Reduce rustc `-L` args used in the new `build-dir` layout by ranger-ross · Pull Request #17168 · rust-lang/cargo

ranger-ross · 2026-07-02T11:20:51Z

What does this PR try to resolve?

With the new build-dir enabled in rust-lang/rust there are have been a few reported issues due to long commands / PATH.
This PR attempts to mitigate this by doing the following:

Fix a bug where the current build unit's out dir would be added the rustc args even though it is never needed. (due to the first iteration in the recursive add_dep_arg fn).
- This probably wont make much different for long PATHs but it should not passed.
Avoid passing build-script build unit's out dir which contains the build-script binary as it would very odd to link against a build script. This also has the effect of not recursively including other [build-dependencies] the build-script unit depended on.

I included this both in the PR as they are small and related but happy to split into dedicated PRs if desired :)

How to test and review this PR?

See the included tests. Best reviewed commit by commit

cc: @Kobzol

rustbot · 2026-07-02T11:20:55Z

r? @epage

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Owners of files modified in this PR: @ehuss, @epage, @weihanglo
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Kobzol · 2026-07-02T12:04:55Z

Maybe as another optimization (because each dependency with a build script will generate two -Ls even after this PR), Cargo could check if the build script's $OUT_DIR is empty, and if yes, in that case do not generate the -L for the out directory of the build script's execution? 🤔

Kobzol · 2026-07-02T19:28:43Z

I think that another -L that we could skip is the out directory of each direct dependency of the compiled crate. Consider a hello world crate foo that depends e.g. on log and env_logger, using the new build dir layout. It generates this final rustc call (I omitted some flags):

rustc --crate-name foo --edition=2024 src/main.rs --crate-type bin
--emit=dep-info,link -C embed-bitcode=no -C debuginfo=2
--out-dir /tmp/foo/target/debug/build/foo/ea1ee4071e58254b/out
-C incremental=/tmp/foo/target/debug/incremental
-L dependency=/tmp/foo/target/debug/build/aho-corasick/c53d038be9b9715d/out
-L dependency=/tmp/foo/target/debug/build/anstream/4bdda5cd8d2978b2/out
-L dependency=/tmp/foo/target/debug/build/log/a7c7b57a5f6aeb19/out
-L dependency=/tmp/foo/target/debug/build/env_logger/c5ba55a17507a07e/out 
--extern env_logger=/tmp/foo/target/debug/build/env_logger/c5ba55a17507a07e/out/libenv_logger-c5ba55a17507a07e.rlib
--extern log=/tmp/foo/target/debug/build/log/a7c7b57a5f6aeb19/out/liblog-a7c7b57a5f6aeb19.rlib

Notice how cargo passes both -L dependency=log and --extern log=log.rlib. Is that actually necessary? It shouldn't be for the .rlib file itself, because when you pass something via --extern, it does not actually need -L (see this comment).

So if that is the case, we could skip each -L flag for a dependency that is already passed through --extern, and only use -L for indirect dependencies. That being said, I'm not sure if there can be something else in the direct dependency's out directory that rustc might have to depend on through -L. Do you know? I'll also try to ask bjorn or Vadim.

Kobzol · 2026-07-02T20:00:40Z

I tested it both with depending on .rlib and dylib (.so), with and without -Zno-embed-metadata, and everything seems to work even without the -L for direct dependencies.

… to rustc

rustbot · 2026-07-08T13:08:25Z

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 · 2026-07-08T13:17:03Z

dont merge this yet, I am still testing it and I think there might be some issues hidden in other tests since -Zbuild-dir-new-layout no longer defaults to ON.

@rustbot author

ranger-ross · 2026-07-08T13:56:24Z

Notice how cargo passes both -L dependency=log and --extern log=log.rlib. Is that actually necessary? It shouldn't be for the .rlib file itself, because when you pass something via --extern, it does not actually need -L (see this comment).

So if that is the case, we could skip each -L flag for a dependency that is already passed through --extern, and only use -L for indirect dependencies. That being said, I'm not sure if there can be something else in the direct dependency's out directory that rustc might have to depend on through -L. Do you know? I'll also try to ask bjorn or Vadim.

Oh interesting, I am not sure. That sounds like a reasonable optimization we could make if we indeed don't need to depend on anything through -L for direct deps

ranger-ross · 2026-07-08T14:45:57Z

I tried removing -L args from direct dependencies and the Cargo testsuite passed (minus a few tests due to cmd line arg snapshots)
So might be possible to do, though maybe better to do that in its own PR.

I finished checking the remaining tests and I believe the latest version is now correct and all of the tests pass.

@rustbot ready

@weihanglo

Backports: * #17191 * #17168 * #17179 Was only planning to backport #17191 but it builds on tests added in #17168 so was getting merge conflicts. Given they are both bug fixes and the new build-dir layout is still unstable, I figured it would be simpler to backport both fixes. r? @weihanglo

Update cargo submodule 29 commits in 59800466c5c41c444d264b1010b4d57e85a7117f..3efb1f477e99b42974b982d939fd100303cdf7db 2026-07-07 15:52:22 +0000 to 2026-07-17 23:53:19 +0000 - refactor(context): normalize in `homedir` instead (rust-lang/cargo#17222) - chore(ci): reflect doc folder move in book deployment (rust-lang/cargo#17235) - refactor(ops): Have cargo-metadata's ops match the command name (rust-lang/cargo#17233) - chore: Flatten src (rust-lang/cargo#17231) - chore: Flatten `src` (rust-lang/cargo#17230) - chore(ci): remove stale libsecret packages (rust-lang/cargo#17229) - perf: Lazily initialize git2 fetch transports (rust-lang/cargo#17226) - test(trim-paths): re-enable lldb debugger tests (rust-lang/cargo#17223) - Include SBOM outputs in fingerprints (rust-lang/cargo#17216) - Update cfg_aliases to 0.2.2 (rust-lang/cargo#17225) - test(trim-paths): exercise GDB on windows-gnu (rust-lang/cargo#17221) - feat(profile): Disable incremental compilation under CI by default (rust-lang/cargo#17220) - Remove myself from review rotation (rust-lang/cargo#17219) - Fix typo in comment in sync (rust-lang/cargo#17217) - docs(ref): Improve handing of built-in profiles (rust-lang/cargo#17213) - fix: dont apply host-config gating to stable behavior (rust-lang/cargo#17198) - chore(deps): update msrv (rust-lang/cargo#17192) - test: fix race in cargo_compile_with_invalid_code_in_deps (rust-lang/cargo#17203) - Rename `-Zno-embed-metadata` to `-Zembed-metadata=no` (rust-lang/cargo#17149) - fix(source): incorrect duplicate package warning (rust-lang/cargo#17204) - Fix manifest schema generation: `TomlDebugInfo` enum-variants doesn't renamed (rust-lang/cargo#17202) - Reduce library search path length in new build dir layout (rust-lang/cargo#17191) - fix(install): Move --debug to Compilation options (rust-lang/cargo#17199) - chore(ci): dogfood `build.warnings` (rust-lang/cargo#17195) - docs(lints): Better match clippy in lint section titles (rust-lang/cargo#17190) - chore: bump to 0.100.0; update changelog (rust-lang/cargo#17189) - docs(ref): Clarify MSRV for lints (rust-lang/cargo#17184) - docs(report): add missing entry for `cargo report future-incompatibilities` (rust-lang/cargo#17188) - Reduce rustc `-L` args used in the new `build-dir` layout (rust-lang/cargo#17168)

rustbot assigned epage Jul 2, 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 Jul 2, 2026

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ranger-ross force-pushed the reduce-path-length branch from a6fc9f2 to b6b1b73 Compare July 2, 2026 14:06

Kobzol mentioned this pull request Jul 6, 2026

Flatten all -L search paths into a single list of files rust-lang/rust#158823

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ranger-ross force-pushed the reduce-path-length branch 2 times, most recently from f8dc895 to 11e2dd5 Compare July 7, 2026 12:01

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test: Added a test verifying we pass the current build unit's out dir…

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ranger-ross force-pushed the reduce-path-length branch from 11e2dd5 to 62ecc64 Compare July 8, 2026 13:08

rustbot added S-waiting-on-author Status: The marked PR is awaiting some action (such as code changes) from the PR author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 8, 2026

ranger-ross added 2 commits July 8, 2026 22:44

fix: Do not pass the current build unit's out dir to with -L to rustc

80f7382

test: Added test showing we pass all build script out dirs to rustc args

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ranger-ross force-pushed the reduce-path-length branch from 62ecc64 to c28162c Compare July 8, 2026 13:48

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: The marked PR is awaiting some action (such as code changes) from the PR author. labels Jul 8, 2026

epage reviewed Jul 8, 2026

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Comment thread src/cargo/core/compiler/mod.rs Outdated

fix: Don't include build script out dirs in rustc args

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ranger-ross force-pushed the reduce-path-length branch from c28162c to 73f20c2 Compare July 8, 2026 16:00

epage approved these changes Jul 8, 2026

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github-merge-queue Bot removed this pull request from the merge queue due to failed status checks Jul 8, 2026

epage added this pull request to the merge queue Jul 8, 2026

Merged via the queue into rust-lang:master with commit dd56971 Jul 8, 2026
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rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 8, 2026

ranger-ross mentioned this pull request Jul 12, 2026

[beta 1.98] Backport new build-dir layout bug fixes #17207

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weihanglo mentioned this pull request Jul 18, 2026

Update cargo submodule rust-lang/rust#159475

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rustbot added this to the 1.99.0 milestone Jul 18, 2026

This was referenced Jul 19, 2026

Zed cannot be compiled with the new build dir layout #17183

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Re-organize build-dir by package + hash, rather than artifact type #15010

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Reduce rustc `-L` args used in the new `build-dir` layout#17168

Reduce rustc `-L` args used in the new `build-dir` layout#17168
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ranger-ross commented Jul 2, 2026

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