Cargo from `rustup which` is broken

[kryptan]

I have hit a strange issue where using cargo reported by rustup which doesn't work. When building dashmap crate it downloads rust 1.65 and tries to build with that but it fails. Even if it succeeded how would it be possible to link crate built with 1.65 with all the other crates built with the latest version?

I'm not sure if this is a rustup or cargo issue.

Here are the exact steps:

Create new crate:

PS D:\test> cargo new project
    Creating binary (application) `project` package
note: see more `Cargo.toml` keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
PS D:\test> cd project

Add dashmap v6.1.0 as a dependency:

PS D:\test\project> cargo add dashmap@6.1.0
    Updating crates.io index
      Adding dashmap v6.1.0 to dependencies
             Features:
             - arbitrary
             - inline
             - raw-api
             - rayon
             - serde
             - typesize
    Updating crates.io index
     Locking 13 packages to latest Rust 1.92.0-nightly compatible versions

Find out cargo location:

PS D:\test\project> rustup which cargo
C:\Users\user\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\cargo.exe

Build with the provided cargo:

PS D:\test\project> C:\Users\user\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\cargo.exe build
   Compiling crossbeam-utils v0.8.21
   Compiling parking_lot_core v0.9.12
   Compiling scopeguard v1.2.0
   Compiling windows-link v0.2.1
   Compiling smallvec v1.15.1
   Compiling cfg-if v1.0.4
   Compiling once_cell v1.21.3
   Compiling hashbrown v0.14.5
   Compiling lock_api v0.4.14
   Compiling dashmap v6.1.0
info: syncing channel updates for '1.65-x86_64-pc-windows-msvc'
info: latest update on 2022-11-03, rust version 1.65.0 (897e37553 2022-11-02)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-std'
info: installing component 'rustc'
info: installing component 'rustfmt'
error: the `-Z unstable-options` flag must also be passed to enable the flag `check-cfg`

error: could not compile `dashmap` (lib)

Caused by:
  process didn't exit successfully: `rustc --crate-name dashmap --edition=2018 C:\Users\user\.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\dashmap-6.1.0\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=119 --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 --check-cfg cfg(docsrs,test) --check-cfg "cfg(feature, values(\"arbitrary\", \"inline\", \"raw-api\", \"rayon\", \"serde\", \"typesize\"))" -C metadata=7088beb6e94b1ca5 -C extra-filename=-e6076f9687df7ba4 --out-dir D:\test\project\target\debug\deps -L dependency=D:\test\project\target\debug\deps --extern cfg_if=D:\test\project\target\debug\deps\libcfg_if-c6c59aeea06f958d.rmeta --extern crossbeam_utils=D:\test\project\target\debug\deps\libcrossbeam_utils-96db4feb57c79985.rmeta --extern hashbrown=D:\test\project\target\debug\deps\libhashbrown-aeca10d661ea0b2e.rmeta --extern lock_api=D:\test\project\target\debug\deps\liblock_api-04e31afff8092457.rmeta --extern once_cell=D:\test\project\target\debug\deps\libonce_cell-ecc86b59685e8d56.rmeta --extern parking_lot_core=D:\test\project\target\debug\deps\libparking_lot_core-e8be9b060a8eb8f9.rmeta --cap-lints allow` (exit code: 1)

Building with the default cargo (C:\Users\user\.cargo\bin\cargo.exe) does work. I was executing cargo from rustup which because this is what evcxr does. If this cargo is not meant to be executed directly than it should be documented somewhere.

dashmap contains rust-toolchain.toml with channel = "1.65", that's probably why rustup downloads rust 1.65.

---

rustup 1.28.2 (e4f3ad6 2025-04-28)
rustc 1.92.0-nightly (844264add 2025-10-14)

[rami3l]

@kryptan Thanks for reporting this issue!

From the first glance rustup seems to be working correctly. A very old version of rustc is invoked, causing failures since the --check-cfg option was not stabilized yet.

The problem here is, the rustup calls are usually recursive, so there is no guarantee that calling cargo from rustup which won't wind up calling rustup further down the line (please note that this is completely out of rustup's control).

Normally this wouldn't be a problem since at the topmost rustup call, the RUSTUP_TOOLCHAIN env var is set to a specific value, say nightly. Once that happens, rust-toolchain.toml will be a no-op.

As such, I suggest that you override the env var before invoking cargo from rustup which and try again.

PS: To clarify, I don't see there is any bug involved... Could be an interesting argument in #4211 though, since the automatic installation could've been stopped by RUSTUP_AUTO_INSTALL=0.

cc @Feelfeel20088 for xacrimon/dashmap#342, as indeed this would be more likely to happen on NixOS for whatever reason.

[Feelfeel20088]

@kryptan I fixed this with using rust-overlay. I am still unsure why this is happening but this is my dev-shell for one of my projects that you may find helpfull:

{
  description = "Krousinator dev shell and package";

  inputs = {
    utils.url = "github:numtide/flake-utils";
    nixpkgs.url = "nixpkgs/nixos-unstable";
    rust-overlay.url = "github:oxalica/rust-overlay";
    rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = { self, nixpkgs, utils, rust-overlay, ... }: let 
    overlays = [ (import rust-overlay) ];
    pkgs = import nixpkgs { system = "x86_64-linux"; inherit overlays; };
    krousinator = pkgs.callPackage ./nix/default.nix {};
  in {
    packages = {
      
      
      x86_64-linux = {
        kroushive = krousinator.kroushive;
      };

      
      x86_64-windows = {
        krousinator = krousinator.krousinator;
      };
    };

    devShells.x86_64-linux.default = pkgs.mkShell {
      buildInputs = with pkgs; [
        pkgs.pkg-config
        openssl
        
          
        (rust-bin.stable.latest.default.override {
          extensions = [
            "clippy"
            "rust-src"
            "rust-analyzer"
          ];
          targets = [
            "x86_64-pc-windows-gnu"
            "x86_64-unknown-linux-gnu"
          ];
        })
      ];
    };
    

    # devShells."x86_64-linux".default = pkgs.mkShell {
    #   buildInputs = with pkgs; [
    #     cargo rustc rustfmt clippy rust-analyzer gnumake42
    #   ];

    #   nativeBuildInputs = [pkgs.pkg-config];
    #     env.RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";
    # };
  };
}

(ignore the packages just take the overlay part and the devshell)

[rami3l]

@Feelfeel20088 Thanks for the additional context! Indeed, in this case the key is to either direct rustup to use the same toolchain as the entrypoint like I have suggested above, or bypass rustup altogether via e.g. rust-overlay like what you did.