allow converting an expr to its `Display` impl

[kianenigma]

The scenario is this.

In your test functions, you have:

#[test]
fn test_ls() {
  let process = Command::new(ls).arg("ln");
  // we then test that ls works as expected
}

and we want to show the same example in our docs, but showing let process = Command::new(ls).arg("ln") is not great. Ideally, we want to use Debug/Display impl of the same process variable, which will nicely format it as ls -ln.

I suppose this can be done with

#[test]
fn test_ls() {
  #[docify::export_display]
  let process = Command::new(ls).arg("ln");
  // we then test that ls works as expected
}

This is a very limited use-case. We can easily limit it to only work in assignment expressions, drop the lhs, assert rhs is a Command etc.

[sam0x17]

so how would it look in the rendered docs?

[kianenigma]

the example I have should render as ln -ls

[sam0x17]

for this to work the expr it is attached to would have to compile and run within the docify proc macro so it can expand to a string there -- remember we are ultimately in doc comment position, so you have to think of these as compile-time Display impls. Probably what this should be is an attribute attached to a function where the function returns a String, and that function would be expected to compile without issues when copy-pasted into an arbitrary proc macro, in other words it can't use dependencies.