Better Diagnostics When Using Wrong Brackets to Create an Array

[Hpmason]

I ended up spending way too much time debugging the following error, since I was trying to port some C code to rust.

const ARR: [u8; 3] = {
    1, 2, 3
}

Give the error:

error: expected one of `.`, `;`, `?`, `}`, or an operator, found `,`
 --> src/lib.rs:2:6
  |
2 |     1, 2, 3,
  |      ^ expected one of `.`, `;`, `?`, `}`, or an operator

error: aborting due to previous error

error: could not compile `playground`

The error message is very vague and hard to determine the actual error here.
Seeing as though some programming languages like Java and C use {} for arrays, I think it'd make sense for the compiler to suggest using [] if the user is trying to create an array.

[hkmatsumoto]

I suppose adding manual diagnostic somewhere around rustc_parse::parse::parse_block_expr will get the job done.
@rustbot claim