Recover and suggest wrapping in block in const generic value application

[Centril]

cc @varkor @rust-lang/wg-diagnostics

Say we have:

#![feature(const_generics)]

fn foo<const N: usize>() {}

fn bar() {
    foo::<1 + 1>();
}

the parser now greets you with:

error: expected one of `,` or `>`, found `+`
 --> src/lib.rs:6:13
  |
6 |     foo::<1 + 1>();
  |             ^ expected one of `,` or `>` here

It would be good if you got:

error: expected one of `,` or `>`, found `+`
 --> src/lib.rs:6:13
  |
6 |     foo::<1 + 1>();
  |             ^ expected one of `,` or `>` here
  |
  | help: wrap the expression in a block:
6 |     foo::<{1 + 1}>();

or some such.

[oli-obk]

Won't attempting to fix this issue hit exactly the same issues that just allowing foo::<1 + 1>(); hits?

[estebank]

@oli-obk allowing blockless const expressions result in ambiguous grammar for the general case, but there's a subset that is unambiguous, and we get away with best effort attempts for the sake of diagnostics. Being able to parse almost any const expression with the exception of those involving greater than is great for error recovery, but would be terrible for the feature/grammar.