bootstrap: fix panic when repo path contains spaces by switching to CARGO_ENCODED_RUSTFLAGS by Rohan-Singla · Pull Request #158073 · rust-lang/rust

Rohan-Singla · 2026-06-18T10:11:33Z

Problem

./x build panics with a cryptic assertion error when the repository is
checked out under a directory path containing spaces (e.g.
/Users/foo/Open Source/rust):

thread 'main' panicked at src/bootstrap/src/core/builder/cargo.rs:54:9:
assertion left == right failed
left: 2
right: 1

The root cause: when building tools in ToolRustcPrivate or Codegen
mode, bootstrap calls llvm-config --libdir and passes the result as a
-Clink-arg=-L<path> rustflag. The Rustflags::arg() method asserted
that arguments contain no spaces, but if the repo path has a space the
libdir path inherits it and the assertion fires. The error gives no hint
that the path is the problem.

A secondary bug: llvm-config --libdir output has a trailing newline
that was previously stripped accidentally by RUSTFLAGS whitespace
splitting. Nothing was trimming it explicitly.

Fix

Two changes in src/bootstrap/src/core/builder/cargo.rs:

Switch RUSTFLAGS → CARGO_ENCODED_RUSTFLAGS: Change
Rustflags to store args as Vec<String> and join with \x1f
(ASCII unit separator) when setting the env var. Cargo's
CARGO_ENCODED_RUSTFLAGS (stable since Cargo 1.55) uses \x1f as
delimiter, which never appears in filesystem paths, so paths with
spaces are handled correctly. The space-based assertion in arg() is
removed.
Trim llvm-config --libdir output: Explicitly .trim() the
captured stdout so the trailing newline is not included in the linker
search path.

RUSTDOCFLAGS is left as-is (space-joined) since no llvm paths are
added to rustdocflags.

cc: @Kobzol

…LAGS

rustbot · 2026-06-18T10:11:39Z

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

@rustbot author: the review is finished, PR author should check the comments and take action accordingly
@rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: bootstrap
bootstrap expanded to 6 candidates
Random selection from Mark-Simulacrum, clubby789

bjorn3 · 2026-06-18T10:21:17Z

        let rustdocflags = &cargo.rustdocflags.0;
        if !rustdocflags.is_empty() {
-            cargo.command.env("RUSTDOCFLAGS", rustdocflags);
+            cargo.command.env("RUSTDOCFLAGS", rustdocflags.join(" "));


This can use CARGO_ENCODED_RUSTDOCFLAGS.

View changes since the review

Ah , let me see..

Done! Updated to use CARGO_ENCODED_RUSTDOCFLAGS with the \x1f separator for RUSTDOCFLAGS as well, consistent with how RUSTFLAGS is now handeled.

…paths

rustbot · 2026-06-18T12:14:48Z

miri is developed in its own repository. If possible, consider making this change to rust-lang/miri instead.

cc @rust-lang/miri

Mark-Simulacrum · 2026-06-18T12:21:40Z

Can you compare with a previous PR in this area (#156096) and evaluate whether there's anything from there that should also be done here? I'm also curious if we can get some kind of testing in place for this. Maybe one of the CI runners can adjust its build directory to have a space in it?

Rohan-Singla · 2026-06-18T14:39:08Z

Can you compare with a previous PR in this area (#156096) and evaluate whether there's anything from there that should also be done here? I'm also curious if we can get some kind of testing in place for this. Maybe one of the CI runners can adjust its build directory to have a space in it?

Sure i will look into it !

…ing>

Rohan-Singla · 2026-06-18T17:42:08Z

Updated to use String with \x1f as the internal delimiter (matching #156096's approach) and added the \x1f assertion in arg(). For the CI test, I can add --set build.build-dir with a space in the path to one existing Linux job in jobs.yml would that be the right approach?

cc : @Mark-Simulacrum

Kobzol · 2026-06-19T07:20:39Z

This looks reasonable to me; at this point it is essentially the same as #156096.

I don't think that we have to test this in CI, it will just create another testing hack in a bash script that we can't run locally through bootstrap. We don't test much more important things in bootstrap on our CI.. 😆

Rohan-Singla · 2026-06-19T09:17:57Z

This looks reasonable to me; at this point it is essentially the same as #156096.

I don't think that we have to test this in CI, it will just create another testing hack in a bash script that we can't run locally through bootstrap. We don't test much more important things in bootstrap on our CI.. 😆

Thanks! Happy to skip the CI test then. Let me know if there's anything else needed before this can be merged.

Mark-Simulacrum · 2026-06-21T19:40:08Z

        let rustdocflags = &cargo.rustdocflags.0;
        if !rustdocflags.is_empty() {
-            cargo.command.env("RUSTDOCFLAGS", rustdocflags);
+            cargo.command.env("CARGO_ENCODED_RUSTDOCFLAGS", rustdocflags);


Could we explicitly unset RUSTFLAGS/RUSTDOCFLAGS as well, just to avoid any confusion over those having different values? I assume Cargo will prioritize the encoded form but not sure if downstream things (e.g., build.rs scripts) will do so.

View changes since the review

Done, I now explicitly unset RUSTFLAGS and RUSTDOCFLAGS before setting the encoded forms, so cargo and any build.rs scripts only see CARGO_ENCODED_RUSTFLAGS/CARGO_ENCODED_RUSTDOCFLAGS.

Any flags from the caller's environment have already been folded into the Rustflags struct via propagate_cargo_env, so nothing is lost.

Mark-Simulacrum · 2026-06-21T19:40:27Z

r? Kobzol

rustbot · 2026-06-21T19:40:31Z

Kobzol is not on the review rotation at the moment.
They may take a while to respond.

bootstrap: fix RUSTFLAGS with spaces in paths via CARGO_ENCODED_RUSTF…

9363038

…LAGS

rustbot assigned Mark-Simulacrum Jun 18, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Jun 18, 2026

bjorn3 reviewed Jun 18, 2026

View reviewed changes

This comment has been minimized.

Sign in to view

bootstrap: also use CARGO_ENCODED_RUSTDOCFLAGS to support spaces in …

63d3395

…paths

This comment has been minimized.

Sign in to view

⏺ bootstrap: reset CARGO_ENCODED_RUSTFLAGS in Miri test runner

386748b

bootstrap: use \x1f-delimited String for Rustflags instead of Vec<Str…

8085cd3

…ing>

Mark-Simulacrum reviewed Jun 21, 2026

View reviewed changes

rustbot assigned Kobzol and unassigned Mark-Simulacrum Jun 21, 2026

unset RUSTFLAGS/RUSTDOCFLAGS when setting encoded forms

b4a7c3e

Uh oh!

Conversation

Rohan-Singla commented Jun 18, 2026 • edited by Kobzol Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Problem

Fix

Uh oh!

rustbot commented Jun 18, 2026

Uh oh!

bjorn3 Jun 18, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Rohan-Singla Jun 18, 2026

Choose a reason for hiding this comment

Uh oh!

Rohan-Singla Jun 18, 2026

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

This comment has been minimized.

rustbot commented Jun 18, 2026

Uh oh!

Mark-Simulacrum commented Jun 18, 2026

Uh oh!

Rohan-Singla commented Jun 18, 2026

Uh oh!

Rohan-Singla commented Jun 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Kobzol commented Jun 19, 2026

Uh oh!

Rohan-Singla commented Jun 19, 2026

Uh oh!

Mark-Simulacrum Jun 21, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Rohan-Singla Jun 21, 2026

Choose a reason for hiding this comment

Uh oh!

Mark-Simulacrum commented Jun 21, 2026

Uh oh!

rustbot commented Jun 21, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

6 participants

Rohan-Singla commented Jun 18, 2026 •

edited by Kobzol

Loading

bjorn3 Jun 18, 2026 •

edited by rustbot

Loading

Rohan-Singla commented Jun 18, 2026 •

edited

Loading

Mark-Simulacrum Jun 21, 2026 •

edited by rustbot

Loading