must_cast: fix test failures and fmt.

[zachs18]

All doctests in src/main.rs were failing under miri due to rust-lang/miri#2423. As a temporary workaround, instead mark those tests as should_panic under miri, and compile_fail only under not(miri) (b275f7d).

Two doctests in src/must.rs were failing for unrelated reasons (4809de5).

(Also cargo fmt on src/must.rs)

[zachs18]

Shorter/clearer version of cfg(not(miri))-ing out the compile_fails, adapted from @danielhenrymantilla via discord:

#[cfg(not(miri))]
macro_rules! compile_fail {() => (
    "``​`compile_fail,E0080"
)}

#[cfg(miri)]
macro_rules! compile_fail {() => (
    "``​`should_panic"
)}

at the beginning of src/must.rs, so as to:

#[doc = compile_fail!()]

(I'm not at a computer atm)

[Lokathor]

Yeah i like the macro, please use that and also put in a comment to the miri issue as to why the macro is there at all.

[zachs18]

I changed it to the macro version, and added a description of why the workaround is needed in a comment.