Xargo does not work with rust-src

[joepie91]

When installing both rust and rust-src from the rust-overlay (specifically, from the nightly channel), after installing Xargo (cargo install xargo), it will fail to run:

$ xargo --help
error: `rust-src` component not found. Run `rustup component add rust-src`.
note: run with `RUST_BACKTRACE=1` for a backtrace

An strace -f suggests that Xargo is incorrectly looking for the src in the main rust derivation output, not the rust-src output:

stat("/nix/store/i9yr504pkw2q7l80mp51pri89pyq50rf-rust-1.20.0-nightly-2017-07-24-598eddf4f/lib/rustlib/src/rust/src/libstd/Cargo.toml", 0x7fffb2fa9c30) = -1 ENOENT (No such file or directory)
stat("/nix/store/i9yr504pkw2q7l80mp51pri89pyq50rf-rust-1.20.0-nightly-2017-07-24-598eddf4f/lib/rustlib/src", 0x7fffb2fa9c30) = -1 ENOENT (No such file or directory)
write(2, "error: ", 7error: )                  = 7
write(2, "`rust-src` component not found. "..., 68`rust-src` component not found. Run `rustup component add rust-src`.) = 68

I don't know if this is strictly speaking a problem with the rust-overlay, but I'd imagine that more tools might be making the same assumption as to where to find the sources.

Workaround

The full process to get it working:

1. Install the nixpkgs.rustChannels.nightly.rust-src package (or a rust-src from your channel of choice) from the Mozilla overlay.
2. Add the following to your system configuration: environment.pathsToLink = [ "/lib/rustlib/src" ];
3. Add the following to your .bashrc: export XARGO_RUST_SRC="/run/current-system/sw/lib/rustlib/src/rust/src"

This probably is more impure than it should be, but at least it makes Xargo work. Alternative suggestions are very welcome, a 'proper' fix even moreso :)

[jameysharp]

I've been using this expression:

(rustChannels.nightly.rust.override { extensions = [ "rust-src" ]; })

More broadly, here's the shell.nix I'm using to also get a GCC toolchain for ARM microcontrollers:

with import <nixos> {};
runCommand "dummy" {
  buildInputs = [
    (rustChannels.nightly.rust.override { extensions = [ "rust-src" ]; })
    gcc-arm-embedded
  ];
} ""

[nbp]

@joepie91 Can you confirm if the solution proposed by @jameysharp works for you?

[crawford]

I've successfully used the above solution for a while. It's worth noting that Xargo is no longer required, so now I use the following (installing rust-std for thumbv7m-none-eabi instead of rust-src):

let
  moz_overlay = import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz);
  nixpkgs = import <nixpkgs> { overlays = [ moz_overlay ]; };
in
  with nixpkgs;
  stdenv.mkDerivation {
    name = "project";
    buildInputs = [
      (rustChannels.nightly.rust.override {
        targets = [ "thumbv7m-none-eabi" ];
        extensions = [ "rust-std" "rustfmt-preview" ];
      })
      gcc-arm-embedded
    ];
  }

This can probably be closed.