Supress swapping lhs and rhs in equality suggestion in extern macro

[xizheyin]

Fixes #139050

I have tried to find the initial call point via find_oldest_ancestor_in_same_ctxt, but it terminates inside the debug_assert_eq macro and doesn't get the span we were expecting.

For this reason #139316 (comment) , the test can not show the diff.
I test it locally.
In master branch:

error[E0412]: cannot find type `Item` in this scope
 --> src/main.rs:5:5
  |
5 |     Item: Eq + Debug, //~ ERROR cannot find type `Item` in this scope [E0412]
  |     ^^^^ not found in this scope

error[E0308]: mismatched types
 --> src/main.rs:7:35
  |
7 |     debug_assert_eq!(iter.next(), Some(value)); //~ ERROR  mismatched types [E0308]
  |                                   ^^^^^^^^^^^ expected `Option<<I as Iterator>::Item>`, found `Option<&<I as Iterator>::Item>`
  |
  = note: expected enum `Option<_>`
             found enum `Option<&_>`
  = note: `Option<&<I as Iterator>::Item>` implements `PartialEq<Option<<I as Iterator>::Item>>`
help: consider swapping the equality
 --> /home/nju/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/macros/mod.rs:46:22
  |
46-                 if !(*left_val == *right_val) {
46+                 if !(*right_val == *left_val) {
  |

error[E0308]: mismatched types
 --> src/main.rs:8:29
  |
8 |     assert_eq!(iter.next(), Some(value)); //~ ERROR  mismatched types [E0308]
  |                             ^^^^^^^^^^^ expected `Option<<I as Iterator>::Item>`, found `Option<&<I as Iterator>::Item>`
  |
  = note: expected enum `Option<_>`
             found enum `Option<&_>`
  = note: `Option<&<I as Iterator>::Item>` implements `PartialEq<Option<<I as Iterator>::Item>>`
help: consider swapping the equality
 --> /home/nju/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/macros/mod.rs:46:22
  |
46-                 if !(*left_val == *right_val) {
46+                 if !(*right_val == *left_val) {
  |

error: aborting due to 3 previous errors

In this branch:

error[E0412]: cannot find type `Item` in this scope
 --> src/main.rs:5:5
  |
5 |     Item: Eq + Debug, //~ ERROR cannot find type `Item` in this scope [E0412]
  |     ^^^^ not found in this scope

error[E0308]: mismatched types
 --> src/main.rs:7:35
  |
7 |     debug_assert_eq!(iter.next(), Some(value)); //~ ERROR  mismatched types [E0308]
  |                                   ^^^^^^^^^^^ expected `Option<<I as Iterator>::Item>`, found `Option<&<I as Iterator>::Item>`
  |
  = note: expected enum `Option<_>`
             found enum `Option<&_>`

error[E0308]: mismatched types
 --> src/main.rs:8:29
  |
8 |     assert_eq!(iter.next(), Some(value)); //~ ERROR  mismatched types [E0308]
  |                             ^^^^^^^^^^^ expected `Option<<I as Iterator>::Item>`, found `Option<&<I as Iterator>::Item>`
  |
  = note: expected enum `Option<_>`
             found enum `Option<&_>`

error: aborting due to 3 previous errors

[compiler-errors]

If the test can't actually demonstrate the change, then there's no reason to make this into two commits. Please squash this into one.

[xizheyin]

Pretty fast review!

@rustbot ready

[xizheyin]

Now I checked both.

[compiler-errors]

There is another copy of suggest_swapping_lhs_and_rhs in rustc_trait_selection. Could you please add this check to that, too, since I think it suffers from the same issue.

[xizheyin]

Yeah, I'll also look for a test case for that tomorrow.

[compiler-errors]

I don't really think it needs a test case, but if you want to add one it's probably something like:

upstream crate:

macro_rules! eq {
    ($a:expr) => { $a == () }
}

downstream crate:

struct Foo;

impl PartialEq<Foo> for () {
    fn eq(&self, _: &Foo) -> bool { todo!() }
}

fn main() {
    let _ = eq!(Foo);
}

Pay attention to the fact that it's a trait error and there may already be some macro suppression going on there 🤔

[xizheyin]

I submitted #144268. We can find the correct span with the newly added methods, thus not needing to suppress this suggestion.