* doc: migrate lib.trivial to use doc-comments
* Apply suggestions from code review
---------
Co-authored-by: Daniel Sidhion <DanielSidhion@users.noreply.github.com>
The deprecation warnings in lib were wildly inconsistent. Different
formulations were used in different places for the same meaning. Some warnings
used builtins.trace instead of lib.warn, which prevents silencing; one even
only had a comment instead. Make everything more uniform.
That way each expression uses the final version of other lib.trivial
declarations.
For instance, when replacing `versionSuffix` with the string `"fnord"`
in a lib overlay, `trivial.version` uses `"fnord"` as suffix now rather
than `pre-git`.
This reverts commit b67ee6e861.
https://github.com/NixOS/nixpkgs/issues/202244
error: a string that refers to a store path cannot be appended to a path, at /etc/nixos/nix/nixos-unstable/lib/sources.nix:193:30
appears to happen when there's a nixpkgs git submodule
> So one of the things that is different for a git submodule is that the .git folder isn't a folder, it's a textfile that contains (in my case) this:
> $ cat nix/nixos-unstable/.git
> gitdir: ../../.git/modules/nixpkgs
It gives a warning on the lazy-trees branch of Nix
(NixOS/nix#6530)
one of these was also giving me an error (the one in lib/trivial probably)
```
$ nix build
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at /home/artturin/nixgits/my-nixpkgs/lib/modules.
nix:349:99
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/.git' and then accessing it is dep
recated, at /home/artturin/nixgits/my-nixpkgs/lib/sources.nix:35:32
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc.nix'
and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc-activ
ation.nix' and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at «stdin»:0
error: cannot decode virtual path '/nix/store/virtual0000000000000000000000005-source'
(use '--show-trace' to show detailed location information)
```
It's a common pattern in Nixpkgs to want to emit a warning in certain
cases, but not actually change behaviours.
This is often expressed as either
if cond then lib.warn "Don't do that thing" x else x
Or
(if cond then lib.warn "Don't do that thing" else lib.id) x
Neither of which really expresses the intent here, because it looks
like 'x' is being chosen conditionally.
To make this clearer, I introduce a "warnIf" function, which makes it
clear that the only thing being affected by the condition is whether
the warning is generated, not the value being returned.
