From c440b8667bf0bdd307e18c27860daeb2214cd4d1 Mon Sep 17 00:00:00 2001
From: sideshowbarker <mike@w3.org>
Date: Fri, 29 May 2026 17:56:14 +0900
Subject: [PATCH] CI: Avoid in-place nightly toolchain update in lint setup
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Problem: A new upstream nightly Rust release made all CI lint code
checks fail in the Set Up Environment step

Cause: CI runs “rustup toolchain install nightly --component rustfmt”.
The runner container ships nightly with rustfmt and clippy preinstalled.
But “rustup toolchain install” treats an existing channel as an upgrade
target, and tries to refresh it whenever a newer nightly is available.
The refresh removes the old clippy component first, renaming
/opt/rustup/toolchains/.../share/doc/clippy into /opt/rustup/tmp/, and
those two directories live on different overlayfs mounts in the
container — so the rename fails with EXDEV.

Fix: Replace “rustup toolchain install nightly --component rustfmt” with
“rustup component add rustfmt --toolchain nightly”. That adds the
component to the existing toolchain without checking for or applying a
channel update — so the cross-mount-rename path is never reached.
Because the container already provisions rustfmt + clippy at build time,
the new command is effectively a no-op on healthy containers, and fails
loudly only if a future container loses the nightly toolchain.
---
 .github/actions/setup/action.yml | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/.github/actions/setup/action.yml b/.github/actions/setup/action.yml
index 3a24be139d..932352c0f0 100644
--- a/.github/actions/setup/action.yml
+++ b/.github/actions/setup/action.yml
@@ -114,11 +114,15 @@ runs:
       if: ${{ inputs.os == 'macOS' || inputs.os == 'Android' }}
       uses: dtolnay/rust-toolchain@stable
 
-      # TODO: Remove once `imports_granularity` rustfmt option is stable. https://rust-lang.github.io/rustfmt/?version=v1.9.0&search=group#imports_granularity
-    - name: 'Install Nightly Rust toolchain for linting'
+      # Avoid 'rustup toolchain install nightly'. When a newer nightly is published, it triggers an in-place toolchain
+      # update whose component swap renames files from /opt/rustup/toolchains/ to /opt/rustup/tmp/. And those land on
+      # different overlayfs mounts inside the runner container — producing "Invalid cross-device link (os error 18)".
+      # TODO: Remove once 'imports_granularity' rustfmt option is stable:
+      #       https://rust-lang.github.io/rustfmt/?version=v1.9.0&search=group#imports_granularity
+    - name: 'Ensure rustfmt is available on the nightly toolchain'
       if: ${{ inputs.type == 'lint' }}
       shell: bash
-      run: rustup toolchain install nightly --component rustfmt
+      run: rustup component add rustfmt --toolchain nightly
 
     - name: 'Install wasm-tools'
       if: ${{ inputs.type == 'build' }}