feat: implement just_version() function

[nyurik]

implement #2616

Note that this also allows #2290

_required_just_ver := if semver_matches(just_version(), '>=1.43.0') == 'true' {
  'yes'
} else {
  error('just version 1.43.0 or greater is required, found ' + just_version())
}

[nyurik]

@casey hi, are there any blockers or concerns with this? Thx!

[laniakea64]

Sorry if I'm missing something, but so far the proposed use cases for this function revolve around detecting whether the running just version has the features the justfile requires. But when a too-old just version is presented with a syntax or function that requires a later just version, it will fail with a compile error before the justfile's version check is evaluated. So as casey said in #2290 (comment) , a just function cannot fulfill those use cases.

Are there any other use cases for just_version()?

[nyurik]

ironically, my biggest use case is NOT to do version check, but rather to reliably report the version from inside. All my just-using projects have this section. I need it to work in CI before anything else - just so that I can debug bigger issues.

# Print environment info
env-info:
    @echo "Running for '{{main_crate}}' crate {{if ci_mode == '1' {'in CI mode'} else {'in dev mode'} }} on {{os()}} / {{arch()}}"
    {{just_executable()}} --version
    rustc --version
    cargo --version
    rustup --version

What I recently discovered is that this breaks on Windows - because just_executable() returns Windows path, but if the script runs using bash, it creates all sorts of problems. So if we have {{just_version()}}, I will have more flexibility in the future.

Do note that with every function we will have the "before" and "after" -- so anyone who wants to use any new function will have to ensure that the version of the just executable will handle it. Not much we can do about it "in retrospect" other than to create some external wrappers.

[casey]

See comments!

[casey]

We should remove this, since this is a fragile way to enforce a particular just version, since the justfile may fail to compile at all with the older version.

[casey]

Suggested change

	Ok(env!("CARGO_PKG_VERSION").to_string())
	Ok(env!("CARGO_PKG_VERSION").into())

[casey]

Let's add a single extra test for this, instead of making these tests more complicated.

[casey]

I would leave off the e.g.

Suggested change

	- `just_version()`<sup>master</sup> — The version of `just` being run, e.g. `"1.42.5"`.
	- `just_version()`<sup>master</sup> — The version of `just` being run.