Avoid passing search path (-L) args when they are passed as --extern by ranger-ross · Pull Request #17410 · rust-lang/cargo

ranger-ross · 2026-08-30T04:17:24Z

What does this PR try to resolve?

This PR reduces the -L args passed rustc by omitting direct dependencies as they are passed via --extern. (which makes also passing via -L unnecessary)

See #t-cargo > Reducing number of arguments passed with the new build dir for context.

I tested this change on a handful of open source projects to see the results and this generally reduces the amount of args by 5-20%.

Notable results:

Zed reduced top level crate args from 1189 to 1136. A 5% reduction
Vector reduced top level crate args from 934 to 867. A 7% reduction

How to test and review this PR?

Added a new test to explicitly verify this behavior.

r? @weihanglo

cc: @Kobzol @petrochenkov

Kobzol

Thanks for working on this! The logic looks simple enough, though the double checks for proc macros/build scripts are a bit annoying.

This is more a question for Vadim, but I assume that the order of the -L flags is not important?

View changes since this review

Kobzol · 2026-08-30T05:47:58Z

+            if v.target.proc_macro() {
+                continue;
+            }
+            if !visited.contains(v) {


With this contains:

Already visited: hashmap lookup

Not visited: hashmap lookup + stack push + clone of Unit + hashmap lookup in the next iter

Without the contains:

Already visited: stack push + clone of Unit + hashmap lookup in the next iter

Not visited: stack push + clone of Unit + hashmap lookup in the next iter

With switching this to insert, and removing the insert from start of the loop:

Already visited: hashmap lookup + clone of Unit

Not visited: hashmap lookup + clone of Unit

Probably there are more unvisited hits than visited hits, and the lookup isn't that expensive. Just wanted to note some alternatives. I can try benchmarking it later.

Nevermind, doing the path cloning/sorting is way more expensive than this loop on Zed. No need to look into this further, IMO.

petrochenkov · 2026-08-30T10:00:37Z

I assume that the order of the -L flags is not important?

The lookup is supposed to search the directories in the order of -L flags.
For native libraries it's important, but IIRC for Rust libraries there was an ambiguity error if there are several candidates in different directories, although I'm not sure, need to check the code.

rustbot · 2026-08-31T06:47:57Z

This PR was rebased onto a different master 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.

@ranger-ross

Looked into this while profiling rust-lang#17410 ([context](https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/Reducing.20number.20of.20arguments.20passed.20with.20the.20new.20build.20dir/with/620216426)). Those two changes reduce the total duration of collecting dep dirs from ~56ms to ~40ms on Zed. Not a huge win, but for such a small change.. The `format!` change is a bit annoying (there the win was ~5ms), but the `build_dir` change is IMO straightforward and just worth it to reduce an unnecessary `PathBuf` copy (there the win was ~10ms). r? @ranger-ross

weihanglo

Looks good

View changes since this review

weihanglo · 2026-08-31T16:02:13Z

+    //
+    // FIXME: Remove the `-L dependency=[ROOT]/foo/target/debug/build/my-direct-dep/[HASH]/out` once fixed
+    p.cargo("-v build")
+        .enable_mac_dsym()


I probably forgot. Why is enable_mac_dsym needed?

I am also trying to remember why, I always add it because I copy/paste existing testing as a skeleton when writing.

If I recall correctly, the split debug info files on macos have really nasty names that make them not play nice with the build-dir file-system snapshot testing tool.
But its probably not needed here. If the commit message from a4f0988 is to believed it should be faster?
I'll remove it and see if CI passes

weihanglo · 2026-08-31T16:23:00Z

+    // directly. So instead we only sort valid Utf8 paths as &str's which is a bit faster.
+    // We do not allow non utf8 paths when building, so we take advantage of that by treating them
+    // as a single value when sorting/deduplicating.
+    paths.sort_unstable_by(|a, b| match (a.to_str(), b.to_str()) {


Not sure if we want to include some links and numbers here for future references.

(git blame helps that, though it is a bit cumbersome to use)

weihanglo

@ranger-ross feel free to @rustbot merge when you feel it is ready :)

View changes since this review

…xtern

ranger-ross · 2026-09-01T13:01:44Z

@rustbot merge

Update cargo submodule 26 commits in e8cb624d5701824f46a2ec5873cfd59ee3d2f66c..b2e9d5f9db3fb1c454ab84f10c16508984a266e2 2026-08-22 00:23:45 +0000 to 2026-09-02 14:49:16 +0000 - fix(parser): Resolve theoretical use-after-free (rust-lang/cargo#17428) - fix(trim-paths)!: remove default scope from release profile (rust-lang/cargo#17424) - fix(git): Use git's 429 retry, when available (rust-lang/cargo#17422) - Avoid passing search path (-L) args when they are passed as --extern (rust-lang/cargo#17410) - chore(deps): update crate-ci/typos action to v1.50.0 (rust-lang/cargo#17417) - test: Move -Z onto its own line (rust-lang/cargo#17416) - chore(triagebot): enable `@rustbot merge/delegate` (rust-lang/cargo#17415) - Micro-optimize two package dir functions (rust-lang/cargo#17413) - perf: Do not build SBOM if user has not set build.sbom (rust-lang/cargo#17412) - feat(manifest)!: implement feature-metadata RFC3416 (rust-lang/cargo#15056) - Cargo profiling improvements (rust-lang/cargo#17411) - test(git): Remove gix override run in CI and the mode in code (rust-lang/cargo#17405) - perf(git): Reduce extra work when using git-cli (rust-lang/cargo#17406) - feat(resolver): Stabilize min-publish-age (rust-lang/cargo#17335) - fix(git): Remove ref status update when showing progress (rust-lang/cargo#17400) - revert: refactor: move sysroot lookup to GlobalContext (rust-lang/cargo#17401) - fix(run): Printing a new line to avoid overwriting error code after \r (rust-lang/cargo#17373) - fix(trim-paths): custom workspace-relative member paths remap (rust-lang/cargo#17366) - fix(home): rustdoc lint (rust-lang/cargo#17394) - feat(diag): Stabilize cargo-lints (rust-lang/cargo#17298) - chore(deps): Update partial_ref to v0.3.4 (rust-lang/cargo#17392) - refactor: remove ad-hoc `subslice_range` (rust-lang/cargo#17390) - docs(changelog): move build-dir new layout to Changed (rust-lang/cargo#17387) - chore(deps): update msrv (1 version) to v1.98 (rust-lang/cargo#17386) - docs: Use mdbook admonitions (rust-lang/cargo#17384) - chore(ci): exclude resolver-tests from intra doc link checks (rust-lang/cargo#17385)

@ranger-ross

Looked into this while profiling rust-lang/cargo#17410 ([context](https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/Reducing.20number.20of.20arguments.20passed.20with.20the.20new.20build.20dir/with/620216426)). Those two changes reduce the total duration of collecting dep dirs from ~56ms to ~40ms on Zed. Not a huge win, but for such a small change.. The `format!` change is a bit annoying (there the win was ~5ms), but the `build_dir` change is IMO straightforward and just worth it to reduce an unnecessary `PathBuf` copy (there the win was ~10ms). r? @ranger-ross

rustbot assigned weihanglo Aug 30, 2026

rustbot added A-build-execution Area: anything dealing with executing the compiler S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 30, 2026

Kobzol reviewed Aug 30, 2026

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test: Enable dylib search path tests on stable

5d69ed7

ranger-ross force-pushed the deduplicate-rustc-args branch from 14a65d4 to afd71df Compare August 31, 2026 06:47

Kobzol mentioned this pull request Aug 31, 2026

Micro-optimize two package dir functions #17413

Merged

weihanglo approved these changes Aug 31, 2026

View reviewed changes

weihanglo approved these changes Sep 1, 2026

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ranger-ross added 3 commits September 1, 2026 21:53

test: Added test for direct dependencies search path args

005affe

perf: Avoid passing search path (-L) args when they are passed as --e…

86e7c16

…xtern

perf: Optimized search paths sorting and deduplicating

5334f1f

ranger-ross force-pushed the deduplicate-rustc-args branch from 6f618bd to 5334f1f Compare September 1, 2026 12:53

rustbot enabled auto-merge September 1, 2026 13:01

rustbot added this pull request to the merge queue Sep 1, 2026

Merged via the queue into rust-lang:master with commit 3167045 Sep 1, 2026
28 checks passed

rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 1, 2026

weihanglo mentioned this pull request Sep 1, 2026

Update cargo submodule rust-lang/rust#161789

Merged

rustbot added this to the 1.100.0 milestone Sep 2, 2026

ranger-ross mentioned this pull request Sep 4, 2026

Cargo cross workspace cache rust-lang/goals#626

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Avoid passing search path (-L) args when they are passed as --extern - #17410

Avoid passing search path (-L) args when they are passed as --extern#17410
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