Constraints

What jinx does not do, and the deviations it knows about. The ones that matter for nixpkgs come first.

Pure evaluation only

jinx evaluates the way nix-instantiate --pure-eval would, and a little stricter:

• No import-from-derivation. nixpkgs bans it (ci/eval sets allow-import-from-derivation = false), and jinx has no builder to call. Importing or reading a store path that is not already on disk is an error.
• No fetching. fetchTarball, fetchGit, fetchTree, fetchurl (the builtin, not the nixpkgs function) and getFlake are errors. nixpkgs does not need them; its fetchers are derivations.
• No environment unless asked. getEnv is "", as under --pure-eval; jinx eval --impure and eval-jobs --impure read the process environment the way nix-instantiate does by default, which the parity run needs because a few packages (drbd) embed getEnv "SHELL" in their derivations. currentTime is an error, currentSystem is the --system jinx was given, nixPath is empty except for <nix/...>, which resolves to the corepkgs Nix embeds (fetchurl.nix, needed by the stdenv bootstrap).
• storePath and scopedImport are errors. Neither appears on the path nixpkgs' evaluation takes.

Compiled trees only

import resolves to a compiled module. A path outside every compiled tree is an error; there is no interpreter to fall back to. jinx eval compiles on demand into a cache so that any file on the machine works; a published nixpkgs tree is compiled ahead.

Derivation kinds

Input-addressed and fixed-output derivations, which is all of nixpkgs. __contentAddressed and __impure are errors (experimental in Nix too), and dynamic derivations are not represented.

Known deviations

• Strings are UTF-16 internally. stringLength, substring and hashString act on the UTF-8 bytes, which matches Nix; a substring that cuts through a multibyte character yields U+FFFD where Nix yields the raw bytes, and match runs on code units, so a regex that counts non-ASCII characters can differ. ASCII, which is what nixpkgs' regexes and slices touch, agrees.
• Regular expressions are translated from POSIX ERE to JavaScript and run by V8. Whole-match results agree with libstdc++; captures inside repeated groups and split on alternations whose branches are prefixes of one another can differ. src/runtime/regex.ts lists the cases with Nix's answers.
• Error messages carry Nix's wording where a test depends on it, but not its positions or traces.
• genericClosure compares keys as Nix does, so 1 and 1.0 collide.

Target Nix version

The runtime mirrors Nix 2.35.2: its builtins, its langVersion of 6, its hashing. Derivation hashing has not changed across Nix versions and is what parity measures; builtins that newer Nix adds are added here when nixpkgs starts to use them.

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