#[derive(Debug)] fails when a generic parameter shares the name of the generic type

[nekodjin]

I tried this code:

#[derive(Debug)]
pub struct Irrelevant<Irrelevant> {
    irrelevant: Irrelevant,
}

I expected to see this happen:
Code should compile normally as expected, with a correct fmt::Debug implementation. Omitting the derive shows that rustc accepts this as valid and intelligible code. We know that rustc is interpreting the type of irrelevant as the type parameter and not the parent struct, as the latter would produce an error since it would produce a recursive type with infinite size which would cause an error. As rustc defines the rust language, the derive macro should also handle this code correctly.

Instead, this happened:
Compilation errors with the following diagnostic:

error[E0574]: expected struct, variant or union type, found type parameter `Irrelevant`
 --> main.rs:3:10
  |
3 | #[derive(Debug)]
  |          ^^^^^ not a struct, variant or union type
  |
  = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0109]: type arguments are not allowed for this type
 --> main.rs:4:23
  |
3 | #[derive(Debug)]
  |          ----- in this derive macro expansion
4 | pub struct Irrelevant<Irrelevant> {
  |                       ^^^^^^^^^^ type argument not allowed
  |
  = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

Meta

I tested this and got the same result on the following versions:

rustc 1.61.0 (fe5b13d68 2022-05-18)
binary: rustc
commit-hash: fe5b13d681f25ee6474be29d748c65adcd91f69e
commit-date: 2022-05-18
host: x86_64-unknown-linux-gnu
release: 1.61.0
LLVM version: 14.0.0

rustc 1.63.0-nightly (ee160f2f5 2022-05-23)
binary: rustc
commit-hash: ee160f2f5e73b6f5954bc33f059c316d9e8582c4
commit-date: 2022-05-23
host: x86_64-unknown-linux-gnu
release: 1.63.0-nightly
LLVM version: 14.0.4

[nekodjin]

Running with macro backtrace does not provide any enlightening information as the macro is a compiler builtin.

[Noratrieb]

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

pub struct Irrelevant<Irrelevant> {
    irrelevant: Irrelevant,
}

impl<Irrelevant: Debug> Debug for Irrelevant<Irrelevant> {
    fn fmt(&self, f: &mut Formatter) -> ::core::fmt::Result {
        match self {
            Irrelevant { irrelevant } => {
                f.debug_struct("Irrelevant")
                    .field("irrelevant", irrelevant)
                    .finish()
            }
        }
    }
}

Expanding and cleaning up the expansion yields this, which causes the same errors.
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1118974f80862e77fe76fa5be67901cf

[Noratrieb]

So I don't think this is a bug, since the issue is caused by exactly the name resolution specifics that allow the struct in the first place.

So this is a bad diagnostic at worst.

[fmease]

From my perspective this is indeed a bug. The built-in macro does not respect hygiene. The output can be fixed by prepending self:: to the implementing type. Playground. This is what the macro should do, too.

[nekodjin]

I agree. It's hard to see this as anything other than a bug, as I see it.

[Noratrieb]

Hm true, they could indeed use self::. Or they could even use the actual Self.

[fmease]

@rustbot claim

[fmease]

Ah, what a shame, self:: of course doesn't work if the type and the impl are inside of a function 🤦‍♂️ 😞

[fmease]

@estebank Do you think this hygiene issue was potentially caused by #90519 or was it likely already present before that?

[estebank]

@fmease it predates it, it just had worse output:

error[E0574]: expected struct, variant or union type, found type parameter `Irrelevant`
 --> <source>:1:10
  |
1 | #[derive(Debug)]
  |          ^^^^^ not a struct, variant or union type
  |
  = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0109]: type arguments are not allowed for this type
 --> <source>:1:10
  |
1 | #[derive(Debug)]
  |          ^^^^^ type argument not allowed
  |
  = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0210]: type parameter `Irrelevant` must be used as the type parameter for some local type (e.g., `MyStruct<Irrelevant>`)
 --> <source>:1:10
  |
1 | #[derive(Debug)]
  |          ^^^^^ type parameter `Irrelevant` must be used as the type parameter for some local type
  |
  = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
  = note: only traits defined in the current crate can be implemented for a type parameter

error: aborting due to 3 previous errors

The issue with the clashing of the literal struct inside of fmt can be addressed by changing from the name to Self. For the type in the impl block, I'm less certain. As shown above, prepending self:: won't work in any non-mod scope.

---

As a separate concern, I would like us to improve E0109 to account for this case and recommend you changing the type param Irrelevant so it doesn't clash with the type Irrelevant, regardless of whether it happens in a derive macro or not.