Suggest swapping equality on E0277

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2126,6 +2126,12 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 &mut vec![],
                 &mut Default::default(),
             );
+            self.suggest_swapping_lhs_and_rhs(
+                err,
+                obligation.predicate,
+                obligation.param_env,
+                obligation.cause.code(),
+            );
             self.suggest_unsized_bound_if_applicable(err, obligation);
             if let Some(span) = err.span.primary_span()
                 && let Some(mut diag) =

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -4843,6 +4843,53 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         );
         true
     }
+    pub(crate) fn suggest_swapping_lhs_and_rhs<T>(
+        &self,
+        err: &mut Diag<'_>,
+        predicate: T,
+        param_env: ty::ParamEnv<'tcx>,
+        cause_code: &ObligationCauseCode<'tcx>,
+    ) where
+        T: Upcast<TyCtxt<'tcx>, ty::Predicate<'tcx>>,
+    {
+        let tcx = self.tcx;
+        let predicate = predicate.upcast(tcx);
+        match *cause_code {
+            ObligationCauseCode::BinOp {
+                lhs_hir_id,
+                rhs_hir_id: Some(rhs_hir_id),
+                rhs_span: Some(rhs_span),
+                ..
+            } if let Some(typeck_results) = &self.typeck_results
+                && let hir::Node::Expr(lhs) = tcx.hir_node(lhs_hir_id)
+                && let hir::Node::Expr(rhs) = tcx.hir_node(rhs_hir_id)
+                && let Some(lhs_ty) = typeck_results.expr_ty_opt(lhs)
+                && let Some(rhs_ty) = typeck_results.expr_ty_opt(rhs) =>
+            {
+                if let Some(pred) = predicate.as_trait_clause()
+                    && tcx.is_lang_item(pred.def_id(), LangItem::PartialEq)
+                    && self
+                        .infcx
+                        .type_implements_trait(pred.def_id(), [rhs_ty, lhs_ty], param_env)
+                        .must_apply_modulo_regions()
+                {
+                    let lhs_span = tcx.hir().span(lhs_hir_id);
+                    let sm = tcx.sess.source_map();
+                    if let Ok(rhs_snippet) = sm.span_to_snippet(rhs_span)
+                        && let Ok(lhs_snippet) = sm.span_to_snippet(lhs_span)
+                    {
+                        err.note(format!("`{rhs_ty}` implements `PartialEq<{lhs_ty}>`"));
+                        err.multipart_suggestion(
+                            "consider swapping the equality",
+                            vec![(lhs_span, rhs_snippet), (rhs_span, lhs_snippet)],
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.

tests/ui/suggestions/partialeq_suggest_swap_on_e0277.rs

@@ -0,0 +1,11 @@
+struct T(String);
+
+impl PartialEq<String> for T {
+    fn eq(&self, other: &String) -> bool {
+        &self.0 == other
+    }
+}
+
+fn main() {
+    String::from("Girls Band Cry") == T(String::from("Girls Band Cry")); //~ can't compare `String` with `T` [E0277]
+}

tests/ui/suggestions/partialeq_suggest_swap_on_e0277.stderr

@@ -0,0 +1,24 @@
+error[E0277]: can't compare `String` with `T`
+  --> $DIR/partialeq_suggest_swap_on_e0277.rs:10:36
+   |
+LL |     String::from("Girls Band Cry") == T(String::from("Girls Band Cry"));
+   |                                    ^^ 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<ByteStr>`
+             `String` implements `PartialEq<ByteString>`
+             `String` implements `PartialEq<Cow<'_, str>>`
+             `String` implements `PartialEq<str>`
+             `String` implements `PartialEq`
+   = note: `T` implements `PartialEq<String>`
+help: consider swapping the equality
+   |
+LL -     String::from("Girls Band Cry") == T(String::from("Girls Band Cry"));
+LL +     T(String::from("Girls Band Cry")) == String::from("Girls Band Cry");
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.