Overflowed stack

[JustForFun88]

I tried this code:

struct CharBuffer {
    buf: [u8; 4],
    len: usize,
}

impl From<CharBuffer> for char {
    #[inline]
    fn from(char: CharBuffer) -> Self {
        char.into()
    }
}

fn main() {
    let foo = CharBuffer{ buf: [0; 4], len: 0 };
    let bar: char = char::from(foo);
}

I expected to see this happen:

error[E0277]: the trait bound `char: From<CharBuffer>` is not satisfied
 --> src/main.rs:9:9
  |
9 |         char.into()
  |         ^^^^ ---- required by a bound introduced by this call
  |         |
  |         the trait `From<CharBuffer>` is not implemented for `char`
  |
  = note: required for `CharBuffer` to implement `Into<char>`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error

Instead, this happened:

thread 'main' has overflowed its stack

[bjorn3]

I can reproduce this on the playground when using nightly, but not locally.

[albertlarsan68]

It is the created binary that crashes, not the rustc process.

It can be explained by the fact that there is a recursive call in the impl From<CharBuffer> for char.

[bjorn3]

That explains why I couldn't reproduce it locally 😅 I didn't run the executable. Only rustc.

[JustForFun88]

I know that in general it is a problem of recursive call. I was just confused by the silent compilation.

The compiler didn't even warn that there was a recursive call of the same function.

[albertlarsan68]

The bug may be this, that there is no diagnostic concerning this unconditional recursion.

@rustbot label +A-diagnostics +D-terse

[quat1024]

This is a case of #57965, the unconditional_recursion lint not detecting mutual recursion.

See also: #98217

[jruderman]

I think this can be closed as a duplicate of #98217