In reply to @colemickens:matrix.org
how is there not a single example of how to set RUST_SRC_PATH when using craneLib.devShell?

If your using nixpkgs to populate craneLib like this in their examples

pkgs = nixpkgs.legacyPackages.${system};
craneLib = crane.mkLib pkgs;

It looks like it would pull cargo, clippy, rustc, and rustfmt from nixpkgs. So I guess the RUST_SRC_PATH = "${pkgs.rustPlatform.rustLibSrc}"; is the closest thing to correct but yeah it does feel a bit off

Are you using nixpkgs or another toolchain provider?

if you're using both oxalica/rust-overlay and ipetkov/crane, e.g. to pick crane's toolchain from rust-overlay, then you can do something like the following to pick the correct rust src: (shown using different scaffolding than flake-utils because i don't like it)

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";

    crane.url = "github:ipetkov/crane";

    rust-overlay = {
      url = "github:oxalica/rust-overlay";
      inputs.nixpkgs.follows = "nixpkgs";
    };

  };

  outputs =
    {
      nixpkgs,
      crane,
      rust-overlay,
      ...
    }:
    let
      inherit (nixpkgs) lib;
      inherit (lib.attrsets) genAttrs;

      forAllSystems =
        maker:
        genAttrs lib.systems.flakeExposed (
          system:
          maker rec {
            inherit system;
            pkgs = import nixpkgs {
              inherit system;
              overlays = [ rust-overlay.overlays.default ];
            };
            rust = rustFor pkgs;
            crane = craneFor pkgs;
          }
        );

      rustFor = pkgs: pkgs.rust-bin.stable.latest;
      craneFor = pkgs: (crane.mkLib pkgs).overrideToolchain (pkgs': (rustFor pkgs').default);
    in
    {
      packages = forAllSystems (
        { crane, ... }:
        {
          default = crane.buildPackage {
            # ...
          };
        }
      );

      devShells = forAllSystems (
        { crane, rust, ... }: {
          default = crane.devShell {
            # ...

            "RUST_SRC_PATH" = "${rust.rust-src}/lib/rustlib/src/rust/library";
          };
        }
      );
    };
}