Detect when negative literal indices are used and suggest appropriate code

[estebank]

No description provided.

[rust-highfive]

r? @davidtwco

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[meithecatte]

Suggested test case (no error should trigger):

use core::ops::Index;
struct X;
impl Index<i32> for X {
    type Output = ();
    
    fn index(&self, _: i32) -> &() {
        &()
    }
}

fn main() {
    X[-1];
}

[estebank]

@NieDzejkob added your case to the test.

[davidtwco]

This is looking really good, some minor changes. r=me after fixing.

[estebank]

@bors r=davidtwco

[bors]

📌 Commit e19d82f has been approved by davidtwco