Add fromToolchainName to targets

[soywod]

This would allow combining components from a specific version in a cross compilation context:

[toolchain]
channel = "1.77.0"
profile = "default"
components = [ "rust-src", "rust-analyzer" ]

let
  name = (pkgs.lib.importTOML ./rust-toolchain.toml).toolchain.channel;
  fenixPackage = fenix.packages.${buildPlatform};
  toolchain = fenixPackage.fromToolchainName { inherit name sha256; };
  targetToolchain = fenixPackage.targets.${targetPlatform}.fromToolchainName { inherit name sha256; };
in
fenixPackage.combine [
  toolchain.rustc
  toolchain.cargo
  targetToolchain.rust-std
];

[soywod]

Any news about it?

[shymega]

@soywod You should probably revert 86c0fc2, and rebase against fenix/main, otherwise we end up with merge commits in a PR :)

[shymega]

I'm in favour of this PR, fwiw.

[soywod]

Thank you @shymega 😉 I just pushed the rebase.