Cannot convert string to path

[yajo]

Describe the bug

When you have a string that contains nix store info, it's impossible to convert it to a Path.

Steps To Reproduce

# flake.nix
{
  outputs = {
    self,
    nixpkgs,
  }: {
    packages.x86_64-linux.default = let
      absoluteNixpkgs = /. + nixpkgs;
    in
      assert builtins.isPath absoluteNixpkgs;
        nixpkgs.legacyPackages.x86_64-linux.runCommandLocal "test" ''
          true
        '';
  };
}

Now build it:

nix build --show-trace
error:
       … in the condition of the assert statement
         at /nix/store/v99q15bqy6lrm4wiqjwsp0ig9vl1g949-source/mrchef/nix/flake.nix:9:7:
            8|     in
            9|       assert builtins.isPath absoluteNixpkgs;
             |       ^
           10|         nixpkgs.legacyPackages.x86_64-linux.runCommandLocal "test" ''

       … while calling the 'isPath' builtin
         at /nix/store/v99q15bqy6lrm4wiqjwsp0ig9vl1g949-source/mrchef/nix/flake.nix:9:14:
            8|     in
            9|       assert builtins.isPath absoluteNixpkgs;
             |              ^
           10|         nixpkgs.legacyPackages.x86_64-linux.runCommandLocal "test" ''

       error: a string that refers to a store path cannot be appended to a path
       at /nix/store/v99q15bqy6lrm4wiqjwsp0ig9vl1g949-source/mrchef/nix/flake.nix:7:28:
            6|     packages.x86_64-linux.default = let
            7|       absoluteNixpkgs = /. + nixpkgs;
             |                            ^
            8|     in

Expected behavior

I should be able to convert it to a path, and work with it from there onwards.

However, builtins.toPath returns a String, as shown in #1074. And the docs say to use this new method I'm using, but it also doesn't work as shown here.

Metadata

nix-env (Nix) 2.24.12

Additional context

I'm trying to use paths because they react appropriately directory traversal when concatenating with ../../ or similar stuff. These are however prohibited in other library functions in nixpkgs.lib.path.subpath, so I cannot just use strings.

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

---

Add 👍 to issues you find important.

[yajo]

I'm trying to use paths because they react appropriately directory traversal when concatenating with ../../ or similar stuff. These are however prohibited in other library functions in nixpkgs.lib.path.subpath, so I cannot just use strings.

FWIW, the deprecated builtins.toPath function gets a String and returns it normalized:

nix-repl> builtins.toPath "/a/b/c/..///d"                       
"/a/b/d"

So, it works for me as a workaround of this issue. Hopefully it will never be removed: NixOS/nixpkgs#40937 (comment)

Indeed, the name is confusing and the operations it does are not easy to understand, but I found no substitute for this use case.