Steps:

• Make a branch up to date with origin/master
• Run ./mach rustup, which updates the rust-toolchain file
• Run ./mach build
• See many warnings like

  warning: `extern` fn uses type `script_runtime::JSContext`, which is not FFI-safe
     --> components/script/script_runtime.rs:829:56
      |
  829 | unsafe extern "C" fn report_stream_error_callback(_cx: *mut JSContext, error_code: usize) {
      |                                                        ^^^^^^^^^^^^^^ not FFI-safe
      |
      = note: `#[warn(improper_ctypes)]` on by default
      = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
      = note: this struct has unspecified layout

• Find a way to fix them. This may involve upgrading rust-bindgen, or making changes rust-bindgen and then upgrading to a version with those changes
• Revert the rust-toolchain change
• Submit a PR

We don’t need to upgrade the Rust version yet, but the next time we do those warnings would block landing the upgrade unless we change CI to allow build warnings.

Is suspect that rust-lang/rust#65134 is what introduced these warnings.