docs(tvix): add reconstructed changelog for the Nix language r/5662

This was put together by digging through the C++ Nix git history as
described in the second paragraph. Note that this is only based on
changes given when the language version was actually increased.

There is no guarantee that there have been no other changes inbetween
that would have warranted an increase as well.

Change-Id: I4ddee0d4ecafa1b3e5e1a867e9700d6c32e936ad
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7836
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>

1 files changed, 60 insertions, 0 deletions

diff --git a/tvix/docs/lang-version.md b/tvix/docs/lang-version.md
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+# Nix language version history
+
+The Nix language (“Nix”) has its own versioning mechanism independent from its
+most popular implementation (“C++ Nix”): `builtins.langVersion`. It has been
+increased whenever the language has changed syntactically or semantically in a
+way that would not be introspectable otherwise. In particular, this does not
+include addition (or removal) of `builtins`, as this can be introspected using
+standard attribute set operations.
+
+Changes to `builtins.langVersion` are best found by viewing the git history of
+C++ Nix using `git log -G 'mkInt\\(v, [0-9]\\)'` for `builtins.langVersion` < 7.
+After that point `git log -G 'v\\.mkInt\\([0-9]+\\)'` should work. To reduce the
+amount of false positives, specify the version number you are interested in
+explicitly.
+
+## 1
+
+The first version of the Nix language is its state at the point when
+`builtins.langVersion` was added in [8b8ee53] which was first released
+as part of C++ Nix 1.2.
+
+## 2
+
+Nix version 2 changed the behavior of `builtins.storePath`: It would now [try to
+substitute the given path if missing][storePath-substitute], instead of creating
+an evaluation failure. `builtins.langVersion` was increased in [e36229d].
+
+## 3
+
+Nix version 3 changed the behavior of the `==` behavior. Strings would now be
+considered [equal even if they had differing string context][equal-no-ctx].
+
+## 4
+
+Nix version 4 [added the float type][float] to the language.
+
+## 5
+
+The [increase of `builtins.langVersion` to 5][langVersion-5] did not signify a
+language change, but added support for structured attributes to the Nix daemon.
+Eelco Dolstra writes as to what changed:
+
+> The structured attributes support. Unfortunately that's not so much a language
+> change as a build.cc (i.e. daemon) change, but we don't really have a way to
+> express that...
+
+Probably `builtins.nixVersion` (which was added in version 1) should have been
+used instead.
+
+## 6
+
+Nix version 6 added support for [comparing two lists][list-comparison].
+
+[8b8ee53]: https://github.com/nixos/nix/commit/8b8ee53bc73769bb25d967ba259dabc9b23e2e6f
+[storePath-substitute]: https://github.com/nixos/nix/commit/22d665019a3770148929b7504c73bcdbe025ec12
+[e36229d]: https://github.com/nixos/nix/commit/e36229d27f9ab508e0abf1892f3e8c263d2f8c58
+[equal-no-ctx]: https://github.com/nixos/nix/commit/ee7fe64c0ac00f2be11604a2a6509eb86dc19f0a
+[float]: https://github.com/nixos/nix/commit/14ebde52893263930cdcde1406cc91cc5c42556f
+[langVersion-5]: https://github.com/nixos/nix/commit/8191992c83bf4387b03c5fdaba818dc2b520462d
+[list-comparison]: https://github.com/nixos/nix/commit/09471d2680292af48b2788108de56a8da755d661