OS-specific attributes on all top-level declarations

[cspotcode]

Is it intuitive to support OS-specific attributes on all top-level declarations in a justfile, not just recipes?

The OS is hardcoded into the executable, so the logic is straightforward:

E.g just.exe parses and completely ignores anything with [macos] or [linux] on it

This simplifies things overall and implements multiple tickets:

• set windows-shell := can be deprecated, replaced by [windows] ; set shell :=
• Get windows-script-interpreter for free, can close Add windows-script-interpreter setting #2944
• Implements Support OS attributes on aliases #1493 & Allow (some) attributes on aliases #2627
• Implements OS-specific attributes on import #2786
• Replace conditional variable declarations:

  • SHEBANG := if os() == 'windows' {
        'busybox sh'
    } else {
        '/usr/bin/env sh'
    }
    recipe:
      #!{{SHEBANG}}

  • becomes:

    [windows]
    SHEBANG := 'busybox sh'
    [unix]
    SHEBANG := '/usr/bin/env sh'
    
    recipe:
      #!{{SHEBANG}}

Design challenges

Requires OS-specific attribute on both windows and unix setting/variable declarations:

[unix] # <-- if you omit this, error "Setting redefined"
set shell := ['bash', '-euc']

[windows]
set shell := ['busybox', 'bash', '-euc']

We should raise the "redefinition" error on Unix, too. Even though we're ignoring the second declaration on Unix, the error warns us that this justfile will fail on Windows.

Alternatively, could allow redefinition if second one has OS attribute. The un-attributed declaration becomes automatically [not-windows].

Limitations

Falls short of enabling if {} else {} logic in settings. But that's currently impossible anyway.

# If anyone has been wanting to do this, they still won't be able to
if which('sh') {
    set shell := ['sh', '-euc']
} else {
    set shell := ['somethingelse', '-euc']
}

[casey]

I think this would be great, especially if it could be done in a generic way, such that it works with all top-level items.

I haven't thought about this in depth, but we would need to turn Ast::items from Vec<Item> into Vec<(Item, AttributeSet)> and then filter out disabled attributes in the analyzer.

I think the initial version should complain in the duplicate setting case you mentioned. So this should be an error:

set shell := ['bash', '-euc']

[windows]
set shell := ['busybox', 'bash', '-euc']

Later, we could relax this, so if there are two enabled settings with one having attributes and the other not, the one without attributes is dropped. This would be backwards compatible, since previously this would have been an error.