Suggest deref on comparison binop RHS even if type is not Copy

Author: compiler-errors

compiler/rustc_hir_typeck/src/demand.rs

@@ -1508,6 +1508,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
                         // `steps` layers of boxes, not just one, but this is easier and most likely.
                         || (checked_ty.is_box() && steps == 1)
+                        // We can always deref a binop that takes its arguments by ref.
+                        || matches!(
+                            self.tcx.hir().get_parent(expr.hir_id),
+                            hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Binary(op, ..), .. })
+                                if !op.node.is_by_value()
+                        )
                     {
                         let deref_kind = if checked_ty.is_box() {
                             "unboxing the value"

tests/ui/inference/deref-suggestion.rs

@@ -72,4 +72,13 @@ fn main() {
     } else {
         &0
     };
+
+    #[derive(PartialEq, Eq)]
+    struct Foo;
+    let foo = Foo;
+    let bar = &Foo;
+
+    if foo == bar {
+    //~^ ERROR mismatched types
+    }
 }

tests/ui/inference/deref-suggestion.stderr

@@ -175,6 +175,19 @@ LL | ||     };
    |  |_____`if` and `else` have incompatible types
    |        expected `i32`, found `&{integer}`
 
-error: aborting due to 13 previous errors
+error[E0308]: mismatched types
+  --> $DIR/deref-suggestion.rs:81:15
+   |
+LL |     if foo == bar {
+   |        ---    ^^^ expected `Foo`, found `&Foo`
+   |        |
+   |        expected because this is `Foo`
+   |
+help: consider dereferencing the borrow
+   |
+LL |     if foo == *bar {
+   |               +
+
+error: aborting due to 14 previous errors
 
 For more information about this error, try `rustc --explain E0308`.