Using into() in the From trait without having implemented Into should generate a compilation error.

[ThibaultEbroussard]

Code

fn main() {
    #[derive(Debug)]
    struct Number(f32);

    impl From<f32> for Number {
        fn from(value: f32) -> Self {
            value.into()
        }
    }

    let value: Number = (10.0).into();
    println!("{value:?}");
}

Current output

Compiling number v0.1.0 (/home/number)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/number`

thread 'main' has overflowed its stack
fatal runtime error: stack overflow
[1]    25988 IOT instruction (core dumped)  cargo run

Desired output

The compiler should detect that we are calling Into for f32 trait which is automatically implemented by From for Number trait in our implementation of the From for Number trait.

Rationale and extra context

No response

Other cases

No response

Anything else?

No response

[riking]

Implementation would be a deny-by-default lint. Not sure whether to recommend Clippy first or straight into rustc.

[Noratrieb]

I agree that this should live in clippy instead of rustc, @matthiaskrgr do you want to move the issue :)?

[y21]

I think a more general version of this is #11032. The blanket Into impl just has U::from(self) in its implementation, so this is basically the same idea: from -> into -> from -> ...