feat(tvix/eval): implement nested keys r/5002

This finishes up the implementation of nested keys after the key
insight that the nesting level does not need to be tracked, and
instead the attribute iterator can simply be retained inside the
structures as is (in an advanced state).

With this implementation, when encountering a nested key, the Tvix
compiler will first analyse whether there is already a matching
binding that can be merged (i.e. a binding that is a literal attribute
set), and perform the merge, or otherwise create a new recursive set
of bindings in which the entry is inserted with the path iterator
advanced beyond the first name component.

With this, all the logic simply applies recursively until there are no
more nested bindings (i.e. until all iterators are "empty").

Note that this has one (potentially insignificant) deviation from Nix
currently: If a non-mergable value is supplied (e.g. `a.b = 1; a =
2;`), Tvix will emit a *runtime* error (whereas it is *parse* time in
Nix) as the branch which could statically analyse this is currently
unreachable. There's a TODO for this, so we can fix it up later.

Change-Id: I53df70e09614ff4281a70b80eac7da3beca12da9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6806
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

0 files changed, 0 insertions, 0 deletions