WTF Error Message with `impl<T> Deref for T

[Havvy]

Play link: http://is.gd/WYlDGu

use std::ops::Deref;

struct Foo;

impl<Foo> Deref for Foo { }

fn main() {}

Error message:

<anon>:5:1: 5:28 error: type parameter `Foo` must be used as the type parameter for some local type (e.g. `MyStruct<T>`); only traits defined in the current crate can be implemented for a type parameter [E0210]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0210
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `core`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `core`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `collections`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `collections`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `core`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `core`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `alloc`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `alloc`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `collections`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `core`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `alloc`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `collections`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `core`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 error: conflicting implementations for trait `core::ops::Deref` [E0119]
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
<anon>:5:1: 5:28 help: see the detailed explanation for E0119
<anon>:5:1: 5:28 note: conflicting implementation in crate `std`
<anon>:5 impl<Foo> Deref for Foo { }
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to 22 previous errors

[apasel422]

The error messages are correct. The existence of struct Foo here is irrelevant, as the impl is saying to implement Deref for all types Foo that are Sized. To make the issue more obvious, the impl could be rewritten as impl<T> Deref for T {} -- this clearly conflicts with all other types U: Sized that implement Deref, including those in the std, core, alloc, and collections crates.

It might be possible to improve the error message to suggest removing the type parameter from the impl when a type parameter shadows a type that is in scope.

Similar to #27218.

[jonas-schievink]

It would be useful if the note actually contained the conflicting impl and not the same code that's already printed by the error. I wonder how hard it would be to reconstruct the impl header from the crate metadata.

It should also probably abort after like 5 conflicts for the same impl and just print note: N more conflicts not shown or something similar.

[Havvy]

Yeah, I wasn't claim the error message was incorrect. Sorry about that. I was more claiming that the error message is unwieldy.

[birkenfeld]

The "type parameter aliases type" part of this issue is #24354.

[Mark-Simulacrum]

Today's error message is much shorter, and a little more helpful. We explicitly call out Foo as a type parameter; and also state that it must be used for some local type -- this sort of helps lead to a "oh, the Foo in the RHS isn't a local type" realization, hopefully. I'm not really sure what the best way of communicating this to the user would be, though. Any thoughts?

error[E0119]: conflicting implementations of trait `std::ops::Deref` for type `&_`:
 --> test.rs:5:1
  |
5 | impl<Foo> Deref for Foo { }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`

error[E0210]: type parameter `Foo` must be used as the type parameter for some local type (e.g. `MyStruct<T>`); only traits defined in the current crate can be implemented for a type parameter
 --> test.rs:5:1
  |
5 | impl<Foo> Deref for Foo { }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error(s)

[Havvy]

The lack of the repeating error counts as a fix for this issue. Marking as closed.

@Mark-Simulacrum, open another issue for that if you feel it deserves it.