rustc.source = "discover" tries to write Cargo.lock in rustc source dir

[GoldsteinE]

My rustc source is installed on read-only filesystem (in Nix store). When I try to use rust-analyzer on out-of-tree project that uses rustc_private crates, I get the following error:

[ERROR][2023-01-06 18:37:49] .../vim/lsp/rpc.lua:734	"rpc"	"rust-analyzer"	"stderr"	'[ERROR rust_analyzer::lsp_utils] rust-analyzer failed to load workspace: Failed to read Cargo metadata for Rust sources: Failed to run `"cargo" "metadata" "--format-version" "1" "--all-features" "--manifest-path" "/nix/store/686jpzdz5zxlfhy682gc2qyf98ri413l-rust-minimal-1.68.0-nightly-2022-12-13/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.toml" "--filter-platform" "x86_64-unknown-linux-gnu"`: `cargo metadata` exited with an error:     Updating crates.io index\nerror: failed to write /nix/store/686jpzdz5zxlfhy682gc2qyf98ri413l-rust-minimal-1.68.0-nightly-2022-12-13/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.lock\n\nCaused by:\n  failed to open: /nix/store/686jpzdz5zxlfhy682gc2qyf98ri413l-rust-minimal-1.68.0-nightly-2022-12-13/lib/rustlib/rustc-src/rust/compiler/rustc/Cargo.lock\n\nCaused by:\n  Read-only file system (os error 30)\n\n'

I think that rust-analyzer shouldn’t try to write anything inside of rustc-src directory, since it can be installed globally (e.g. with Nix or other third-party package manager).

rust-analyzer version: (eg. output of "rust-analyzer: Show RA Version" command, accessible in VSCode via Ctrl/⌘+Shift+P)

rust-analyzer 1.68.0-nightly (37d7de3 2022-12-12)

rustc version: (eg. output of rustc -V)

rustc 1.68.0-nightly (37d7de337 2022-12-12)

relevant settings: (eg. client settings, or environment variables like CARGO, RUSTUP_HOME or CARGO_HOME)

1. rust-analyzer.rustc-source is set to "discover"
2. In Cargo.toml, package.metadata.rust-analyzer.rustc_private is set to true

[bjorn3]

Rust-analyzer runs cargo metadata in the rustc source directory. This updates the Cargo.lock file if it is outdated and due to the way the rustc-src component is created, it is indeed outdated as it mentions the standard library crates too, which only exist in the rust-src component, not the rustc-src component. Cargo doesn't have an option to not update Cargo.lock, other than --locked, which instead errors if Cargo.lock is not up to date.

Closing in favor of rust-lang/cargo#10096