test precedence for setting the search path

[fricklerhandwerk]

this is just to show the principle for now.
the test cases are incomplete and way too manual.
the complete specification should follow this table, and the tests ideally be generated from that automatically:

precedence	hard-coded	nix.conf: nix-path	nix.conf: extra-nix-path	NIX_CONFIG: nix-path	NIX_CONFIG: extra-nix-path	NIX_PATH	--nix-path	--extra-nix-path	-I
hard-coded	x	^override	^append	^override	^append	^override	^override	^append	^append
nix.conf: nix-path		last wins	^append	^override	^append	^override	^override	^append	^append
nix.conf: extra-nix-path			append in order	^override	^append	^override	^override	^append	^append
NIX_CONFIG: nix-path				last wins	^append	^override	^override	^append	^append
NIX_CONFIG: extra-nix-path					append in order	^override	^override	^append	^append
NIX_PATH						x	^override	^append	^append
--nix-path							last wins	^append	^append
--extra-nix-path								append in order	append in order
-I									append in order

Motivation

The current behavior being inconsistent and surprising (#8890 #8784) has spawned multiple competing and conflicting attempts of fixing it:

• Make the NIX_PATH environment variable take precedence over config files #8902
• Restore NIX_PATH taking precedence over nix-path #7871
• getDefaultNixPath: actually respect {restrict,pure}-eval #7689
  • Revert #7689 #7911

Context

developed together with @thufschmitt

this should also resolve the confusion around behavior of restrict-eval, as that is supposed to only allow explicitly defined search paths.

Priorities

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[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-11-13-nix-team-meeting-minutes-103/35400/1

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-path-is-not-recognized/38404/6

[fricklerhandwerk]

Note: Update documentation accordingly once this is fixed https://github.com/NixOS/nix/pull/9113/files#r1487801059

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-05-06-nix-team-meeting-minutes-143/45021/1