borrowck cares about field names with disjoint enum downcasts

[arielb1]

STR

This compiles:

pub enum Foo {
    X { foo: u32 },
    Y { bar: u32 }
}

pub fn foo(mut x: Foo) {
    let mut y = None;
    let mut z = None;
    if let Foo::X { ref foo } = x {
        z = Some(foo);
    }
    if let Foo::Y { ref mut bar } = x {
        y = Some(bar);
    }
    drop((y, z));
}

fn main() {}

If you change the fields to have identical names, it doesn't:

pub enum Foo {
    X { foo: u32 },
    Y { foo: u32 }
}

pub fn foo(mut x: Foo) {
    let mut y = None;
    let mut z = None;
    if let Foo::X { ref foo } = x {
        z = Some(foo); //~ NOTE previous borrow of `x.foo` occurs here
    }
    if let Foo::Y { ref mut foo } = x {
        y = Some(foo); //~ ERROR cannot borrow `x.foo` as mutable
    }
    drop((y, z));
}

fn main() {}

Expected

Field names aren't supposed to be significant between different variants.

[pnkfelix]

that is amusing

[bluss]

Fixed in #27929, which includes a test.