Cargo "failed to write Cargo.lock" - Read-only Source in CI

Cargo needed to write or update Cargo.lock but the workspace is read-only or owned by another user. Resolution wants to record the lockfile and cannot, so the build fails before compiling.

What this error means

cargo fails with failed to write <path>/Cargo.lock and a Read-only file system (os error 30) or Permission denied cause. Common in containers that mount the source read-only or check it out as root.

error: failed to write `/src/Cargo.lock`

Caused by:
  Read-only file system (os error 30)

Common causes

Workspace mounted read-only

A container or CI step mounts the source tree read-only, so cargo cannot create or update Cargo.lock during resolution.

Source owned by another user

The checkout is owned by root (or a different user) while the build runs unprivileged, blocking the lockfile write.

How to fix it

Build with --locked so no write is needed

A committed, current lockfile plus --locked means cargo never rewrites it.

cargo build --locked   # uses Cargo.lock as-is, no write

Make the workspace writable

Mount the source read-write, or fix ownership so cargo can write the lockfile.

sudo chown -R "$(id -u):$(id -g)" .
# or remount the source read-write in the container

How to prevent it

• Commit Cargo.lock and build with --locked so no lockfile write occurs in CI.
• Mount the source read-write when resolution may update the lockfile.
• Keep checkout ownership consistent with the build user.

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