add more tests for type-instantiation `pat::<T>` syntax in patterns

[pnkfelix]

Spawned off of issue #22544

Consider this code:

pub struct Foo<T>(T, T);

impl<T: ::std::fmt::Display> Foo<T> {
    fn foo(&self) {
        match *self {
            Foo::<T>(ref x, ref y) =>  // <=== check out this line!
                println!("Goodbye, World! {} {}", x, y)
        }
    }
}

fn main() {
    let f = Foo(2,3);
    f.foo();
}

The above compiles and runs in the playpen today.

We probably shouldn't accept the use of Foo::<T> in a pattern, right?

(Is there some context where we would want that, i.e. as some sort of feedback into type-inference? I dunno, I still remain to be convinced that type ascription in patterns is generally useful.)

[pnkfelix]

Nominating. (Removing support for this is arguably P-backcompat-lang. It is also arguably "just a bug", i.e. "polish at best".)

I suspect it is not too hard for someone familiar with the parser to fix, though.

I am mainly nominating it to answer the question: Is it actually buggy syntax or not?

Now that patterns fully participate in type inference, that can come in useful like in this example:

let Foo::<uint>(a, b) = Default::default();

[bombless]

I wrote a PoC for this and after make check I found that this behavior is intended.
See
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/generic-tag-match.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/generic-tag-values.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/match-bot.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/match-join.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/nested-matchs.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/shadow.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/simple-generic-match.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/simple-match-generic-tag.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/size-and-align.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/use-uninit-match.rs
https://github.com/rust-lang/rust/blob/master/src/test/run-pass/use-uninit-match2.rs

[pnkfelix]

@brson points out that we might nonetheless need further testing of the cases here; @alexcrichton relayed an anecdote that these used to be ignored, but clearly are not today (at least not in all cases).

Tagging as E-needstest, removing nomination without milestone.