Allow filtering string with contexts (e.g. flake inputs) if contexts are in state.allowedPaths

[ilkecan]

Is your feature request related to a problem? Please describe.
Currently filtering a string with context is not possible as both builtins.filterSource¹ and builtins.path² throws an error in the form of string '<string>' cannot refer to other paths unless the context is empty.

This is a problem in general; filtering a path that is already filtered is not possible, as described in here. But it has a bigger implication with flakes and that is flake inputs cannot be filtered. This is important because flakes are not always inside the source repository they belong³, and source filtering has to be omitted in that case.

Describe the solution you'd like
I think this

... check if the context's paths are in state.allowedPaths rather than just checking if it is empty ...

as proposed by @tomberek in here would solve the problem. But I don't have enough knowledge with the codebase to say this wouldn't break anything.

Describe alternatives you've considered
While not a solution, one could use builtins.unsafeDiscardStringContext avoid the problem. But obviously, such things shouldn't be necessary.

Additional context
I wasn't sure whether to open this as a bug or as a feature request. Even if this isn't a bug, I think this deserves a high priority. Only other discussion I could find about this is this issue, which is unfortunate because it got sidetracked by another avoidable problem. @maisiliym was the main person that was talking about this problem⁴⁵⁶.

Footnotes

1. https://github.com/NixOS/nix/blob/53e479428958b39a126ce15de85d7397fdcfe2e1/src/libexpr/primops.cc#L1904-L1908 ↩

2. https://github.com/NixOS/nix/blob/53e479428958b39a126ce15de85d7397fdcfe2e1/src/libexpr/primops.cc#L1991-L1995 ↩

3. See this as an example of a flake residing outside the source repository because the PR to add it to the upstream is not accepted. ↩

4. https://github.com/NixOS/nix/issues/3732#issuecomment-721478348 ↩

5. https://github.com/NixOS/nix/issues/3732#issuecomment-728683410 ↩

6. https://github.com/NixOS/nix/issues/3732#issuecomment-732028019 ↩