WTF Error Message with `impl<T> Deref for T #28981

Havvy · 2015-10-12T04:15:17Z

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

The text was updated successfully, but these errors were encountered:

apasel422 · 2015-10-12T11:31:17Z

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 · 2015-10-12T12:57:48Z

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 · 2015-10-12T18:03:05Z

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

This makes the error message in #28981 a bit shorter (152 to 115 lines). Previous output (the local impl was always printed twice when it conflicted with an external impl): ``` test.rs:3:1: 3:23 error: conflicting implementations for trait `core::ops::Deref` [E0119] test.rs:3 impl<T> Deref for T {} ^~~~~~~~~~~~~~~~~~~~~~ test.rs:3:1: 3:23 help: run `rustc --explain E0119` to see a detailed explanation test.rs:3:1: 3:23 note: conflicting implementation in crate `std` test.rs:3 impl<T> Deref for T {} ^~~~~~~~~~~~~~~~~~~~~~ ``` Output after this patch: ``` test.rs:3:1: 3:23 error: conflicting implementations for trait `core::ops::Deref` [E0119] test.rs:3 impl<T> Deref for T {} ^~~~~~~~~~~~~~~~~~~~~~ test.rs:3:1: 3:23 help: run `rustc --explain E0119` to see a detailed explanation note: conflicting implementation in crate `std` ```

birkenfeld · 2016-05-02T09:03:17Z

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

Mark-Simulacrum · 2017-06-01T19:30:54Z

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 · 2017-06-01T20:36:50Z

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.

huonw added the A-diagnostics Area: Messages for errors, warnings, and lints label Oct 12, 2015

jonas-schievink mentioned this issue Oct 27, 2015

Don't print the same impl twice on ext. overlap #29404

Merged

steveklabnik removed the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Mar 9, 2017

Havvy closed this as completed Jun 1, 2017

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WTF Error Message with `impl<T> Deref for T #28981

WTF Error Message with `impl<T> Deref for T #28981

Havvy commented Oct 12, 2015

apasel422 commented Oct 12, 2015

jonas-schievink commented Oct 12, 2015

Havvy commented Oct 12, 2015

birkenfeld commented May 2, 2016

Mark-Simulacrum commented Jun 1, 2017

Havvy commented Jun 1, 2017

WTF Error Message with `impl<T> Deref for T #28981

WTF Error Message with `impl<T> Deref for T #28981

Comments

Havvy commented Oct 12, 2015

apasel422 commented Oct 12, 2015

jonas-schievink commented Oct 12, 2015

Havvy commented Oct 12, 2015

birkenfeld commented May 2, 2016

Mark-Simulacrum commented Jun 1, 2017

Havvy commented Jun 1, 2017