docs(tvix/eval): start doc about problematic/weird lang behavior r/5008

The idea is that we can keep track of the more unexpected behavior,
behavior that maybe should not be a thing at all and behavior we are not
sure about yet.

Change-Id: I70933f00af1230a7ab9d30e917b61199fe571caf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6803
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 39 insertions, 0 deletions

diff --git a/tvix/eval/docs/language-issues.md b/tvix/eval/docs/language-issues.md
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+# Nix language issues
+
+In the absence of a language standard, what Nix (the language) is, is prescribed
+by the behavior of the C++ Nix implementation. Still, there are reasons not to
+accept some behavior:
+
+* Tvix aims for nixpkgs compatibility only. This means we can ignore behavior in
+  edge cases nixpkgs doesn't trigger as well as obscure features it doesn't use
+  (e.g. `__overrides`).
+* Some behavior of the Nix evaluator seems to be unintentional or an
+  implementation detail leaking out into language behavior.
+
+Especially in the latter case, it makes sense to raise the respective issue and
+maybe to get rid of the behavior in all implementations for good. Below is an
+(incomplete) list of such issues:
+
+* [Behaviour of nested attribute sets depends on definition order][i7111]
+* [Partially constructed attribute sets are observable during dynamic attr names construction][i7012]
+* [Nix parsers merges multiple attribute set literals for the same key incorrectly depending on definition order](i7115)
+
+On the other hand, there is behavior that seems to violate one's expectation
+about the language at first, but has good enough reasons from an implementor's
+perspective to keep them:
+
+* Dynamic keys are forbidden in `let` and `inherit`. This makes sure that we
+  only need to do runtime identifier lookups for `with`. More dynamic (i.e.
+  runtime) lookups would make the scoping system even more complicated as well
+  as hurt performance.
+* Dynamic attributes of `rec` sets are not added to its scope. This makes sense
+  for the same reason.
+* Dynamic and nested attributes in attribute sets don't get merged. This is a
+  tricky one, but avoids doing runtime (recursive) merges of attribute sets.
+  Instead all necessary merging can be inferred statically, i.e. the C++ Nix
+  implementation already merges at parse time, making nested attribute keys
+  syntactic sugar effectively.
+
+[i7111]: https://github.com/NixOS/nix/issues/7111
+[i7012]: https://github.com/NixOS/nix/issues/7012
+[i7115]: https://github.com/NixOS/nix/issues/7115