cargo metadata fails in read only filesystem

[StealthyKamereon]

Problem

cargo metadata fails when the target Cargo.toml is inside a read-only filesystem.

I'm on Nixos and I'd like to write a compiler plugin so I need rustc sources.
In particular, I need rust-analyzer to load rustc sources.
To do so, RA uses cargo metadata to locate the rustc sources.
However, when running cargo metadata it attempts to create a Cargo.lock file which fails because Rust is installed in the nix store which is read-only.

Here is the command output :

$ "/nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/bin/cargo" "metadata" "--format-version" "1" "--manifest-path" "/nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.toml" "--filter-platform" "x86_64-unknown-linux-gnu"
    Updating crates.io index
error: failed to write /nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.lock

Caused by:
  failed to open: /nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.lock

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

Here is the rust-analyzer log from which I got the command :

2024-08-10T20:19:02.972263Z ERROR project_model::workspace: Failed to read Cargo metadata from rustc source at /nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.toml e=Failed to run `cd "<project dir>" && RUSTUP_TOOLCHAIN="/nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20" "/nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/bin/cargo" "metadata" "--format-version" "1" "--manifest-path" "/nix/store/v0769idpz76l6b3xff8ps98bcsy5ymjb-rust-default-1.80.0-nightly-2024-05-20/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.toml" "--filter-platform" "x86_64-unknown-linux-gnu"

Steps

1. Have or install Nix:

curl -L https://nixos.org/nix/install | sh

2. Create a directory for testing and the corresponding nix shell:

mkdir cargo-bug-ro-fs && cd cargo-bug-ro-fs

Create a shell.nix with the following content:

{ nixpkgs ? import <nixpkgs> { }}:

let
rustOverlay = builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz";
pinnedPkgs = nixpkgs.fetchFromGitHub {
  owner  = "NixOS";
  repo   = "nixpkgs";
  rev    = "b3fcfcfabd01b947a1e4f36622bbffa3985bdac6";
  sha256 = "0va68ddkrqb6j349rsbkjhdq7m243kg9njcqhnx9p2sbrcl2g5l8";
};
pkgs = import pinnedPkgs {
  overlays = [ (import rustOverlay) ];
};
rust_pkg = pkgs.rust-bin.nightly."2024-05-20".default.override {
  extensions = ["rust-src" "rustc" "rustc-dev" "llvm-tools" "rust-analyzer" "cargo"];
};
in
pkgs.mkShell {
  buildInputs = with pkgs; [
    (
      rust_pkg
    )
    openssl
    pkg-config
  ];

  RUST_PKG="${rust_pkg}";
}

3. Enter the shell and run the following command:

nix-shell
cargo metadata --manifest-path $RUST_PKG/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.toml

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.80.0-nightly (0de7f2ec6 2024-05-17)
release: 1.80.0-nightly
commit-hash: 0de7f2ec6c39d68022e6b97a39559d2f4dbf3930
commit-date: 2024-05-17
host: x86_64-unknown-linux-gnu
libgit2: 1.7.2 (sys:0.18.3 vendored)
libcurl: 8.6.0-DEV (sys:0.4.72+curl-8.6.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: NixOS 24.11.0 [64-bit]

[StealthyKamereon]

Resolves rust-lang/rust-analyzer#17560
Resolves cognitive-engineering-lab/rustc_plugin#26

[weihanglo]

Thanks for the detailed use case. Really appreciate that!

This is a duplicate of #10096 as far as I can tell. We are working on providing a new flag --lockfile-path for an alternative location to write lockfile. That is expected to improve the situation. The implementation is almost done in #14326

I am going to close this in favour of #10096. Let's discuss the problem in one place then!