Why are region parameters not allowed on type parameters?

[bblum]

unsafe fn transmute_region<T>(x: T/&a) -> T/&b {
    unsafe { unsafe::transmute(x) }
}

refuses to compile with:

trans.rs:1:33: 1:34 error: region parameters are not allowed on this type
trans.rs:1 unsafe fn transmute_region<T>(x: T/&a) -> T/&b {
                                            ^
trans.rs:1:42: 1:43 error: region parameters are not allowed on this type
trans.rs:1 unsafe fn transmute_region<T>(x: T/&a) -> T/&b {
                                                     ^

Is there some unsafety lurking, is it impossible to implement, or is it just not implemented?

[nikomatsakis]

It would be akin to writing T<S>. Type variables do not themselves take region parameters, only type paths (like, the name of a type def'n or a nominal type) do. Basically you can use the /& notation anywhere you could attach a type parameter. The bounds on type variables should be able to be region-bounded---but that is not fully implemented.

[bblum]

sure - I guess that'd be getting pretty close to higher-kinded polymorphism (or perhaps exactly that). I'm not sure I follow the last thing, though - do you mean region-bounded traits, like T: foo/&a?

[nikomatsakis]

Yes, I mean precisely that.