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builtins: fix builtins.langVersion docs to state it's deprecated

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This builtin was always a problem and nixpkgs uses it in exactly one
place, to give up if the Nix version is absurdly old. It has no other
use cases, and doesn't work in a multi-implementation world anyway.

Change-Id: I03c36e118591029e2ef14b091fe14a311c66a08a
This commit is contained in:
Jade Lovelace 2024-05-15 21:40:39 -07:00
parent 7cfaf057e3
commit 6d84d6bdc1
2 changed files with 29 additions and 14 deletions
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14
doc/manual/rl-next/nixversion-fake.md
View file

@ -1,13 +1,17 @@
---
synopsis: "`builtins.nixVersion` now returns a fixed value \"2.18.3-lix\""
cls: 558
synopsis: "`builtins.nixVersion` and `builtins.langVersion` return fixed values"
cls: [558, 1144]
credits: jade
category: Breaking Changes
---
`builtins.nixVersion` now returns a fixed value `"2.18.3-lix"`. This prevents
feature detection assuming that features that exist in Nix post-Lix-branch-off
might exist, even though the Lix version is greater than the Nix version.
`builtins.nixVersion` now returns a fixed value `"2.18.3-lix"`.
`builtins.langVersion` returns a fixed value `6`, matching CppNix 2.18.
This prevents feature detection assuming that features that exist in Nix
post-Lix-branch-off might exist, even though the Lix version is greater than
the Nix version.
In the future, check for builtins for feature detection. If a feature cannot be
detected by *those* means, please file a Lix bug.

29
src/libexpr/primops.cc
View file

@ -4355,10 +4355,13 @@ void EvalState::createBaseEnv()
.doc = R"(
Legacy version of Nix. Always returns "2.18.3-lix" on Lix.
To determine if features exist, Nix scripts should instead use direct
means of feature detection, such as checking for existence of
builtins they want to use. Doing so allows for much better compatibility
across implementations.
Code in the Nix language should use other means of feature detection
like detecting the presence of builtins, rather than trying to find
the version of the Nix implementation, as there may be other Nix
implementations with different feature combinations.
If the feature you want to write compatibility code for cannot be
detected by any means, please file a Lix bug.
)",
});
@ -4377,15 +4380,23 @@ void EvalState::createBaseEnv()
)",
});
/* Language version. This should be increased every time a new
language feature gets added. It's not necessary to increase it
when primops get added, because you can just use `builtins ?
primOp' to check. */
/* Legacy language version.
* This is fixed at 6, and will never change in the future on Lix.
* A better language versioning construct needs to be built instead. */
v.mkInt(6);
addConstant("__langVersion", v, {
.type = nInt,
.doc = R"(
The current version of the Nix language.
The legacy version of the Nix language. Always is `6` on Lix,
matching Nix 2.18.
Code in the Nix language should use other means of feature detection
like detecting the presence of builtins, rather than trying to find
the version of the Nix implementation, as there may be other Nix
implementations with different feature combinations.
If the feature you want to write compatibility code for cannot be
detected by any means, please file a Lix bug.
)",
});