Make flake metadata evaluation behaviour more discoverable

[ckiee]

See commit messages. Builds and works for me in a nix develop shell.

[ckiee]

cc #4945

[tomberek]

These commits seem to imply the entire flake.nix cannot perform evaluation. Closer would be to say that a flake.nix contains up to three attributes at the top-level: inputs, nixConfig, and outputs. Where inputs and nixConfig must be simple key-value attrsets with no function evaluation. Outputs must be a function definition for which the function body can perform any pure Nix evaluation (modulo the usage of --impure,--restrict-eval, etc).

[thufschmitt]

Thanks, that's a great addition :)

I'm a bit annoyed by the pointer to a github issue, though. If there's some valuable information in the issue that's missing from the manual, then it should be ported to the manual rather than linked from the source code

[ckiee]

I'm a bit annoyed by the pointer to a github issue, though. If there's some valuable information in the issue that's missing from the manual, then it should be ported to the manual rather than linked from the source code

That would be nice and it did annoy me a bit too but I'm not sure how to do that. flake.md is a manual page, not a reference guide or something else. It seems weird to include a ## Flake metadata evaluation or whatnot in what seems like more of a casual introduction. ("As an example, [...]")

If you have any ideas on how to make that fit in that'd be nice.

[edolstra]

We definitely shouldn't link to GitHub issues in error messages, except maybe if there is an actual bug, which is not the case here.

[ckiee]

We definitely shouldn't link to GitHub issues in error messages, except maybe if there is an actual bug, which is not the case here.

Are you sure?