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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 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
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@ -1,13 +1,17 @@
--- ---
synopsis: "`builtins.nixVersion` now returns a fixed value \"2.18.3-lix\"" synopsis: "`builtins.nixVersion` and `builtins.langVersion` return fixed values"
cls: 558 cls: [558, 1144]
credits: jade credits: jade
category: Breaking Changes category: Breaking Changes
--- ---
`builtins.nixVersion` now returns a fixed value `"2.18.3-lix"`. This prevents `builtins.nixVersion` now returns a fixed value `"2.18.3-lix"`.
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.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 In the future, check for builtins for feature detection. If a feature cannot be
detected by *those* means, please file a Lix bug. 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"( .doc = R"(
Legacy version of Nix. Always returns "2.18.3-lix" on Lix. Legacy version of Nix. Always returns "2.18.3-lix" on Lix.
To determine if features exist, Nix scripts should instead use direct Code in the Nix language should use other means of feature detection
means of feature detection, such as checking for existence of like detecting the presence of builtins, rather than trying to find
builtins they want to use. Doing so allows for much better compatibility the version of the Nix implementation, as there may be other Nix
across implementations. 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 /* Legacy language version.
language feature gets added. It's not necessary to increase it * This is fixed at 6, and will never change in the future on Lix.
when primops get added, because you can just use `builtins ? * A better language versioning construct needs to be built instead. */
primOp' to check. */
v.mkInt(6); v.mkInt(6);
addConstant("__langVersion", v, { addConstant("__langVersion", v, {
.type = nInt, .type = nInt,
.doc = R"( .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.
)", )",
}); });