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

Cargo.lock

@@ -4580,6 +4580,7 @@ checksum = "8ba09476327c4b70ccefb6180f046ef588c26a24cf5d269a9feba316eb4f029f"
 name = "rustc_trait_selection"
 version = "0.0.0"
 dependencies = [
+ "itertools",
  "rustc_ast",
  "rustc_attr",
  "rustc_data_structures",

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2441,7 +2441,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         }
                     };
 
-                    // Suggest dereferencing the lhs for expressions such as `&T == T`
+                    // Suggest dereferencing the lhs for expressions such as `&T <= T`
                     if let Some(hir::Node::Expr(hir::Expr {
                         kind: hir::ExprKind::Binary(_, lhs, ..),
                         ..

compiler/rustc_hir_typeck/src/op.rs

@@ -47,7 +47,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             if let Some(lhs_deref_ty) = self.deref_once_mutably_for_diagnostic(lhs_ty) {
                 if self
                     .lookup_op_method(
-                        lhs_deref_ty,
+                        (lhs, lhs_deref_ty),
                         Some((rhs, rhs_ty)),
                         Op::Binary(op, IsAssign::Yes),
                         expected,
@@ -58,7 +58,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // emitted a better suggestion during error handling in check_overloaded_binop.
                     if self
                         .lookup_op_method(
-                            lhs_ty,
+                            (lhs, lhs_ty),
                             Some((rhs, rhs_ty)),
                             Op::Binary(op, IsAssign::Yes),
                             expected,
@@ -246,7 +246,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         });
 
         let result = self.lookup_op_method(
-            lhs_ty,
+            (lhs_expr, lhs_ty),
             Some((rhs_expr, rhs_ty_var)),
             Op::Binary(op, is_assign),
             expected,
@@ -391,7 +391,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     |err: &mut DiagnosticBuilder<'_, _>, lhs_deref_ty: Ty<'tcx>| {
                         if self
                             .lookup_op_method(
-                                lhs_deref_ty,
+                                (lhs_expr, lhs_deref_ty),
                                 Some((rhs_expr, rhs_ty)),
                                 Op::Binary(op, is_assign),
                                 expected,
@@ -424,7 +424,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                      rhs_new_mutbl: Option<ast::Mutability>| {
                         if self
                             .lookup_op_method(
-                                lhs_adjusted_ty,
+                                (lhs_expr, lhs_adjusted_ty),
                                 Some((rhs_expr, rhs_adjusted_ty)),
                                 Op::Binary(op, is_assign),
                                 expected,
@@ -479,7 +479,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
                 let is_compatible_after_call = |lhs_ty, rhs_ty| {
                     self.lookup_op_method(
-                        lhs_ty,
+                        (lhs_expr, lhs_ty),
                         Some((rhs_expr, rhs_ty)),
                         Op::Binary(op, is_assign),
                         expected,
@@ -578,7 +578,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         // suggestion for the user.
                         let errors = self
                             .lookup_op_method(
-                                lhs_ty,
+                                (lhs_expr, lhs_ty),
                                 Some((rhs_expr, rhs_ty)),
                                 Op::Binary(op, is_assign),
                                 expected,
@@ -779,7 +779,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Expectation<'tcx>,
     ) -> Ty<'tcx> {
         assert!(op.is_by_value());
-        match self.lookup_op_method(operand_ty, None, Op::Unary(op, ex.span), expected) {
+        match self.lookup_op_method((ex, operand_ty), None, Op::Unary(op, ex.span), expected) {
             Ok(method) => {
                 self.write_method_call_and_enforce_effects(ex.hir_id, ex.span, method);
                 method.sig.output()
@@ -865,7 +865,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn lookup_op_method(
         &self,
-        lhs_ty: Ty<'tcx>,
+        (lhs_expr, lhs_ty): (&'tcx hir::Expr<'tcx>, Ty<'tcx>),
         opt_rhs: Option<(&'tcx hir::Expr<'tcx>, Ty<'tcx>)>,
         op: Op,
         expected: Expectation<'tcx>,
@@ -910,8 +910,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let cause = self.cause(
             span,
             traits::BinOp {
+                lhs_hir_id: lhs_expr.hir_id,
+                rhs_hir_id: opt_rhs_expr.map(|expr| expr.hir_id),
                 rhs_span: opt_rhs_expr.map(|expr| expr.span),
-                is_lit: opt_rhs_expr.is_some_and(|expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
+                rhs_is_lit: opt_rhs_expr
+                    .is_some_and(|expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
                 output_ty: expected.only_has_type(self),
             },
         );

compiler/rustc_middle/src/traits/mod.rs

@@ -429,8 +429,10 @@ pub enum ObligationCauseCode<'tcx> {
     MatchImpl(ObligationCause<'tcx>, DefId),
 
     BinOp {
+        lhs_hir_id: hir::HirId,
+        rhs_hir_id: Option<hir::HirId>,
         rhs_span: Option<Span>,
-        is_lit: bool,
+        rhs_is_lit: bool,
         output_ty: Option<Ty<'tcx>>,
     },
 
@@ -510,6 +512,21 @@ impl<'tcx> ObligationCauseCode<'tcx> {
         base_cause
     }
 
+    /// Returns the base obligation and the base trait predicate, if any, ignoring
+    /// derived obligations.
+    pub fn peel_derives_with_predicate(&self) -> (&Self, Option<ty::PolyTraitPredicate<'tcx>>) {
+        let mut base_cause = self;
+        let mut base_trait_pred = None;
+        while let Some((parent_code, parent_pred)) = base_cause.parent() {
+            base_cause = parent_code;
+            if let Some(parent_pred) = parent_pred {
+                base_trait_pred = Some(parent_pred);
+            }
+        }
+
+        (base_cause, base_trait_pred)
+    }
+
     pub fn parent(&self) -> Option<(&Self, Option<ty::PolyTraitPredicate<'tcx>>)> {
         match self {
             FunctionArgumentObligation { parent_code, .. } => Some((parent_code, None)),

compiler/rustc_trait_selection/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
+itertools = "0.11.0"
 rustc_ast = { path = "../rustc_ast" }
 rustc_attr = { path = "../rustc_attr" }
 rustc_data_structures = { path = "../rustc_data_structures" }