Replacing a variable in Rust with same value yields different result

Question

I could not understand this code. How comes -a yields different result from -7?

fn main() {
    let a: i32 = 7; // or any other integer type
    let b = 4;
                
    assert_eq!((-a).rem_euclid(b), 1);
    assert_eq!(-7_i32.rem_euclid(4), -3);
    assert_eq!(-a, -7_i32);
}

Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3147fbbf7aeeeff5510522f9af2d12f2

It is because in second assertion you negate the result, in first one you negate the operand — Ömer Erden
– Ömer Erden, Commented Jul 3, 2020 at 19:13
@yolenoyer Because it's a basic question that shows no research and is not useful to others. Playing around with the placement of the parentheses would have shown the difference pretty quickly. — Herohtar
– Herohtar, Commented Jul 3, 2020 at 23:18

Emerson Harkin · Accepted Answer · 2020-07-03 19:11:58Z

4

It's because -7_i32.rem_euclid(4) == -(7_i32.rem_euclid(4)). The unary negation has lower precedence than the method call.

answered Jul 3, 2020 at 19:11

Emerson Harkin

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