support for thumbv7em-none-eabihf

[Spaqin]

I tried to introduce flakes to our embedded systems projects, and Fenix seemed like a nice lightweight solution, especially for someone new in the nix ecosystem like me.

Seems like this would be the correct flake code:

      toolchain = fenix.packages.x86_64-linux.toolchainOf {
        date = "2020-10-30";
        sha256 = "0iygcwzh8s0lfdghj5809krvzifc1ii1wm4sd3qqn7s0rz1s14hi";
      };

      rc = fenix.packages.x86_64-linux.combine [
        toolchain.rustc
        toolchain.cargo
        fenix.packages.x86_64-linux.targets.thumbv7em-none-eabihf.latest.rust-std
      ];

      rustPlatform = pkgs.makeRustPlatform {
        cargo = rc;
        rustc = rc;
      };
...

Which does get me to the compiling stage, but unfortunately it errors out later with

       > error[E0463]: can't find crate for `core`
       >   |
       >   = note: the `thumbv7em-none-eabihf` target may not be installed

However, the only package for that platform for crosscompiling is rust-std - which doesn't cover our needs (we'd need rust-src).

How difficult would it be to add support for this, and possibly other embedded development platforms?

[figsoda]

Haave you tried using rust-src from x86_64-linux?

[WizardUli]

This works (or at least I can successfully build https://github.com/rp-rs/rp2040-project-template):
flake.nix:

{
  description = "A simple flake";

  inputs = {
    fenix = {
      url = "github:nix-community/fenix";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    flake-utils.url = "github:numtide/flake-utils";
    # nixpkgs are tracking github:NixOS/nixpkgs/nixos-22.05
  };

  outputs = { self, nixpkgs, fenix, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; overlays = [ fenix.overlay ]; };
      in
      {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            (pkgs.fenix.fromToolchainFile { dir = ./.; })
          ];
        };
      }
    );
}

rust-toolchain.toml:

[toolchain]
channel = "stable"
targets = ["thumbv6m-none-eabi"]
profile = "complete"