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Friendlier compiler message for incorrect trait method signatures #28011

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andersforsgren opened this Issue Aug 26, 2015 · 4 comments

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@andersforsgren @nagisa @birkenfeld @estebank @Mark-Simulacrum @jdm @steveklabnik
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andersforsgren commented Aug 26, 2015

When implementing a trait, it would be useful to see the expected method signature in compiler error messages. Example

impl std::fmt::Display for MyType {
   // Not sure what to put here so I stub it and hope to get help from error
   fn fmt() -> () { } 
}

The current error message for E0186 only lets me partially resolve the issue:

error: method `fmt` has a `&self` declaration in the trait, but not in the impl [E0186]

It would be more friendly to the user if the message showed both the actual and expected method signatures.

Incorrect method signature for trait `Display` method `fmt`
Actual signature    fn fmt() -> ()
Expected signature  fn fmt(&self, &mut Formatter) -> Result<(), Error>

@jdm jdm added the A-diagnostics label Aug 26, 2015

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nagisa commented Aug 26, 2015

I wanted similar behaviour occasionally as well. Looking signatures up in the docs is painful.
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birkenfeld commented May 2, 2016

related: #24626, which requests the same for missing methods in impls.

estebank added a commit to estebank/rust that referenced this issue Sep 10, 2016

@estebank
Show full trait method signatures on E0186 
Given file `foo.rs`:

```rust
struct A {}

impl std::fmt::Display for A {
    fn fmt() -> () {}
}
```

provide the expected method signature:

```bash
error: main function not found

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
 --> foo.rs:4:5
  |
4 |     fn fmt() -> () {}
  |     ^^^^^^^^^^^^^^^^^ expected `&self` in impl
  |
  = note: Expected signature: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;
  = note:    Found signature: fn fmt();

error: aborting due to previous error
```

Fixes rust-lang#28011
04e901a

estebank added a commit to estebank/rust that referenced this issue Sep 10, 2016

@estebank
Show type of missing/incorrect trait methods 
For a given file `foo.rs`:

```rust
use std::str::FromStr;

struct A {}

trait X<T> {
    type Foo;
    const BAR: u32 = 128;

    fn foo() -> T;
    fn bar();
    fn    bay<
        'lifetime,    TypeParameterA
            >(  a   : usize,
                b: u8  );
}

impl std::fmt::Display for A {
}
impl FromStr for A{}

impl X<usize> for A {
}
```

Provide the following output:

```bash
error: main function not found

error[E0046]: not all trait items implemented, missing: `fmt`
  --> foo.rs:18:1
   |
18 | impl std::fmt::Display for A {
   | ^ missing `fmt` in implementation
   |
   = note: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;

error[E0046]: not all trait items implemented, missing: `Err`, `from_str`
  --> foo.rs:20:1
   |
20 | impl FromStr for A{}
   | ^^^^^^^^^^^^^^^^^^^^ missing `Err`, `from_str` in implementation
   |
   = note: type Err;
   = note: fn from_str(&str) -> std::result::Result<Self, <Self as std::str::FromStr>::Err>;

error[E0046]: not all trait items implemented, missing: `Foo`, `foo`, `bar`, `bay`
  --> foo.rs:22:1
   |
22 | impl X<usize> for A {
   | ^ missing `Foo`, `foo`, `bar`, `bay` in implementation
   |
   = note: type Foo;
   = note: fn foo() -> T;
   = note: fn bar();
   = note: fn bay<'lifetime, TypeParameterA>(a: usize, b: u8);

error: aborting due to 3 previous errors
```

Fixes rust-lang#24626

For a given file `foo.rs`:

```rust
struct A {}

impl std::fmt::Display for A {
    fn fmt() -> () {}
}
```

provide the expected method signature:

```bash
error: main function not found

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
 --> foo.rs:4:5
  |
4 |     fn fmt() -> () {}
  |     ^^^^^^^^^^^^^^^^^ expected `&self` in impl
  |
  = note: Expected signature: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;
  = note:    Found signature: fn fmt();

error: aborting due to previous error
```

Fixes rust-lang#28011
d53289f

@estebank estebank referenced this issue Sep 10, 2016

Closed

Include type of missing trait methods in error #36371

estebank added a commit to estebank/rust that referenced this issue Oct 1, 2016

@estebank
Show type of missing/incorrect trait methods 
For a given file `foo.rs`:

```rust
use std::str::FromStr;

struct A {}

trait X<T> {
    type Foo;
    const BAR: u32 = 128;

    fn foo() -> T;
    fn bar();
    fn    bay<
        'lifetime,    TypeParameterA
            >(  a   : usize,
                b: u8  );
}

impl std::fmt::Display for A {
}
impl FromStr for A{}

impl X<usize> for A {
}
```

Provide the following output:

```bash
error: main function not found

error[E0046]: not all trait items implemented, missing: `fmt`
  --> foo.rs:18:1
   |
18 | impl std::fmt::Display for A {
   | ^ missing `fmt` in implementation
   |
   = note: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;

error[E0046]: not all trait items implemented, missing: `Err`, `from_str`
  --> foo.rs:20:1
   |
20 | impl FromStr for A{}
   | ^^^^^^^^^^^^^^^^^^^^ missing `Err`, `from_str` in implementation
   |
   = note: type Err;
   = note: fn from_str(&str) -> std::result::Result<Self, <Self as std::str::FromStr>::Err>;

error[E0046]: not all trait items implemented, missing: `Foo`, `foo`, `bar`, `bay`
  --> foo.rs:22:1
   |
22 | impl X<usize> for A {
   | ^ missing `Foo`, `foo`, `bar`, `bay` in implementation
   |
   = note: type Foo;
   = note: fn foo() -> T;
   = note: fn bar();
   = note: fn bay<'lifetime, TypeParameterA>(a: usize, b: u8);

error: aborting due to 3 previous errors
```

Fixes rust-lang#24626

For a given file `foo.rs`:

```rust
struct A {}

impl std::fmt::Display for A {
    fn fmt() -> () {}
}
```

provide the expected method signature:

```bash
error: main function not found

error[E0186]: method `fmt` has a `&self` declaration in the trait, but not in the impl
 --> foo.rs:4:5
  |
4 |     fn fmt() -> () {}
  |     ^^^^^^^^^^^^^^^^^ expected `&self` in impl
  |
  = note: Expected signature: fn fmt(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error>;
  = note:    Found signature: fn fmt();

error: aborting due to previous error
```

Fixes rust-lang#28011
f9c0319
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estebank commented Nov 12, 2016

Related: #37370.

@steveklabnik steveklabnik removed the T-compiler label Mar 9, 2017

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Mark-Simulacrum commented Jun 1, 2017

Worth pointing out that the signature is relatively easy to get today if the method is removed. Still a problem though.

@estebank estebank referenced this issue Jun 1, 2017

Merged

Show trait method signature when impl differs #42362

bors added a commit that referenced this issue Jun 4, 2017

@bors
Auto merge of #42362 - estebank:type, r=arielb1 
Show trait method signature when impl differs

When the trait's span is available, it is already being used, add a
`note` for the cases where the span isn't available:

<pre>
error[E0053]: <b>method `fmt` has an incompatible type for trait</b>
  --> $DIR/trait_type.rs:17:4
   |
17 |    fn fmt(&self, x: &str) -> () { }
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ types differ in mutability
   |
   = note: expected type `<b>fn(&MyType, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error></b>`
              found type `<b>fn(&MyType, &str)</b>`

error[E0050]: <b>method `fmt` has 1 parameter but the declaration in trait `std::fmt::Display::fmt` has 2</b>
  --> $DIR/trait_type.rs:21:11
   |
21 |    fn fmt(&self) -> () { }
   |           ^^^^^ expected 2 parameters, found 1
   |
   = note: `fmt` from trait: `<b>fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error></b>`

error[E0186]: <b>method `fmt` has a `&self` declaration in the trait, but not in the impl</b>
  --> $DIR/trait_type.rs:25:4
   |
25 |    fn fmt() -> () { }
   |    ^^^^^^^^^^^^^^^^^^ expected `&self` in impl
   |
   = note: `fmt` from trait: `<b>fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error></b>`

error[E0046]: <b>not all trait items implemented, missing: `fmt`</b>
  --> $DIR/trait_type.rs:28:1
   |
28 | impl std::fmt::Display for MyType4 {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `fmt` in implementation
   |
   = note: `fmt` from trait: `<b>fn(&Self, &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error></b>`
</code></pre>

Fix #28011.
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0418fa9

@bors bors closed this in #42362 Jun 4, 2017

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