Bogus error when using `..` in struct expr with all fields present, in a const ("error[E0016]: blocks in constants are limited to items and tail expressions")

[jorendorff]

struct X { a: i32, b: i32 }

#[allow(unused_variables)]
fn main() {
    const DX: X = X { a: 0, b: 0 };
    const X1: X = X { a: 1, ..DX };  // ok
    let   x2    = X { a: 1, b: 2, ..DX };  // ok
    const X3: X = X { a: 1, b: 2, ..DX };  // error[E0016]
}

• It's weird that X1 and x2 are allowed, but not X3.
• Even if that's intentional, the error message seems off.

[jakubadamw]

The provided code compiles fine both in 1.36 and the 2019-08-12 nightly.

I did a quick search through the test suite and there's none for this particular case. Marking as such.

@rustbot modify labels: E-needstest