Warn when intended Deref is not triggered correctly

[jamesmcm]

What it does

Aim would be to warn when intended Deref is not triggered correctly.

E.g. for https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c78429c73f85949bd3799a9fdcaa4d74

use std::ops::Deref;

struct TestType;

impl Deref for TestType {
    type Target = str;

    fn deref(&self) -> &Self::Target {
        "test"
    }
}

impl std::fmt::Display for TestType {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{}", &**self)
    }
}

fn main() {
    let test = TestType {};
    println!("Hello, world!, {}", test);
}

If we write:

        write!(f, "{}", &self)

or

        write!(f, "{}", &*self)

Then the Deref to str is not triggered and we got a stack overflow due to recursively calling Display::fmt

At the moment there are no clippy or rustc warnings or errors, and we just get a stack overflow.

Categories (optional)

• Kind: clippy::suspicious

What is the advantage of the recommended code over the original code

The recommended code correctly triggers the Deref and avoids the stack overflow.

[camsteffen]

We already have to_string_in_display. I think it should be enhanced and renamed to recursive_display_impl, meaning any usage of self as Display in impl Display.

[camsteffen]

Duplicate of #2691