add a lint for useless transmutes

[emberian]

fn main() {
    unsafe { std::cast::transmute::<int, int> ( 52 ) };
}

Should have a warning. In general, transmute::<T, T> should be warned against.

[sanxiyn]

I am not sure about this. In transmute::<T, U>, T can be a platform-specific type, which may or may not be equal to U depending on the platform. It would be annoying to get warned in such cases.

[fhahn]

I think there was a similar problem with #11135 (a lint for unnecessary type casts). This lint defaults to allow at the moment, partly because of the problem with platfrom specific types. There are some thoughts on this problem in the comments.

[pongad]

Would it be feasible to write up a list of platform-specific types? Ideally, users won't be creating new ones, so we can exclude them from the warning.

[fhahn]

I think this could work. Some lints already use static lists, the attr_usage lint uses a a list of attribute names to check if attributes are known to the compiler. I could write a proof of concept patch if this approach seems feasible.

[tshepang]

That sample code should now be:

fn main() {
    unsafe { std::mem::transmute::<int, int> ( 52 ) };
}

[steveklabnik]

/cc @Manishearth , does clippy have one of these?

[Manishearth]

Filed https://github.com/Manishearth/rust-clippy/issues/441