This is an example of "Parse, don't validate" principle [1].
Before, we had a number of `StringSet`s in `DerivationOptions` that
were not *actually* allowed to be arbitrary sets of strings. Instead,
each set member had to be one of:
- a store path
- a CA "downstream placeholder"
- an output name
Only later, in the code that checks outputs, would these strings be
further parsed to match these cases. (Actually, only 2 by that point,
because the placeholders must be rewritten away by then.)
Now, we fully parse everything up front, and have an "honest" data type
that reflects these invariants:
- store paths are parsed, stored as (opaque) deriving paths
- CA "downstream placeholders" are rewritten to the output deriving
paths they denote
- output names are the only arbitrary strings left
Since the first two cases both become deriving paths, that leaves us
with a `std::variant<SingleDerivedPath, String>` data type, which we use
in our sets instead.
Getting rid of placeholders is especially nice because we are replacing
them with something much more internally-structured / transparent.
[1]: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-validate/
Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Otherwise without the change the test fails on nix-2.26 as:
error: derivation contains an illegal reference specifier 'dev'
Note: the error message does not match intended change.
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
