fix(build): Use shared Rust target directory by default (fixes #2442).

[junhaoliao]

Description

Fixes #2442.

Cargo commands launched outside the Taskfile environment, including IDE language servers and coding agents, currently fall back to the repository-local target/ directory and duplicate artifacts already stored under build/rust-targets.

Restore build/rust-targets as Cargo's repository default in .cargo/config.toml. The generated toolchain environment continues to export CARGO_TARGET_DIR from G_RUST_BUILD_DIR, so custom G_BUILD_DIR values still take precedence over the default.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

$ task lint:fix-rust >/dev/null 2>&1 && printf 'task lint:fix-rust: passed\n'
task lint:fix-rust: passed

$ task tests:rust-all >/dev/null 2>&1 && printf 'task tests:rust-all: passed\n'
task tests:rust-all: passed

$ task >/dev/null 2>&1 && printf 'task: passed\n'
task: passed

$ printf 'default=' && env -u CARGO_TARGET_DIR cargo metadata --no-deps --format-version 1 | python3 -c 'import json,sys; print(json.load(sys.stdin)["target_directory"])' && printf 'override=' && CARGO_TARGET_DIR=/tmp/clp-issue-2442-target cargo metadata --no-deps --format-version 1 | python3 -c 'import json,sys; print(json.load(sys.stdin)["target_directory"])' && printf 'task-env=' && sed -n 's/^export CARGO_TARGET_DIR="\(.*\)"$/\1/p' build/toolchains/rust/env
default=/home/junhao/.codex/worktrees/de50/clp/build/rust-targets
override=/tmp/clp-issue-2442-target
task-env=/home/junhao/.codex/worktrees/de50/clp/build/rust-targets

$ git diff --check && printf 'git diff --check: passed\n'
git diff --check: passed

Summary by CodeRabbit

• Chores
  • Updated Rust build output storage to use a dedicated build directory.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 979b93c and 376794b.

📒 Files selected for processing (1)

• .cargo/config.toml

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Walkthrough

Cargo now stores Rust build outputs in build/rust-targets through the project Cargo configuration.

Changes

Cargo build configuration

Layer / File(s)	Summary
Configure Cargo target directory .cargo/config.toml	Cargo uses build/rust-targets for Rust build outputs.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related issues

• Rust language servers can create build artifacts outside build/rust-targets #2442 — The change restores the requested repository-local Cargo target directory.

Suggested reviewers: davidlion

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: using a shared Rust target directory by default.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[Bill-hbrhbr]

I assume this PR is intended to have Cargo invocations outside the CLP toolchain environment use the same target directory as the CLP-managed toolchain.

That being said, if a user runs:

G_BUILD_DIR=/some/other/location task <some_rust_task>

the target-dir added to .cargo/config.toml would no longer match the target directory used by the CLP toolchain.

If supporting custom G_BUILD_DIR values is not a requirement for Rust build artifacts, .cargo/config.toml can be sufficient to serve as the single source of truth for the Rust target directory, and we could remove the CARGO_TARGET_DIR export from build/toolchains/rust/env.

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Anyway, there's no perfect way to make cargo default to the CLP-installed toolchain without requiring some prerequisite setup.