compiletest `string_enum`s have no error reporting

[lolbinarycat]

in common.rs:

        impl FromStr for $name {
            type Err = ();

            fn from_str(s: &str) -> Result<Self, ()> {
                match s {
                    $($repr => Ok(Self::$variant),)*
                    _ => Err(()),
                }
            }
        }

the error type being unit means if you pass an unrecognized option to any string enum flag, you get a very unhelpful error (it just prints ()).

if it was changed to the following:

        impl FromStr for $name {
            type Err = String;

            fn from_str(s: &str) -> Result<Self, String> {
                match s {
                    $($repr => Ok(Self::$variant),)*
                    _ => Err(s.to_string()),
                }
            }
        }

you would at least get an error message that says what the unrecognized argument is.

i would file a PR myself but the build system seems to be falling apart at the seams for me.

[jieyouxu]

In the short term something like Err(format!("unknown option: `{}`", s)) is fine.

[lolbinarycat]

it would be leagues better than the status quo, and probably quite easy.