Fix cargo add overwriting symlinked Cargo.toml files #15281

RaghavenderSingh · 2025-03-08T14:13:41Z

What does this PR try to resolve?

This PR fixes a bug where cargo add breaks symlinks to Cargo.toml files. Currently, when Cargo.toml is a symlink and cargo add is used to add a dependency, the symlink is replaced with a regular file, breaking the link to the original target file.

This issue was reported in #15241 where a user who relies on symlinked Cargo.toml files found that cargo add breaks their workflow.

Fixes #15241

How should we test and review this PR?

I've modified LocalManifest::write() to check if the path is a symlink, and if so, follow it to get the actual target path. This ensures we write to the actual file rather than replacing the symlink.

I've also added a test in tests/testsuite/cargo_add/symlink.rs that:

Creates a symlinked Cargo.toml file
Runs cargo add to add a dependency
Verifies the symlink is preserved and the dependency is added to the target file

I've manually tested this fix and confirmed it works correctly.

rustbot · 2025-03-08T14:13:45Z

r? @weihanglo

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weihanglo · 2025-03-08T17:37:20Z

tests/testsuite/cargo_add/symlink.rs

nit: we prefer contributions split tests into the first commit, with them passing. That way the fix commit will show the change in behavior by how the test changes. This helps test the test and provides an illustration of the PR description for the reviewer and the community at large.

We linked to examples in the contrib docs explaining this

Thanks for the guidance. I've restructured the PR.

Note that the first commit is best to pass CI. That means you'll assert the problematic behavior. The "fix" commit then contains both the fix, and the change in test. See also this PR as an example.

In addition, your test is a functional test, but is placed along with other UI tests. Is it possible to make it a pure file-based UI test, and change the test name to case in order to follow the convention?

(It might not work because cross-platform symlinks are tricky, and git is not good at handling it.)

cc @epage, any thoughts on how to organize these?

This mixing of functional with UI tests seems inline with how we do it elesewhere

weihanglo · 2025-03-09T17:10:32Z

tests/testsuite/cargo_add/symlink.rs

Note that the first commit is best to pass CI. That means you'll assert the problematic behavior. The "fix" commit then contains both the fix, and the change in test. See also this PR as an example.

In addition, your test is a functional test, but is placed along with other UI tests. Is it possible to make it a pure file-based UI test, and change the test name to case in order to follow the convention?

(It might not work because cross-platform symlinks are tricky, and git is not good at handling it.)

weihanglo · 2025-03-09T17:25:08Z

tests/testsuite/cargo_add/symlink.rs

+    }
+
+    // Add a dependency
+    project.cargo("add rand").run();


This is pulling an exteranl dependency from crates.io. We avoid that in cargo's testsuite and instead host a local registry fixure, like

cargo/tests/testsuite/rename_deps.rs

Line 12 in ab1463d

Package::new("bar", "0.2.0").publish();

cargo/tests/testsuite/cargo_add/features/mod.rs

Lines 10 to 16 in ab1463d

cargo_test_support::registry::init();

cargo_test_support::registry::Package::new("your-face", "99999.0.0+my-package")

.feature("nose", &[])

.feature("mouth", &[])

.feature("eyes", &[])

.feature("ears", &[])

.publish();

weihanglo · 2025-03-09T17:26:15Z

tests/testsuite/cargo_add/symlink.rs

cc @epage, any thoughts on how to organize these?

When Cargo.toml is a symlink, cargo add was overwriting it with a regular file. This change follows the symlink and writes to the target file instead, preserving the symlink structure. Fixes rust-lang#15241

epage · 2025-03-10T16:16:30Z

src/cargo/util/toml_mut/manifest.rs

+    /// Write changes back to the file.
    /// Write changes back to the file.


Why the double comment?

epage · 2025-03-10T16:16:40Z

src/cargo/util/toml_mut/manifest.rs

+        // Check if the path is a symlink and follow it if it is
+        let actual_path = if self.path.is_symlink() {
+            std::fs::read_link(&self.path)?
+        } else {
+            self.path.clone()
+        };

+        // Write to the actual target path instead of the symlink
+        cargo_util::paths::write_atomic(&actual_path, new_contents_bytes)


Should this be put in write_atomic?

rustbot assigned weihanglo Mar 8, 2025

rustbot added A-manifest Area: Cargo.toml issues S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 8, 2025

weihanglo reviewed Mar 8, 2025

View reviewed changes

RaghavenderSingh force-pushed the fix-cargo-add-symlinks branch from a45d668 to 53fdb96 Compare March 8, 2025 19:25

weihanglo reviewed Mar 9, 2025

View reviewed changes

test: Add failing test for cargo add breaking symlinks

b5147ac

RaghavenderSingh force-pushed the fix-cargo-add-symlinks branch 2 times, most recently from 2f5fc06 to 465de3c Compare March 9, 2025 19:57

fix: Preserve symlinks when running cargo add

1708e12

When Cargo.toml is a symlink, cargo add was overwriting it with a regular file. This change follows the symlink and writes to the target file instead, preserving the symlink structure. Fixes rust-lang#15241

RaghavenderSingh force-pushed the fix-cargo-add-symlinks branch from 465de3c to 1708e12 Compare March 9, 2025 20:04

epage reviewed Mar 10, 2025

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Fix cargo add overwriting symlinked Cargo.toml files #15281

Fix cargo add overwriting symlinked Cargo.toml files #15281

RaghavenderSingh commented Mar 8, 2025

rustbot commented Mar 8, 2025

weihanglo Mar 8, 2025

RaghavenderSingh Mar 8, 2025

weihanglo Mar 9, 2025

weihanglo Mar 9, 2025

epage Mar 10, 2025

weihanglo Mar 9, 2025

weihanglo Mar 9, 2025

weihanglo Mar 9, 2025

epage Mar 10, 2025

epage Mar 10, 2025

	cargo_test_support::registry::init();
	cargo_test_support::registry::Package::new("your-face", "99999.0.0+my-package")
	.feature("nose", &[])
	.feature("mouth", &[])
	.feature("eyes", &[])
	.feature("ears", &[])
	.publish();

		/// Write changes back to the file.
		/// Write changes back to the file.

Fix cargo add overwriting symlinked Cargo.toml files #15281

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Fix cargo add overwriting symlinked Cargo.toml files #15281

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RaghavenderSingh commented Mar 8, 2025

What does this PR try to resolve?

How should we test and review this PR?

rustbot commented Mar 8, 2025

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