Better error messages for &T == T #40660

iqualfragile · 2017-03-20T02:09:41Z

Something along the lines of "you are trying to compare a refference to to Type, try dereferencing first" would be nice, right now it just has the usual PartialEq not implemented.

estebank · 2017-03-21T01:12:01Z

Would the following output (as proposed in #40565) be enough to make the situation a bit more clear?

error[E0277]: the trait bound `&T: std::cmp::PartialEq<T>>` is not satisfied
  --> $DIR/file.rs:14:5
   |
14 |     &t == t;
   |     ^^^^^^^ can't compare `&T` with `T`

iqualfragile · 2017-03-21T01:18:41Z

that would certainly improve things, but in this case the error message could be even more specific and thereby helpful by providing the information about deref-ing, so i would keep it as a separate issue, i think

Add better error message when == operator is badly used Part of #40660. With the following code: ```rust fn foo<T: PartialEq>(a: &T, b: T) { a == b; } fn main() { foo(&1, 1); } ``` It prints: ``` error[E0277]: the trait bound `&T: std::cmp::PartialEq<T>` is not satisfied --> test.rs:2:5 | 2 | a == b; | ^^^^^^ can't compare `&T` with `T` | = help: the trait `std::cmp::PartialEq<T>` is not implemented for `&T` = help: consider adding a `where &T: std::cmp::PartialEq<T>` bound error: aborting due to previous error ```

estebank · 2017-12-06T19:46:19Z

Current output for:

fn main() {
    let x: usize = 3;
    let y: &usize = &3;
    if x == y {
        println!("asdf");
    }
    if y == x {
        println!("asdf");
    }
    if 3 == &3 {
        println!("asdf");
    }
    if &3 == 3 {
        println!("asdf");
    }
}

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |     if x == y {
  |             ^
  |             |
  |             expected usize, found &usize
  |             help: consider dereferencing the borrow: `*y`
  |
  = note: expected type `usize`
             found type `&usize`

error[E0277]: the trait bound `&usize: std::cmp::PartialEq<usize>` is not satisfied
 --> src/main.rs:7:10
  |
7 |     if y == x {
  |          ^^ can't compare `&usize` with `usize`
  |
  = help: the trait `std::cmp::PartialEq<usize>` is not implemented for `&usize`

error[E0277]: the trait bound `{integer}: std::cmp::PartialEq<&{integer}>` is not satisfied
  --> src/main.rs:10:10
   |
10 |     if 3 == &3 {
   |          ^^ can't compare `{integer}` with `&{integer}`
   |
   = help: the trait `std::cmp::PartialEq<&{integer}>` is not implemented for `{integer}`

error[E0277]: the trait bound `&{integer}: std::cmp::PartialEq<{integer}>` is not satisfied
  --> src/main.rs:13:11
   |
13 |     if &3 == 3 {
   |           ^^ can't compare `&{integer}` with `{integer}`
   |
   = help: the trait `std::cmp::PartialEq<{integer}>` is not implemented for `&{integer}`

There's room for improvement.

estebank · 2019-03-08T16:21:56Z

Current output

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |     if x == y {
  |             ^
  |             |
  |             expected usize, found &usize
  |             help: consider dereferencing the borrow: `*y`
  |
  = note: expected type `usize`
             found type `&usize`

error[E0277]: can't compare `&usize` with `usize`
 --> src/main.rs:7:10
  |
7 |     if y == x {
  |          ^^ no implementation for `&usize == usize`
  |
  = help: the trait `std::cmp::PartialEq<usize>` is not implemented for `&usize`

error[E0277]: can't compare `{integer}` with `&{integer}`
  --> src/main.rs:10:10
   |
10 |     if 3 == &3 {
   |          ^^ no implementation for `{integer} == &{integer}`
   |
   = help: the trait `std::cmp::PartialEq<&{integer}>` is not implemented for `{integer}`

error[E0277]: can't compare `&{integer}` with `{integer}`
  --> src/main.rs:13:11
   |
13 |     if &3 == 3 {
   |           ^^ no implementation for `&{integer} == {integer}`
   |
   = help: the trait `std::cmp::PartialEq<{integer}>` is not implemented for `&{integer}`

estebank · 2019-09-26T23:43:39Z

CC #45109, #60497, #57621, #52544, #39942, #44695.

rylev · 2020-12-09T17:50:52Z

Triage: current output is only slightly improved over previous output.

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |     if x == y {
  |             ^
  |             |
  |             expected `usize`, found `&usize`
  |             help: consider dereferencing the borrow: `*y`

error[E0277]: can't compare `&usize` with `usize`
 --> src/main.rs:7:10
  |
7 |     if y == x {
  |          ^^ no implementation for `&usize == usize`
  |
  = help: the trait `PartialEq<usize>` is not implemented for `&usize`

error[E0277]: can't compare `{integer}` with `&{integer}`
  --> src/main.rs:10:10
   |
10 |     if 3 == &3 {
   |          ^^ no implementation for `{integer} == &{integer}`
   |
   = help: the trait `PartialEq<&{integer}>` is not implemented for `{integer}`

error[E0277]: can't compare `&{integer}` with `{integer}`
  --> src/main.rs:13:11
   |
13 |     if &3 == 3 {
   |           ^^ no implementation for `&{integer} == {integer}`
   |
   = help: the trait `PartialEq<{integer}>` is not implemented for `&{integer}`

It's not clear what should improve to make this better. Should we try to detect if T implements the trait and if so, suggest dereferencing &T?

estebank · 2021-01-12T20:50:47Z

Should we try to detect if T implements the trait and if so, suggest dereferencing &T?

Yes, I think we should do exactly that. I think that #57621, #52544 and #44695 cover that requirement as well (but lets keep them open and close in one sweep once we fix this).

estebank · 2021-10-18T09:37:35Z

Output with #90006:

error[E0308]: mismatched types
  --> f16.rs:18:13
   |
18 |     if x == y {
   |             ^ expected `usize`, found `&usize`
   |
help: consider dereferencing the borrow
   |
18 |     if x == *y {
   |             +

error[E0369]: can't compare `&usize` with `usize`
  --> f16.rs:21:10
   |
21 |     if y == x {
   |        - ^^ - usize
   |        |
   |        &usize
   |
help: `==` can be applied on `&usize`
   |
21 |     if y == &x {
   |             +
help: `==` can be used on `usize`, you can dereference `y`
   |
21 |     if *y == x {
   |        +

error[E0277]: can't compare `{integer}` with `&{integer}`
  --> f16.rs:24:10
   |
24 |     if 3 == &3 {
   |          ^^ no implementation for `{integer} == &{integer}`
   |
   = help: the trait `PartialEq<&{integer}>` is not implemented for `{integer}`

error[E0277]: can't compare `&{integer}` with `{integer}`
  --> f16.rs:27:11
   |
27 |     if &3 == 3 {
   |           ^^ no implementation for `&{integer} == {integer}`
   |
   = help: the trait `PartialEq<{integer}>` is not implemented for `&{integer}`

estebank · 2023-11-17T19:17:45Z

Another case that should be accounted for (from #67571):

struct MyType<T>
    where T: PartialEq,
{
    data: T,
}

impl<T> MyType<T>
    where T: PartialEq,
{
    fn write(&mut self, data: T) {
        self.data = data;
    }

    fn matches(&self, data: &T) -> bool {
        data == self.data
    }
}

fn main() {
    let foo: MyType<String> = MyType { data: String::from("ABC") };
    let bar = String::from("XYZ");

    if foo.matches(&bar) {
         println!("{:?}", bar);
    }    
}

estebank · 2023-11-17T19:20:23Z

Triage, few changes:

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |     if x == y {
  |        -    ^ expected `usize`, found `&usize`
  |        |
  |        expected because this is `usize`
  |
help: consider dereferencing the borrow
  |
4 |     if x == *y {
  |             +

error[E0277]: can't compare `&usize` with `usize`
 --> src/main.rs:7:10
  |
7 |     if y == x {
  |          ^^ no implementation for `&usize == usize`
  |
  = help: the trait `PartialEq<usize>` is not implemented for `&usize`
  = help: the trait `PartialEq` is implemented for `usize`
  = help: for that trait implementation, expected `usize`, found `&usize`

error[E0277]: can't compare `{integer}` with `&{integer}`
  --> src/main.rs:10:10
   |
10 |     if 3 == &3 {
   |          ^^ no implementation for `{integer} == &{integer}`
   |
   = help: the trait `PartialEq<&{integer}>` is not implemented for `{integer}`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             isize
             i8
             i16
             i32
             i64
             i128
             usize
             u8
           and 6 others

error[E0277]: can't compare `&{integer}` with `{integer}`
  --> src/main.rs:13:11
   |
13 |     if &3 == 3 {
   |           ^^ no implementation for `&{integer} == {integer}`
   |
   = help: the trait `PartialEq<{integer}>` is not implemented for `&{integer}`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             isize
             i8
             i16
             i32
             i64
             i128
             usize
             u8
           and 6 others

sjwang05 · 2023-11-28T07:51:21Z

I've gotten the output to look like this:

fn main() {
    let x: usize = 3;
    let y: &usize = &3;
    _ = y == x;
    _ = &y == x;
    _ = &&y == x;
    _ = 3 == &3;
    _ = &3 == 3;
}

error[E0277]: can't compare `&usize` with `usize`
 --> foo.rs:6:11
  |
6 |     _ = y == x;
  |           ^^ no implementation for `&usize == usize`
  |
  = help: the trait `PartialEq<usize>` is not implemented for `&usize`
help: consider dereferencing here
  |
6 |     _ = *y == x;
  |         +

error[E0277]: can't compare `&&usize` with `usize`        
 --> foo.rs:7:12
  |
7 |     _ = &y == x;
  |            ^^ no implementation for `&&usize == usize`
  |
  = help: the trait `PartialEq<usize>` is not implemented for `&&usize`
help: consider removing the borrow and dereferencing instead
  |
7 -     _ = &y == x;
7 +     _ = *y == x;
  |

error[E0277]: can't compare `&&&usize` with `usize`
 --> foo.rs:8:13
  |
8 |     _ = &&y == x;
  |             ^^ no implementation for `&&&usize == usize`
  |
  = help: the trait `PartialEq<usize>` is not implemented for `&&&usize`
help: consider removing the borrows and dereferencing instead
  |
8 -     _ = &&y == x;
8 +     _ = *y == x;
  |

error[E0277]: can't compare `{integer}` with `&{integer}`
 --> foo.rs:9:11
  |
9 |     _ = 3 == &3;
  |           ^^ no implementation for `{integer} == &{integer}`
  |
  = help: the trait `PartialEq<&{integer}>` is not implemented for `{integer}`
  = help: the following other types implement trait `PartialEq<Rhs>`:
            isize
            i8
            i16
            i32
            i64
            i128
            usize
            u8
          and 6 others

error[E0277]: can't compare `&{integer}` with `{integer}`
  --> foo.rs:10:12
   |
10 |     _ = &3 == 3;
   |            ^^ no implementation for `&{integer} == {integer}`
   |
   = help: the trait `PartialEq<{integer}>` is not implemented for `&{integer}`
help: consider removing the borrow
   |
10 -     _ = &3 == 3;
10 +     _ = 3 == 3;
   |

error: aborting due to 5 previous errors

For more information about this error, try `rustc --explain E0277`.

I'll see if I can get Rhs suggestions working.

@rustbot claim

estebank · 2023-11-28T18:53:45Z

@sjwang05 if you make a new ObligationCauseCode you should be able to keep both lhs and rhs in it to do this.

sjwang05 · 2023-11-29T02:56:33Z

Storing the lhs and rhs HirIds in ObligationCauseCode::BinOp did the trick. Thanks!

GuillaumeGomez added A-frontend Area: Compiler frontend (errors, parsing and HIR) A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools labels Mar 20, 2017

steveklabnik added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. and removed A-docs Area: documentation for any part of the project, including the compiler, standard library, and tools labels Apr 25, 2017

GuillaumeGomez mentioned this issue Apr 26, 2017

Add better error message when == operator is badly used #41559

Merged

Mark-Simulacrum added A-diagnostics Area: Messages for errors, warnings, and lints and removed A-frontend Area: Compiler frontend (errors, parsing and HIR) labels May 7, 2017

Mark-Simulacrum added the C-enhancement Category: An issue proposing an enhancement or a PR with one. label Jul 26, 2017

estebank added E-needs-mentor WG-diagnostics Working group: Diagnostics labels Dec 6, 2017

estebank mentioned this issue Sep 26, 2019

unhelpful diagnostic: trying to compare f64 and &f64 for equality gives E0277 but E0308 would be better #45109

Closed

estebank mentioned this issue Sep 27, 2019

Suggest derefence operator when mismatched types related to reference #39942

Closed

estebank added the D-papercut Diagnostics: An error or lint that needs small tweaks. label Oct 11, 2019

djc mentioned this issue May 3, 2020

Try different Deref/PartialEq orderings before erroring #49454

Open

SNCPlay42 mentioned this issue Sep 28, 2020

Error message for T <binop> U worsens if T has exactly one implementation of the relevant trait #77304

Open

estebank mentioned this issue Oct 18, 2021

More accurate error for binop errors after identifying RHS type #90006

Closed

estebank added the D-newcomer-roadblock Diagnostics: Confusing error or lint; hard to understand for new users. label Jan 5, 2023

jyn514 removed the E-needs-mentor label Jun 30, 2023

estebank mentioned this issue Nov 17, 2023

Suggest dereferencing on E0277 if appropriate #67571

Closed

rustbot assigned sjwang05 Nov 28, 2023

sjwang05 mentioned this issue Nov 29, 2023

Emit better suggestions for &T == T and T == &T #118431

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bors closed this as completed in 2df6406 Dec 26, 2023

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Better error messages for &T == T #40660

Better error messages for &T == T #40660

iqualfragile commented Mar 20, 2017 •

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estebank commented Nov 28, 2023

sjwang05 commented Nov 29, 2023

Better error messages for &T == T #40660

Better error messages for &T == T #40660

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