Uncaught unconditional recursion in From impl

[CyrusAlbright]

Rustc has no issues compiling the following code (playground link):

#![deny(unconditional_recursion)]

struct Dummy;

impl From<u8> for Dummy {
    fn from(u: u8) -> Self {
        u.into()
    }
}

The other way of writing this, however, does (correctly) fail to compile (playground link):

#![deny(unconditional_recursion)]

struct Dummy;

impl From<u8> for Dummy {
    fn from(u: u8) -> Self {
        Dummy::from(u)
    }
}

I assume that type inference is happening after the unconditional_recursion lint causing it to miss this?

[eggyal]

The lint only detects direct recursion (where a function calls itself). It doesn't detect recursion through other functions, so the following will compile (playground):

#![deny(unconditional_recursion)]

pub fn a() {
    b()
}

pub fn b() {
    a()
}

In your example, <Dummy as From<u8>>::from is calling <u8 as Into<Dummy>>::into which then calls back to the former.

[jruderman]

This is a case of #57965 where the recursion goes through trait functions.