"Unnecessary" braces have an influence

[vandenheuvel]

On 1.45.0-nightly (2020-05-29 4bd32c98047a809ba5fd)

fn f() -> usize {
    {1usize} - 1usize
}

Intuitively, I would expect these braces to be redundant and also have no influence, but they do. The compiler expects a () type and then considers the minus sign a unary operator.

warning: unnecessary braces around block return value
 --> src/lib.rs:6:5
  |
6 |     {1usize} - 1usize
  |     ^^^^^^^^ help: remove these braces
  |
  = note: `#[warn(unused_braces)]` on by default

error[E0308]: mismatched types
 --> src/lib.rs:6:6
  |
6 |     {1usize} - 1usize
  |      ^^^^^^ expected `()`, found `usize`

error[E0600]: cannot apply unary operator `-` to type `usize`
 --> src/lib.rs:6:14
  |
6 |     {1usize} - 1usize
  |              ^^^^^^^^ cannot apply unary operator `-`
  |
  = note: unsigned values cannot be negated

It seems to me that either the error message about the braces being redundant is incorrect or misleading, or something more nefarious seems to be going on with scopes, expressions, etc.

[RalfJung]

Potentially related: #59975

[vandenheuvel]

Verified that this bug is still present as of today.

[vandenheuvel]

Verified that this bug is still present as of today.