Suggest swapping LHS and RHS when RHS impls PartialEq<lhs_ty>

[makai410]

Closes: #130495
r? @fee1-dead

[fee1-dead]

Please also add an UI test as described in https://rustc-dev-guide.rust-lang.org/tests/ui.html

[makai410]

Thank you for the detailed and thoughtful review! Your suggestions really helped improve the code. Much appreciated.

[makai410]

Just found when the integer type is not explicitly specified, the suggestion will not be emitted; instead, the following diagnostic message will be emitted:

error[E0277]: can't compare `{integer}` with `T`
  --> $DIR/partialeq_suggest_swap.rs:10:9
   |
LL |     123 == T(123);
   |         ^^ no implementation for `{integer} == T`
   |
   = help: the trait `PartialEq<T>` is not implemented for `{integer}`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             f128
             f16
             f32
             f64
             i128
             i16
             i32
             i64
           and 8 others

error: aborting due to 1 previous error

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

I think we should consider this case, as 123 will be inferred as i32, making T(123) == 123 valid in this context, as far as I can tell.

Also the similar case is when the type is String:

error[E0277]: can't compare `String` with `T`
  --> $DIR/partialeq_suggest_swap.rs:10:26
   |
LL |     String::from("text") == T(String::from("text"));
   |                          ^^ no implementation for `String == T`
   |
   = help: the trait `PartialEq<T>` is not implemented for `String`
   = help: the following other types implement trait `PartialEq<Rhs>`:
             `String` implements `PartialEq<&str>`
             `String` implements `PartialEq<Cow<'_, str>>`
             `String` implements `PartialEq<str>`
             `String` implements `PartialEq`

error: aborting due to 1 previous error

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

[fee1-dead]

no, that case does not need to be handled right now

[makai410]

The message can't compare {Self} with {Rhs} is defined in the standard library via #[rustc_on_unimplemented]

rust/library/core/src/cmp.rs

Lines 238 to 249 in fbab782

	#[lang = "eq"]
	#[stable(feature = "rust1", since = "1.0.0")]
	#[doc(alias = "==")]
	#[doc(alias = "!=")]
	#[rustc_on_unimplemented(
	message = "can't compare `{Self}` with `{Rhs}`",
	label = "no implementation for `{Self} == {Rhs}`",
	append_const_msg
	)]
	#[rustc_diagnostic_item = "PartialEq"]
	pub trait PartialEq<Rhs: ?Sized = Self> {
	/// Tests for `self` and `other` values to be equal, and is used by `==`.

So we may have to extend the #[rustc_on_unimplemented] if we want this suggestion to be emitted.

[fee1-dead]

@bors r+

[bors]

📌 Commit 5791a66 has been approved by fee1-dead

It is now in the queue for this repository.

[fee1-dead]

LGTM, thanks. The remaining cases could be done as a followup.

[bors]

⌛ Testing commit 5791a66 with merge 4d215e2...

[bors]

☀️ Test successful - checks-actions
Approved by: fee1-dead
Pushing 4d215e2 to master...

[rust-timer]

Finished benchmarking commit (4d215e2): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary -1.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.4%	[-4.1%, -2.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-4.1%, 2.1%]	3

Cycles

Results (primary -1.3%, secondary -5.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-2.0%, -0.9%]	3
Improvements ✅ (secondary)	-5.8%	[-7.9%, -2.2%]	3
All ❌✅ (primary)	-1.3%	[-2.0%, -0.9%]	3

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 780.805s -> 780.212s (-0.08%)
Artifact size: 335.24 MiB -> 335.27 MiB (0.01%)