-26T13:56:28Z

src/librustdoc/html/render/write_shared.rs

+                        h.elems.borrow_mut().insert(s.to_owned());
+                        break;
+                    }
+                    let next_h = {


nit: you could just do h = { ... }, instead of creating the new binding. up to you.

One less variable and line, sounds good to me. :)

yotamofek · 2026-01-26T13:57:02Z

src/librustdoc/html/render/write_shared.rs

+                Component::ParentDir => {
+                    if let Some(parent) = h.parent.upgrade() {
+                        h = parent;
+                    }
+                }


nit, up to you:

Suggested change

Component::ParentDir => {

if let Some(parent) = h.parent.upgrade() {

h = parent;

}

}

Component::ParentDir if let Some(parent) = h.parent.upgrade() => {

h = parent;

}

Shorter, I like it.

…:add_path`

rustbot · 2026-01-26T14:05:46Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

GuillaumeGomez · 2026-01-26T14:05:53Z

Applied improved code suggestions. :)

yotamofek · 2026-01-26T14:10:09Z

r=me once CI passes, sorry for the long response time with this one :D

GuillaumeGomez · 2026-01-26T16:24:27Z

@bors r=yotamofek rollup

rust-bors · 2026-01-26T16:24:30Z

📌 Commit bd45311 has been approved by yotamofek

It is now in the queue for this repository.

…uwer Rollup of 7 pull requests Successful merges: - #151290 (Recover from struct literals with placeholder or empty path) - #148187 (Remove uses of `&mut CmResolver`) - #151368 (Rustdoc performance improvements) - #151374 (some more rustc_borrowck cleanups) - #151536 (Fix sanitizer target builds on CI) - #151626 (Remove `Deref<Target = TyCtxt>` from `QueryCtxt`) - #151661 (Suggest changing `iter`/`into_iter` when the other was meant)

@yotamofek

Rollup merge of #151368 - GuillaumeGomez:librustdoc-perf, r=yotamofek Rustdoc performance improvements A few things I had sitting around for a while. Let's check what perf says about them r? @yotamofek

rustbot assigned yotamofek Jan 19, 2026

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rust-bors bot pushed a commit that referenced this pull request Jan 19, 2026

Auto merge of #151368 - GuillaumeGomez:librustdoc-perf, r=<try>

aa5a68b

Rustdoc performance improvements

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 19, 2026

yotamofek reviewed Jan 19, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 19, 2026

GuillaumeGomez force-pushed the librustdoc-perf branch from f660783 to 09235ca Compare January 19, 2026 17:22

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GuillaumeGomez force-pushed the librustdoc-perf branch from 09235ca to 22f7c1a Compare January 19, 2026 20:00

yotamofek reviewed Jan 26, 2026

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GuillaumeGomez force-pushed the librustdoc-perf branch from 22f7c1a to 290bb6c Compare January 26, 2026 13:51

yotamofek reviewed Jan 26, 2026

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GuillaumeGomez added 3 commits January 26, 2026 14:59

Remove a string comparison and reduce number of clones in `Hierarchy:…

3757ce6

…:add_path`

Replace regex with find calls

dd42a9f

Improve code

bd45311

GuillaumeGomez force-pushed the librustdoc-perf branch from 290bb6c to bd45311 Compare January 26, 2026 14:05

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 Jan 26, 2026

JonathanBrouwer mentioned this pull request Jan 26, 2026

Rollup of 7 pull requests #151701

Merged

rust-bors bot merged commit 621c712 into rust-lang:main Jan 27, 2026
11 checks passed

rustbot added this to the 1.95.0 milestone Jan 27, 2026

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Rustdoc performance improvements #151368

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Overall result: no relevant changes - no action needed

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