Feature request: add attribute openbsd

[vtamara]

Trying to compile a recent version of optimism on OpenBSD/adJ 7.6beta1 I found that it uses just.

On OpenBSD/adJ 7.6beta1, it was straightforward to compile just and run its tests except for two tests that depend on attributes for the supported platforms (linux, windows, macos):

 % uname -a
OpenBSD selah.pasosdeJesus.org 7.6 APRENDIENDODEJESUS.MP#1 amd64
% rustc --version
rustc 1.81.0 (eeb90cda1 2024-09-04) (built from a source tarball)
% git clone [email protected]:casey/just.git/
...
% cargo build
...
% cargo install --path .
...
% RUST_BACKTRACE=1 LANG=POSIX cargo test
...

failures:                                                                                                                                   
                                                                                                                                                                
---- attributes::multiple_attributes_one_line stdout ----                                                                                                       
Bad stderr: Diff < left / right > :                                                                                                                             
<error: Justfile contains no recipes.                                                                                                                           
>exit 1                                                                                                                                                         
                                                                                                                                                                
                                                                               
thread 'attributes::multiple_attributes_one_line' panicked at tests/attributes.rs:59:6:                                                                         
Output mismatch.                                                               
stack backtrace:                                                               
   0: rust_begin_unwind                                                        
   1: core::panicking::panic_fmt                                               
   2: integration::test::Test::run                                             
             at ./tests/test.rs:293:7                                          
   3: integration::attributes::multiple_attributes_one_line                  
             at ./tests/attributes.rs:48:3                                     
   4: integration::attributes::multiple_attributes_one_line::{{closure}}                                                                                        
             at ./tests/attributes.rs:47:34                                                                                                                     
   5: core::ops::function::FnOnce::call_once                                    
             at /usr/obj/ports/rust-1.81.0/rustc-1.81.0-src/library/core/src/ops/function.rs:250:5                                                              
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
                                                                                                                                                                
---- os_attributes::os stdout ----                                                                                                                              
thread 'os_attributes::os' panicked at tests/os_attributes.rs:59:7:                                                                                             
unexpected os family                                                                                                                                            
stack backtrace:                                                                                                                                                
   0: rust_begin_unwind                                                                                                                                         
   1: core::panicking::panic_fmt                                               
   2: integration::os_attributes::os                                                                                                                            
             at ./tests/os_attributes.rs:59:7                                  
   3: integration::os_attributes::os::{{closure}}                              
             at ./tests/os_attributes.rs:35:8                                  
   4: core::ops::function::FnOnce::call_once                                   
             at /usr/obj/ports/rust-1.81.0/rustc-1.81.0-src/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
                                                                               
                                                                               
failures:                                                                                                                                                       
    attributes::multiple_attributes_one_line                                                                                                                    
    os_attributes::os                                                           
                                                                                                                                                                
test result: FAILED. 850 passed; 2 failed; 7 ignored; 0 measured; 0 filtered out; finished in 20.05s
                                                                                                                                                                
error: test failed, to rerun pass `--test integration`

Since Rust supports OpenBSD: https://doc.rust-lang.org/rustc/platform-support/openbsd.html, IMHO it makes sense to add initial support for OpenBSD in just by adding an attribute openbsd to enable recipes on OpenBSD.

[casey]

Fixed in #2497.