Positive coverage: prove_existential_var_eq / existential-existential

17 tests exercised this rule:

Source location: tests/borrowck.rs:2105

fn foo() -> i32 {
    exists<'r0, 'r1> {
        let result: i32;
        {
            let v1: i32 = 22_i32;
            let v2: &'r0 i32 = &'r1 v1;
            result = *v2;
        }
        return result;
    }
}

Source location: tests/borrowck.rs:2474

fn foo () -> u32 {
    exists<'l_p, 'l_q, 'loan_0, 'loan_1, 'loan_2, 'loan_3> {
        let a: u32 = 0_u32;
        let b: u32 = 0_u32;
        // In Rustc, the 1-tuple is needed for some reason
        // Niko does not 100% understand, else rustc is able to
        // see that this program is safe.
        let q: &'l_q mut u32 = &mut 'loan_0 a;
        let p: &'l_p mut u32 = &mut 'loan_1 a;
        if true {
            p = &mut 'loan_1 a;
            q = &mut 'loan_2 b;
        } else {
            p = &mut 'loan_3 b;
        }
        *q = 1_u32;
        return *p;
    }
}

Source location: tests/borrowck.rs:2664

fn foo() -> u32 {
    exists<'r0, 'r1> {
        'a: loop {
            let x: i32 = 0_i32;
            let r: &'r0 i32 = &'r1 x;
            let _y: i32 = *r;
            continue 'a;
        }
    }
}

Source location: tests/borrowck.rs:3027

fn bar() -> u32 {
    exists<'r0, 'r1> {
        let v: u32 = 1_u32;
        let p: &'r0 u32 = &'r1 v;
        foo(0_u32);
        return *p;
    }
}