preserve name of type parameter when printing

[nikomatsakis]

Right now type parameters are printed as 'a or 'b where a and b are derived from the index of the type parameter in the list of type parameters. The user gave them names, we should print them! Part of the problem is that they may not be readily available; in the ty_param variant we have the index and the def-id. I guess the latter ought to be enough to recover the name the user gave.

The relevant code is in rustc::util::ppaux

[catamorphism]

Reproduced as of b60a0be

[graydon]

Reproduced with:

fn foo<T>(x: T) -> int {
    return x;
}

fn main() {
    foo("hello");
}

produces error:

3:11: 3:12 error: mismatched types: expected `int` but found `'a` (expected int but found type parameter)

[pcwalton]

I don't believe this is backwards incompatible, renominating.

[graydon]

accepted for production-ready milestone

[pnkfelix]

linking to the metabug #3195 so that this ticket gets an entry (with status) in its comments area.