Allow trait-less impls on primitive types when in the corresponding libcore module for each type

[bstrie]

I filed #6045 before I realized that you have to make useless traits to implement methods on primitives. However, we already use lang items to identify core::char as the "canonical" char module, yes? Is it possible then to allow impls directly upon char from within core::char, since we know that it won't have anonymous impls anywhere else? This is what I'd like to be able to do:

impl char {
    fn anything() { ... }
}

without having to make a useless trait first:

trait UselessCharTrait {
    fn anything();
}

impl UselessCharTrait for char {
    fn anything() { ... }
}

This is somewhat more pertinent now that #3048 is WONTFIX.

[emberian]

Still relevant. I thought a potential solution was to move the builtin types to std::unstable, and do like struct char(std::unstable::builtin_char), but @thestinger pointed out newtypes were still buggy. He came up with the idea of #[lang="char"] struct char {priv c: u32}

[thestinger]

This would be incredibly nice to have, in order to clear out a bunch of useless one use traits.

[eddyb]

I would rather have the compiler provide the type, but it could be "owned" by the crate which defines it.

#[lang="char"] struct char;
// `char` is now a type defined in the same crate,
// but behaves just like the current built-in type.
impl char {...}

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#286