print=check-cfg
The tracking issue for this feature is: #125704.
This option of the --print flag print the list of all the expected cfgs.
This is related to the --check-cfg flag which allows specifying arbitrary expected
names and values.
This print option works similarly to --print=cfg (modulo check-cfg specifics).
| --check-cfg | --print=check-cfg | 
|---|---|
| cfg(foo) | foo | 
| cfg(foo, values("bar")) | foo="bar" | 
| cfg(foo, values(none(), "bar")) | foo&foo="bar" | 
| check-cfg specific syntax | |
| cfg(foo, values(any()) | foo=any() | 
| cfg(foo, values()) | foo= | 
| cfg(any()) | any() | 
| none | any()=any() | 
To be used like this:
rustc --print=check-cfg -Zunstable-options lib.rs