`format!` debug fails even though Debug is implemented

[vitorenesduarte]

I tried this code:

use std::fmt::Debug;

trait Trait {
    type A: Debug;
}

struct S1 {}

impl Trait for S1 {
    type A = u64;
}

#[derive(Debug)]
struct S2<T: Trait> {
    a: T::A,
}

fn main() {
    let s = S2::<S1> { a: 10 };
    format!("{:?}", s);
}

When trying to compile it, I get:

error[E0277]: `S1` doesn't implement `std::fmt::Debug`
  --> src/main.rs:20:21
   |
20 |     format!("{:?}", s);
   |                     ^ `S1` cannot be formatted using `{:?}`
   |
   = help: the trait `std::fmt::Debug` is not implemented for `S1`
   = note: add `#[derive(Debug)]` or manually implement `std::fmt::Debug`
   = note: required because of the requirements on the impl of `std::fmt::Debug` for `S2<S1>`
   = note: required by `std::fmt::Debug::fmt

The[derive(Debug)] alone seems to work since the program compiles without the format!.

Meta

rustc --version --verbose:

rustc 1.43.1 (8d69840ab 2020-05-04)
binary: rustc
commit-hash: 8d69840ab92ea7f4d323420088dd8c9775f180cd
commit-date: 2020-05-04
host: x86_64-apple-darwin
release: 1.43.1
LLVM version: 9.0

[vitorenesduarte]

I've tried to remove the #[derive(Debug)] and add the obvious implementation. The code now compiles.

use std::fmt::{self, Debug};

trait Trait {
    type A: Debug;
}

struct S1 {}

impl Trait for S1 {
    type A = u64;
}

struct S2<T: Trait> {
    a: T::A,
}

impl<T: Trait> Debug for S2<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{:?}", self)
    }
}

fn main() {
    let s = S2::<S1> { a: 10 };
    format!("{:?}", s);
}

However, when I run it, I get:

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

[DutchGhost]

use std::fmt::Debug;

trait Trait {
    type A: Debug;
}

struct S1 {}

impl Trait for S1 {
    type A = u64;
}

#[derive(Debug)]
struct S2<T: Trait> {
    a: T::A,
}

The generated Debug impl expanded is:

#[automatically_derived]
#[allow(unused_qualifications)]
impl<T: ::core::fmt::Debug + Trait> ::core::fmt::Debug for S2<T>
where
    T::A: ::core::fmt::Debug,
{
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        match *self {
            S2 { a: ref __self_0_0 } => {
                let mut debug_trait_builder = f.debug_struct("S2");
                let _ = debug_trait_builder.field("a", &&(*__self_0_0));
                debug_trait_builder.finish()
            }
        }
    }
}

SinceS1 does NOT implement Debug, S2<S1> does not either, so a call to format!() fails

[DutchGhost]

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

Here you call write!("{}", self), which calls the debug implementation of S2<T>, which calls the debug impl of S2<T> which calls....infinite recursion!

[jonas-schievink]

You are running into #26925 and #57965, so closing in favor of those

[vitorenesduarte]

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

Here you call write!("{}", self), which calls the debug implementation of S2<T>, which calls the debug impl of S2<T> which calls....infinite recursion!

Thanks for pointing it out. Replacing self with self.a solves the issue.