Allow to lookup Elixir source files under configured location

[smaximov]

Greetings!

elixir_sense uses mod.module_info(:compile)[:source] in order to retrieve the source file for a module. While it works for regular Elixir files and dependencies, it fails for the Elixir sources (i.e., for the applications like :elixir, :ex_unit, :eex that ship with Elixir) if Elixir was built in a sandboxed environment. That happens, for example, on NixOS as it returns a path to a build location that no longer exists:

iex(1)> String.module_info(:compile)[:source]
'/build/source/lib/elixir/lib/string.ex'

("/build/source" refers to a location that contained Elixir source while it was being built inside a sandbox.)

Despite that, on NixOS it's easy to install the Elixir source as a package and provide the path to it as an environment variable.

Click me to get NixOS-specific info!

flake.nix:

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

  outputs = inputs@{ flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit inputs; } {
      systems =
        [ "x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin" ];
      perSystem = { config, self', inputs', pkgs, system, ... }: {
        devShells.default = with pkgs;
          mkShell {
            name = "elixir_ls";

            # Define packages that will be available for our project
            buildInputs = [ elixir erlang elixir_ls ];

            ERL_AFLAGS = "-kernel shell_history enabled";

            # Define envvar $ELIXIR_SRC that points to Elixir sources, it looks like this:
            # /nix/store/cq9kb2qq08y95j2ym4by6f3jk0ix9sni-source
            ELIXIR_SRC = "${elixir.src}";
          };
      };
    };
}

This PR makes ElixirSense.Location to look up Elixir sources under the configured location as a fallback. This will allow for code navigation to Elixir sources to work on environments like NixOS.

Sadly, I have no idea how to write tests for this, so any suggestion is appreciated. Meanwhile I tested it manually on my machine in an IEX session:

iex(1)> ElixirSense.Location.find_mod_fun_source(String, :length, 1)
nil
iex(2)> ElixirSense.Location.find_type_source(DateTime, :t, 0)
nil
iex(3)> elixir_src = System.get_env("ELIXIR_SRC")
"/nix/store/cq9kb2qq08y95j2ym4by6f3jk0ix9sni-source"
iex(4)> Application.put_env(:elixir_sense, :elixir_src, elixir_src)
:ok
iex(5)> ElixirSense.Location.find_mod_fun_source(String, :length, 1)
%ElixirSense.Location{
  type: :function,
  file: "/nix/store/cq9kb2qq08y95j2ym4by6f3jk0ix9sni-source/lib/elixir/lib/string.ex",
  line: 2017,
  column: 3
}
iex(6)> ElixirSense.Location.find_type_source(DateTime, :t, 0)
%ElixirSense.Location{
  type: :typespec,
  file: "/nix/store/cq9kb2qq08y95j2ym4by6f3jk0ix9sni-source/lib/elixir/lib/calendar/datetime.ex",
  line: 125,
  column: 3
}

If this PR is merged, I will open another one in https://github.com/elixir-lsp/elixir-ls that will read $ELIXIR_SRC at the server startup and configure :elixir_sense accordingly.

[smaximov]

I added two basic tests, but they will work only in environments that don't have Elixir sources. Please let me know what you think.

[lukaszsamson]

Nice, that was something on my todo list.

I added two basic tests, but they will work only in environments that don't have Elixir sources. Please let me know what you think.

Fake elixir source file is a right way here

Would you mind creating a similar PR adding OTP source location?