VSCode rust-analyzer WARN cargo metadata failed

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I’m new to developing on nix and I’m running into a problem with rust-analyzer.
It previously complained about a lot of things, but I worked them out with this shell.nix file:

with import <nixpkgs> { };

stdenv.mkDerivation {
  name = "rust-env";

  nativeBuildInputs = [
    # Build-time dependencies
    rustc
    cargo
    llvmPackages.clang
    glibc
    glibc.dev
    pkg-config
    libclang
    ffmpeg
  ];

  buildInputs = [
    # Run-time dependencies
    ffmpeg
  ];

  # Environment variables
  LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
  TMPDIR = "";
}

I also added the nix environment selector extension and installed direnv.
Now everything compiles without warnings, but rust-analyzer still displays a warning for some reason. Here it is in full:

WARN `cargo metadata` failed and returning succeeded result with `--no-deps` error=`cargo metadata` exited with an error:     Updating crates.io index
error: failed to create directory `/nix/store/wf9lnfvq4x0b1zh22i2q9b1vnrk0i8pj-rust-lib-src/rust-analyzer9311-4`

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


Stack backtrace:
   0: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
   1: project_model::cargo_workspace::FetchMetadata::exec
   2: project_model::sysroot::Sysroot::load_workspace
   3: std::sys::backtrace::__rust_begin_short_backtrace
   4: core::ops::function::FnOnce::call_once{{vtable.shim}}
   5: <std::sys::thread::unix::Thread>::new::thread_start
   6: start_thread
   7: __clone3

It also generates directories like proc-macro-srv9e6f-0 in the project root, which I’m not sure is intended.
Here are the dependencies for the project:

[dependencies]
clap = { version = "4.6.6", features = ["derive"] }
ffmpeg-next = "9.0.0"

Can someone explain what I’m doing wrong and how to fix it?