Diagnostics Improvement: Same Body in Branches

[jamesmunns]

I tried this code:

#[derive(PartialEq, Eq, Debug)]
struct SortedEdge {
    a: usize,
    b: usize,
}

impl SortedEdge {
    fn new(a: usize, b: usize) -> SortedEdge {
        if b > a {
            SortedEdge { a, b }
        } else {
            // uh oh!
            // We really want:
            //   SortedEdge { a: b, b: a }
            SortedEdge { b, a }
        }
    }
}

fn main() {
    let x = SortedEdge::new(1, 2);
    let y = SortedEdge::new(2, 1);
    
    // The points should be equal after sorting indicies
    assert_eq!(x, y);
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=cc7599a28cfe7dd105f3f4d609c47b47

I expected to see this happen:

A compiler warning/diagnostic that both of my branches were equivalent.

Instead, this happened:

No warning was emitted

Meta

This happens on Rust 1.47.0 stable and Rust nightly 2020-11-18

CC @estebank, and the related discussion on twitter

[matthiaskrgr]

Clippy has a lint that is supposed to detect this https://rust-lang.github.io/rust-clippy/master/index.html#if_same_then_else but it does not catch this example (false negative I guess..)

[Y-Nak]

The false-negative @matthiaskrgr pointed was indirectly solved in this PR.