compiler/rustc_hir_typeck/src/op.rs

   1 //! Code related to processing overloaded binary and unary operators.
   2
   3 use super::method::MethodCallee;
   4 use super::{has_expected_num_generic_args, FnCtxt};
   5 use crate::check::Expectation;
   6 use rustc_ast as ast;
   7 use rustc_errors::{self, struct_span_err, Applicability, Diagnostic};
   8 use rustc_hir as hir;
   9 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  10 use rustc_infer::traits::ObligationCauseCode;
  11 use rustc_middle::ty::adjustment::{
  12     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
  13 };
  14 use rustc_middle::ty::print::with_no_trimmed_paths;
  15 use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt, TypeFolder, TypeSuperFoldable, TypeVisitable};
  16 use rustc_session::errors::ExprParenthesesNeeded;
  17 use rustc_span::source_map::Spanned;
  18 use rustc_span::symbol::{sym, Ident};
  19 use rustc_span::Span;
  20 use rustc_trait_selection::infer::InferCtxtExt;
  21 use rustc_trait_selection::traits::error_reporting::suggestions::TypeErrCtxtExt as _;
  22 use rustc_trait_selection::traits::{FulfillmentError, TraitEngine, TraitEngineExt};
  23 use rustc_type_ir::sty::TyKind::*;
  24
  25 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  26     /// Checks a `a <op>= b`
  27     pub fn check_binop_assign(
  28         &self,
  29         expr: &'tcx hir::Expr<'tcx>,
  30         op: hir::BinOp,
  31         lhs: &'tcx hir::Expr<'tcx>,
  32         rhs: &'tcx hir::Expr<'tcx>,
  33         expected: Expectation<'tcx>,
  34     ) -> Ty<'tcx> {
  35         let (lhs_ty, rhs_ty, return_ty) =
  36             self.check_overloaded_binop(expr, lhs, rhs, op, IsAssign::Yes, expected);
  37
  38         let ty =
  39             if !lhs_ty.is_ty_var() && !rhs_ty.is_ty_var() && is_builtin_binop(lhs_ty, rhs_ty, op) {
  40                 self.enforce_builtin_binop_types(lhs.span, lhs_ty, rhs.span, rhs_ty, op);
  41                 self.tcx.mk_unit()
  42             } else {
  43                 return_ty
  44             };
  45
  46         self.check_lhs_assignable(lhs, "E0067", op.span, |err| {
  47             if let Some(lhs_deref_ty) = self.deref_once_mutably_for_diagnostic(lhs_ty) {
  48                 if self
  49                     .lookup_op_method(
  50                         lhs_deref_ty,
  51                         Some(rhs_ty),
  52                         Some(rhs),
  53                         Op::Binary(op, IsAssign::Yes),
  54                         expected,
  55                     )
  56                     .is_ok()
  57                 {
  58                     // If LHS += RHS is an error, but *LHS += RHS is successful, then we will have
  59                     // emitted a better suggestion during error handling in check_overloaded_binop.
  60                     if self
  61                         .lookup_op_method(
  62                             lhs_ty,
  63                             Some(rhs_ty),
  64                             Some(rhs),
  65                             Op::Binary(op, IsAssign::Yes),
  66                             expected,
  67                         )
  68                         .is_err()
  69                     {
  70                         err.downgrade_to_delayed_bug();
  71                     } else {
  72                         // Otherwise, it's valid to suggest dereferencing the LHS here.
  73                         err.span_suggestion_verbose(
  74                             lhs.span.shrink_to_lo(),
  75                             "consider dereferencing the left-hand side of this operation",
  76                             "*",
  77                             Applicability::MaybeIncorrect,
  78                         );
  79                     }
  80                 }
  81             }
  82         });
  83
  84         ty
  85     }
  86
  87     /// Checks a potentially overloaded binary operator.
  88     pub fn check_binop(
  89         &self,
  90         expr: &'tcx hir::Expr<'tcx>,
  91         op: hir::BinOp,
  92         lhs_expr: &'tcx hir::Expr<'tcx>,
  93         rhs_expr: &'tcx hir::Expr<'tcx>,
  94         expected: Expectation<'tcx>,
  95     ) -> Ty<'tcx> {
  96         let tcx = self.tcx;
  97
  98         debug!(
  99             "check_binop(expr.hir_id={}, expr={:?}, op={:?}, lhs_expr={:?}, rhs_expr={:?})",
 100             expr.hir_id, expr, op, lhs_expr, rhs_expr
 101         );
 102
 103         match BinOpCategory::from(op) {
 104             BinOpCategory::Shortcircuit => {
 105                 // && and || are a simple case.
 106                 self.check_expr_coercable_to_type(lhs_expr, tcx.types.bool, None);
 107                 let lhs_diverges = self.diverges.get();
 108                 self.check_expr_coercable_to_type(rhs_expr, tcx.types.bool, None);
 109
 110                 // Depending on the LHS' value, the RHS can never execute.
 111                 self.diverges.set(lhs_diverges);
 112
 113                 tcx.types.bool
 114             }
 115             _ => {
 116                 // Otherwise, we always treat operators as if they are
 117                 // overloaded. This is the way to be most flexible w/r/t
 118                 // types that get inferred.
 119                 let (lhs_ty, rhs_ty, return_ty) = self.check_overloaded_binop(
 120                     expr,
 121                     lhs_expr,
 122                     rhs_expr,
 123                     op,
 124                     IsAssign::No,
 125                     expected,
 126                 );
 127
 128                 // Supply type inference hints if relevant. Probably these
 129                 // hints should be enforced during select as part of the
 130                 // `consider_unification_despite_ambiguity` routine, but this
 131                 // more convenient for now.
 132                 //
 133                 // The basic idea is to help type inference by taking
 134                 // advantage of things we know about how the impls for
 135                 // scalar types are arranged. This is important in a
 136                 // scenario like `1_u32 << 2`, because it lets us quickly
 137                 // deduce that the result type should be `u32`, even
 138                 // though we don't know yet what type 2 has and hence
 139                 // can't pin this down to a specific impl.
 140                 if !lhs_ty.is_ty_var()
 141                     && !rhs_ty.is_ty_var()
 142                     && is_builtin_binop(lhs_ty, rhs_ty, op)
 143                 {
 144                     let builtin_return_ty = self.enforce_builtin_binop_types(
 145                         lhs_expr.span,
 146                         lhs_ty,
 147                         rhs_expr.span,
 148                         rhs_ty,
 149                         op,
 150                     );
 151                     self.demand_suptype(expr.span, builtin_return_ty, return_ty);
 152                 }
 153
 154                 return_ty
 155             }
 156         }
 157     }
 158
 159     fn enforce_builtin_binop_types(
 160         &self,
 161         lhs_span: Span,
 162         lhs_ty: Ty<'tcx>,
 163         rhs_span: Span,
 164         rhs_ty: Ty<'tcx>,
 165         op: hir::BinOp,
 166     ) -> Ty<'tcx> {
 167         debug_assert!(is_builtin_binop(lhs_ty, rhs_ty, op));
 168
 169         // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
 170         // (See https://github.com/rust-lang/rust/issues/57447.)
 171         let (lhs_ty, rhs_ty) = (deref_ty_if_possible(lhs_ty), deref_ty_if_possible(rhs_ty));
 172
 173         let tcx = self.tcx;
 174         match BinOpCategory::from(op) {
 175             BinOpCategory::Shortcircuit => {
 176                 self.demand_suptype(lhs_span, tcx.types.bool, lhs_ty);
 177                 self.demand_suptype(rhs_span, tcx.types.bool, rhs_ty);
 178                 tcx.types.bool
 179             }
 180
 181             BinOpCategory::Shift => {
 182                 // result type is same as LHS always
 183                 lhs_ty
 184             }
 185
 186             BinOpCategory::Math | BinOpCategory::Bitwise => {
 187                 // both LHS and RHS and result will have the same type
 188                 self.demand_suptype(rhs_span, lhs_ty, rhs_ty);
 189                 lhs_ty
 190             }
 191
 192             BinOpCategory::Comparison => {
 193                 // both LHS and RHS and result will have the same type
 194                 self.demand_suptype(rhs_span, lhs_ty, rhs_ty);
 195                 tcx.types.bool
 196             }
 197         }
 198     }
 199
 200     fn check_overloaded_binop(
 201         &self,
 202         expr: &'tcx hir::Expr<'tcx>,
 203         lhs_expr: &'tcx hir::Expr<'tcx>,
 204         rhs_expr: &'tcx hir::Expr<'tcx>,
 205         op: hir::BinOp,
 206         is_assign: IsAssign,
 207         expected: Expectation<'tcx>,
 208     ) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) {
 209         debug!(
 210             "check_overloaded_binop(expr.hir_id={}, op={:?}, is_assign={:?})",
 211             expr.hir_id, op, is_assign
 212         );
 213
 214         let lhs_ty = match is_assign {
 215             IsAssign::No => {
 216                 // Find a suitable supertype of the LHS expression's type, by coercing to
 217                 // a type variable, to pass as the `Self` to the trait, avoiding invariant
 218                 // trait matching creating lifetime constraints that are too strict.
 219                 // e.g., adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
 220                 // in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
 221                 let lhs_ty = self.check_expr(lhs_expr);
 222                 let fresh_var = self.next_ty_var(TypeVariableOrigin {
 223                     kind: TypeVariableOriginKind::MiscVariable,
 224                     span: lhs_expr.span,
 225                 });
 226                 self.demand_coerce(lhs_expr, lhs_ty, fresh_var, Some(rhs_expr), AllowTwoPhase::No)
 227             }
 228             IsAssign::Yes => {
 229                 // rust-lang/rust#52126: We have to use strict
 230                 // equivalence on the LHS of an assign-op like `+=`;
 231                 // overwritten or mutably-borrowed places cannot be
 232                 // coerced to a supertype.
 233                 self.check_expr(lhs_expr)
 234             }
 235         };
 236         let lhs_ty = self.resolve_vars_with_obligations(lhs_ty);
 237
 238         // N.B., as we have not yet type-checked the RHS, we don't have the
 239         // type at hand. Make a variable to represent it. The whole reason
 240         // for this indirection is so that, below, we can check the expr
 241         // using this variable as the expected type, which sometimes lets
 242         // us do better coercions than we would be able to do otherwise,
 243         // particularly for things like `String + &String`.
 244         let rhs_ty_var = self.next_ty_var(TypeVariableOrigin {
 245             kind: TypeVariableOriginKind::MiscVariable,
 246             span: rhs_expr.span,
 247         });
 248
 249         let result = self.lookup_op_method(
 250             lhs_ty,
 251             Some(rhs_ty_var),
 252             Some(rhs_expr),
 253             Op::Binary(op, is_assign),
 254             expected,
 255         );
 256
 257         // see `NB` above
 258         let rhs_ty = self.check_expr_coercable_to_type(rhs_expr, rhs_ty_var, Some(lhs_expr));
 259         let rhs_ty = self.resolve_vars_with_obligations(rhs_ty);
 260
 261         let return_ty = match result {
 262             Ok(method) => {
 263                 let by_ref_binop = !op.node.is_by_value();
 264                 if is_assign == IsAssign::Yes || by_ref_binop {
 265                     if let ty::Ref(region, _, mutbl) = method.sig.inputs()[0].kind() {
 266                         let mutbl = match mutbl {
 267                             hir::Mutability::Not => AutoBorrowMutability::Not,
 268                             hir::Mutability::Mut => AutoBorrowMutability::Mut {
 269                                 // Allow two-phase borrows for binops in initial deployment
 270                                 // since they desugar to methods
 271                                 allow_two_phase_borrow: AllowTwoPhase::Yes,
 272                             },
 273                         };
 274                         let autoref = Adjustment {
 275                             kind: Adjust::Borrow(AutoBorrow::Ref(*region, mutbl)),
 276                             target: method.sig.inputs()[0],
 277                         };
 278                         self.apply_adjustments(lhs_expr, vec![autoref]);
 279                     }
 280                 }
 281                 if by_ref_binop {
 282                     if let ty::Ref(region, _, mutbl) = method.sig.inputs()[1].kind() {
 283                         let mutbl = match mutbl {
 284                             hir::Mutability::Not => AutoBorrowMutability::Not,
 285                             hir::Mutability::Mut => AutoBorrowMutability::Mut {
 286                                 // Allow two-phase borrows for binops in initial deployment
 287                                 // since they desugar to methods
 288                                 allow_two_phase_borrow: AllowTwoPhase::Yes,
 289                             },
 290                         };
 291                         let autoref = Adjustment {
 292                             kind: Adjust::Borrow(AutoBorrow::Ref(*region, mutbl)),
 293                             target: method.sig.inputs()[1],
 294                         };
 295                         // HACK(eddyb) Bypass checks due to reborrows being in
 296                         // some cases applied on the RHS, on top of which we need
 297                         // to autoref, which is not allowed by apply_adjustments.
 298                         // self.apply_adjustments(rhs_expr, vec![autoref]);
 299                         self.typeck_results
 300                             .borrow_mut()
 301                             .adjustments_mut()
 302                             .entry(rhs_expr.hir_id)
 303                             .or_default()
 304                             .push(autoref);
 305                     }
 306                 }
 307                 self.write_method_call(expr.hir_id, method);
 308
 309                 method.sig.output()
 310             }
 311             // error types are considered "builtin"
 312             Err(_) if lhs_ty.references_error() || rhs_ty.references_error() => self.tcx.ty_error(),
 313             Err(errors) => {
 314                 let (_, trait_def_id) =
 315                     lang_item_for_op(self.tcx, Op::Binary(op, is_assign), op.span);
 316                 let missing_trait = trait_def_id
 317                     .map(|def_id| with_no_trimmed_paths!(self.tcx.def_path_str(def_id)));
 318                 let (mut err, output_def_id) = match is_assign {
 319                     IsAssign::Yes => {
 320                         let mut err = struct_span_err!(
 321                             self.tcx.sess,
 322                             expr.span,
 323                             E0368,
 324                             "binary assignment operation `{}=` cannot be applied to type `{}`",
 325                             op.node.as_str(),
 326                             lhs_ty,
 327                         );
 328                         err.span_label(
 329                             lhs_expr.span,
 330                             format!("cannot use `{}=` on type `{}`", op.node.as_str(), lhs_ty),
 331                         );
 332                         self.note_unmet_impls_on_type(&mut err, errors);
 333                         (err, None)
 334                     }
 335                     IsAssign::No => {
 336                         let message = match op.node {
 337                             hir::BinOpKind::Add => {
 338                                 format!("cannot add `{rhs_ty}` to `{lhs_ty}`")
 339                             }
 340                             hir::BinOpKind::Sub => {
 341                                 format!("cannot subtract `{rhs_ty}` from `{lhs_ty}`")
 342                             }
 343                             hir::BinOpKind::Mul => {
 344                                 format!("cannot multiply `{lhs_ty}` by `{rhs_ty}`")
 345                             }
 346                             hir::BinOpKind::Div => {
 347                                 format!("cannot divide `{lhs_ty}` by `{rhs_ty}`")
 348                             }
 349                             hir::BinOpKind::Rem => {
 350                                 format!("cannot mod `{lhs_ty}` by `{rhs_ty}`")
 351                             }
 352                             hir::BinOpKind::BitAnd => {
 353                                 format!("no implementation for `{lhs_ty} & {rhs_ty}`")
 354                             }
 355                             hir::BinOpKind::BitXor => {
 356                                 format!("no implementation for `{lhs_ty} ^ {rhs_ty}`")
 357                             }
 358                             hir::BinOpKind::BitOr => {
 359                                 format!("no implementation for `{lhs_ty} | {rhs_ty}`")
 360                             }
 361                             hir::BinOpKind::Shl => {
 362                                 format!("no implementation for `{lhs_ty} << {rhs_ty}`")
 363                             }
 364                             hir::BinOpKind::Shr => {
 365                                 format!("no implementation for `{lhs_ty} >> {rhs_ty}`")
 366                             }
 367                             _ => format!(
 368                                 "binary operation `{}` cannot be applied to type `{}`",
 369                                 op.node.as_str(),
 370                                 lhs_ty
 371                             ),
 372                         };
 373                         let output_def_id = trait_def_id.and_then(|def_id| {
 374                             self.tcx
 375                                 .associated_item_def_ids(def_id)
 376                                 .iter()
 377                                 .find(|item_def_id| {
 378                                     self.tcx.associated_item(*item_def_id).name == sym::Output
 379                                 })
 380                                 .cloned()
 381                         });
 382                         let mut err = struct_span_err!(self.tcx.sess, op.span, E0369, "{message}");
 383                         if !lhs_expr.span.eq(&rhs_expr.span) {
 384                             err.span_label(lhs_expr.span, lhs_ty.to_string());
 385                             err.span_label(rhs_expr.span, rhs_ty.to_string());
 386                         }
 387                         self.note_unmet_impls_on_type(&mut err, errors);
 388                         (err, output_def_id)
 389                     }
 390                 };
 391
 392                 let mut suggest_deref_binop = |lhs_deref_ty: Ty<'tcx>| {
 393                     if self
 394                         .lookup_op_method(
 395                             lhs_deref_ty,
 396                             Some(rhs_ty),
 397                             Some(rhs_expr),
 398                             Op::Binary(op, is_assign),
 399                             expected,
 400                         )
 401                         .is_ok()
 402                     {
 403                         let msg = &format!(
 404                             "`{}{}` can be used on `{}` if you dereference the left-hand side",
 405                             op.node.as_str(),
 406                             match is_assign {
 407                                 IsAssign::Yes => "=",
 408                                 IsAssign::No => "",
 409                             },
 410                             lhs_deref_ty,
 411                         );
 412                         err.span_suggestion_verbose(
 413                             lhs_expr.span.shrink_to_lo(),
 414                             msg,
 415                             "*",
 416                             rustc_errors::Applicability::MachineApplicable,
 417                         );
 418                     }
 419                 };
 420
 421                 let is_compatible = |lhs_ty, rhs_ty| {
 422                     self.lookup_op_method(
 423                         lhs_ty,
 424                         Some(rhs_ty),
 425                         Some(rhs_expr),
 426                         Op::Binary(op, is_assign),
 427                         expected,
 428                     )
 429                     .is_ok()
 430                 };
 431
 432                 // We should suggest `a + b` => `*a + b` if `a` is copy, and suggest
 433                 // `a += b` => `*a += b` if a is a mut ref.
 434                 if !op.span.can_be_used_for_suggestions() {
 435                     // Suppress suggestions when lhs and rhs are not in the same span as the error
 436                 } else if is_assign == IsAssign::Yes
 437                     && let Some(lhs_deref_ty) = self.deref_once_mutably_for_diagnostic(lhs_ty)
 438                 {
 439                     suggest_deref_binop(lhs_deref_ty);
 440                 } else if is_assign == IsAssign::No
 441                     && let Ref(_, lhs_deref_ty, _) = lhs_ty.kind()
 442                 {
 443                     if self.type_is_copy_modulo_regions(
 444                         self.param_env,
 445                         *lhs_deref_ty,
 446                         lhs_expr.span,
 447                     ) {
 448                         suggest_deref_binop(*lhs_deref_ty);
 449                     }
 450                 } else if self.suggest_fn_call(&mut err, lhs_expr, lhs_ty, |lhs_ty| {
 451                     is_compatible(lhs_ty, rhs_ty)
 452                 }) || self.suggest_fn_call(&mut err, rhs_expr, rhs_ty, |rhs_ty| {
 453                     is_compatible(lhs_ty, rhs_ty)
 454                 }) || self.suggest_two_fn_call(
 455                     &mut err,
 456                     rhs_expr,
 457                     rhs_ty,
 458                     lhs_expr,
 459                     lhs_ty,
 460                     |lhs_ty, rhs_ty| is_compatible(lhs_ty, rhs_ty),
 461                 ) {
 462                     // Cool
 463                 }
 464
 465                 if let Some(missing_trait) = missing_trait {
 466                     if op.node == hir::BinOpKind::Add
 467                         && self.check_str_addition(
 468                             lhs_expr, rhs_expr, lhs_ty, rhs_ty, &mut err, is_assign, op,
 469                         )
 470                     {
 471                         // This has nothing here because it means we did string
 472                         // concatenation (e.g., "Hello " + "World!"). This means
 473                         // we don't want the note in the else clause to be emitted
 474                     } else if lhs_ty.has_non_region_param() {
 475                         // Look for a TraitPredicate in the Fulfillment errors,
 476                         // and use it to generate a suggestion.
 477                         //
 478                         // Note that lookup_op_method must be called again but
 479                         // with a specific rhs_ty instead of a placeholder so
 480                         // the resulting predicate generates a more specific
 481                         // suggestion for the user.
 482                         let errors = self
 483                             .lookup_op_method(
 484                                 lhs_ty,
 485                                 Some(rhs_ty),
 486                                 Some(rhs_expr),
 487                                 Op::Binary(op, is_assign),
 488                                 expected,
 489                             )
 490                             .unwrap_err();
 491                         if !errors.is_empty() {
 492                             for error in errors {
 493                                 if let Some(trait_pred) =
 494                                     error.obligation.predicate.to_opt_poly_trait_pred()
 495                                 {
 496                                     let output_associated_item = match error.obligation.cause.code()
 497                                     {
 498                                         ObligationCauseCode::BinOp {
 499                                             output_ty: Some(output_ty),
 500                                             ..
 501                                         } => {
 502                                             // Make sure that we're attaching `Output = ..` to the right trait predicate
 503                                             if let Some(output_def_id) = output_def_id
 504                                                 && let Some(trait_def_id) = trait_def_id
 505                                                 && self.tcx.parent(output_def_id) == trait_def_id
 506                                             {
 507                                                 Some(("Output", *output_ty))
 508                                             } else {
 509                                                 None
 510                                             }
 511                                         }
 512                                         _ => None,
 513                                     };
 514
 515                                     self.err_ctxt().suggest_restricting_param_bound(
 516                                         &mut err,
 517                                         trait_pred,
 518                                         output_associated_item,
 519                                         self.body_id,
 520                                     );
 521                                 }
 522                             }
 523                         } else {
 524                             // When we know that a missing bound is responsible, we don't show
 525                             // this note as it is redundant.
 526                             err.note(&format!(
 527                                 "the trait `{missing_trait}` is not implemented for `{lhs_ty}`"
 528                             ));
 529                         }
 530                     }
 531                 }
 532                 err.emit();
 533                 self.tcx.ty_error()
 534             }
 535         };
 536
 537         (lhs_ty, rhs_ty, return_ty)
 538     }
 539
 540     /// Provide actionable suggestions when trying to add two strings with incorrect types,
 541     /// like `&str + &str`, `String + String` and `&str + &String`.
 542     ///
 543     /// If this function returns `true` it means a note was printed, so we don't need
 544     /// to print the normal "implementation of `std::ops::Add` might be missing" note
 545     fn check_str_addition(
 546         &self,
 547         lhs_expr: &'tcx hir::Expr<'tcx>,
 548         rhs_expr: &'tcx hir::Expr<'tcx>,
 549         lhs_ty: Ty<'tcx>,
 550         rhs_ty: Ty<'tcx>,
 551         err: &mut Diagnostic,
 552         is_assign: IsAssign,
 553         op: hir::BinOp,
 554     ) -> bool {
 555         let str_concat_note = "string concatenation requires an owned `String` on the left";
 556         let rm_borrow_msg = "remove the borrow to obtain an owned `String`";
 557         let to_owned_msg = "create an owned `String` from a string reference";
 558
 559         let is_std_string = |ty: Ty<'tcx>| {
 560             ty.ty_adt_def()
 561                 .map_or(false, |ty_def| self.tcx.is_diagnostic_item(sym::String, ty_def.did()))
 562         };
 563
 564         match (lhs_ty.kind(), rhs_ty.kind()) {
 565             (&Ref(_, l_ty, _), &Ref(_, r_ty, _)) // &str or &String + &str, &String or &&str
 566                 if (*l_ty.kind() == Str || is_std_string(l_ty))
 567                     && (*r_ty.kind() == Str
 568                         || is_std_string(r_ty)
 569                         || matches!(
 570                             r_ty.kind(), Ref(_, inner_ty, _) if *inner_ty.kind() == Str
 571                         )) =>
 572             {
 573                 if let IsAssign::No = is_assign { // Do not supply this message if `&str += &str`
 574                     err.span_label(op.span, "`+` cannot be used to concatenate two `&str` strings");
 575                     err.note(str_concat_note);
 576                     if let hir::ExprKind::AddrOf(_, _, lhs_inner_expr) = lhs_expr.kind {
 577                         err.span_suggestion_verbose(
 578                             lhs_expr.span.until(lhs_inner_expr.span),
 579                             rm_borrow_msg,
 580                             "",
 581                             Applicability::MachineApplicable
 582                         );
 583                     } else {
 584                         err.span_suggestion_verbose(
 585                             lhs_expr.span.shrink_to_hi(),
 586                             to_owned_msg,
 587                             ".to_owned()",
 588                             Applicability::MachineApplicable
 589                         );
 590                     }
 591                 }
 592                 true
 593             }
 594             (&Ref(_, l_ty, _), &Adt(..)) // Handle `&str` & `&String` + `String`
 595                 if (*l_ty.kind() == Str || is_std_string(l_ty)) && is_std_string(rhs_ty) =>
 596             {
 597                 err.span_label(
 598                     op.span,
 599                     "`+` cannot be used to concatenate a `&str` with a `String`",
 600                 );
 601                 match is_assign {
 602                     IsAssign::No => {
 603                         let sugg_msg;
 604                         let lhs_sugg = if let hir::ExprKind::AddrOf(_, _, lhs_inner_expr) = lhs_expr.kind {
 605                             sugg_msg = "remove the borrow on the left and add one on the right";
 606                             (lhs_expr.span.until(lhs_inner_expr.span), "".to_owned())
 607                         } else {
 608                             sugg_msg = "create an owned `String` on the left and add a borrow on the right";
 609                             (lhs_expr.span.shrink_to_hi(), ".to_owned()".to_owned())
 610                         };
 611                         let suggestions = vec![
 612                             lhs_sugg,
 613                             (rhs_expr.span.shrink_to_lo(), "&".to_owned()),
 614                         ];
 615                         err.multipart_suggestion_verbose(
 616                             sugg_msg,
 617                             suggestions,
 618                             Applicability::MachineApplicable,
 619                         );
 620                     }
 621                     IsAssign::Yes => {
 622                         err.note(str_concat_note);
 623                     }
 624                 }
 625                 true
 626             }
 627             _ => false,
 628         }
 629     }
 630
 631     pub fn check_user_unop(
 632         &self,
 633         ex: &'tcx hir::Expr<'tcx>,
 634         operand_ty: Ty<'tcx>,
 635         op: hir::UnOp,
 636         expected: Expectation<'tcx>,
 637     ) -> Ty<'tcx> {
 638         assert!(op.is_by_value());
 639         match self.lookup_op_method(operand_ty, None, None, Op::Unary(op, ex.span), expected) {
 640             Ok(method) => {
 641                 self.write_method_call(ex.hir_id, method);
 642                 method.sig.output()
 643             }
 644             Err(errors) => {
 645                 let actual = self.resolve_vars_if_possible(operand_ty);
 646                 if !actual.references_error() {
 647                     let mut err = struct_span_err!(
 648                         self.tcx.sess,
 649                         ex.span,
 650                         E0600,
 651                         "cannot apply unary operator `{}` to type `{}`",
 652                         op.as_str(),
 653                         actual
 654                     );
 655                     err.span_label(
 656                         ex.span,
 657                         format!("cannot apply unary operator `{}`", op.as_str()),
 658                     );
 659
 660                     if operand_ty.has_non_region_param() {
 661                         let predicates = errors.iter().filter_map(|error| {
 662                             error.obligation.predicate.to_opt_poly_trait_pred()
 663                         });
 664                         for pred in predicates {
 665                             self.err_ctxt().suggest_restricting_param_bound(
 666                                 &mut err,
 667                                 pred,
 668                                 None,
 669                                 self.body_id,
 670                             );
 671                         }
 672                     }
 673
 674                     let sp = self.tcx.sess.source_map().start_point(ex.span);
 675                     if let Some(sp) =
 676                         self.tcx.sess.parse_sess.ambiguous_block_expr_parse.borrow().get(&sp)
 677                     {
 678                         // If the previous expression was a block expression, suggest parentheses
 679                         // (turning this into a binary subtraction operation instead.)
 680                         // for example, `{2} - 2` -> `({2}) - 2` (see src\test\ui\parser\expr-as-stmt.rs)
 681                         err.subdiagnostic(ExprParenthesesNeeded::surrounding(*sp));
 682                     } else {
 683                         match actual.kind() {
 684                             Uint(_) if op == hir::UnOp::Neg => {
 685                                 err.note("unsigned values cannot be negated");
 686
 687                                 if let hir::ExprKind::Unary(
 688                                     _,
 689                                     hir::Expr {
 690                                         kind:
 691                                             hir::ExprKind::Lit(Spanned {
 692                                                 node: ast::LitKind::Int(1, _),
 693                                                 ..
 694                                             }),
 695                                         ..
 696                                     },
 697                                 ) = ex.kind
 698                                 {
 699                                     err.span_suggestion(
 700                                         ex.span,
 701                                         &format!(
 702                                             "you may have meant the maximum value of `{actual}`",
 703                                         ),
 704                                         format!("{actual}::MAX"),
 705                                         Applicability::MaybeIncorrect,
 706                                     );
 707                                 }
 708                             }
 709                             Str | Never | Char | Tuple(_) | Array(_, _) => {}
 710                             Ref(_, lty, _) if *lty.kind() == Str => {}
 711                             _ => {
 712                                 self.note_unmet_impls_on_type(&mut err, errors);
 713                             }
 714                         }
 715                     }
 716                     err.emit();
 717                 }
 718                 self.tcx.ty_error()
 719             }
 720         }
 721     }
 722
 723     fn lookup_op_method(
 724         &self,
 725         lhs_ty: Ty<'tcx>,
 726         other_ty: Option<Ty<'tcx>>,
 727         other_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 728         op: Op,
 729         expected: Expectation<'tcx>,
 730     ) -> Result<MethodCallee<'tcx>, Vec<FulfillmentError<'tcx>>> {
 731         let span = match op {
 732             Op::Binary(op, _) => op.span,
 733             Op::Unary(_, span) => span,
 734         };
 735         let (opname, trait_did) = lang_item_for_op(self.tcx, op, span);
 736
 737         debug!(
 738             "lookup_op_method(lhs_ty={:?}, op={:?}, opname={:?}, trait_did={:?})",
 739             lhs_ty, op, opname, trait_did
 740         );
 741
 742         // Catches cases like #83893, where a lang item is declared with the
 743         // wrong number of generic arguments. Should have yielded an error
 744         // elsewhere by now, but we have to catch it here so that we do not
 745         // index `other_tys` out of bounds (if the lang item has too many
 746         // generic arguments, `other_tys` is too short).
 747         if !has_expected_num_generic_args(
 748             self.tcx,
 749             trait_did,
 750             match op {
 751                 // Binary ops have a generic right-hand side, unary ops don't
 752                 Op::Binary(..) => 1,
 753                 Op::Unary(..) => 0,
 754             },
 755         ) {
 756             return Err(vec![]);
 757         }
 758
 759         let opname = Ident::with_dummy_span(opname);
 760         let method = trait_did.and_then(|trait_did| {
 761             self.lookup_op_method_in_trait(
 762                 span,
 763                 opname,
 764                 trait_did,
 765                 lhs_ty,
 766                 other_ty,
 767                 other_ty_expr,
 768                 expected,
 769             )
 770         });
 771
 772         match (method, trait_did) {
 773             (Some(ok), _) => {
 774                 let method = self.register_infer_ok_obligations(ok);
 775                 self.select_obligations_where_possible(false, |_| {});
 776                 Ok(method)
 777             }
 778             (None, None) => Err(vec![]),
 779             (None, Some(trait_did)) => {
 780                 let (obligation, _) = self.obligation_for_op_method(
 781                     span,
 782                     trait_did,
 783                     lhs_ty,
 784                     other_ty,
 785                     other_ty_expr,
 786                     expected,
 787                 );
 788                 let mut fulfill = <dyn TraitEngine<'_>>::new(self.tcx);
 789                 fulfill.register_predicate_obligation(self, obligation);
 790                 Err(fulfill.select_where_possible(&self.infcx))
 791             }
 792         }
 793     }
 794 }
 795
 796 fn lang_item_for_op(
 797     tcx: TyCtxt<'_>,
 798     op: Op,
 799     span: Span,
 800 ) -> (rustc_span::Symbol, Option<hir::def_id::DefId>) {
 801     let lang = tcx.lang_items();
 802     if let Op::Binary(op, IsAssign::Yes) = op {
 803         match op.node {
 804             hir::BinOpKind::Add => (sym::add_assign, lang.add_assign_trait()),
 805             hir::BinOpKind::Sub => (sym::sub_assign, lang.sub_assign_trait()),
 806             hir::BinOpKind::Mul => (sym::mul_assign, lang.mul_assign_trait()),
 807             hir::BinOpKind::Div => (sym::div_assign, lang.div_assign_trait()),
 808             hir::BinOpKind::Rem => (sym::rem_assign, lang.rem_assign_trait()),
 809             hir::BinOpKind::BitXor => (sym::bitxor_assign, lang.bitxor_assign_trait()),
 810             hir::BinOpKind::BitAnd => (sym::bitand_assign, lang.bitand_assign_trait()),
 811             hir::BinOpKind::BitOr => (sym::bitor_assign, lang.bitor_assign_trait()),
 812             hir::BinOpKind::Shl => (sym::shl_assign, lang.shl_assign_trait()),
 813             hir::BinOpKind::Shr => (sym::shr_assign, lang.shr_assign_trait()),
 814             hir::BinOpKind::Lt
 815             | hir::BinOpKind::Le
 816             | hir::BinOpKind::Ge
 817             | hir::BinOpKind::Gt
 818             | hir::BinOpKind::Eq
 819             | hir::BinOpKind::Ne
 820             | hir::BinOpKind::And
 821             | hir::BinOpKind::Or => {
 822                 span_bug!(span, "impossible assignment operation: {}=", op.node.as_str())
 823             }
 824         }
 825     } else if let Op::Binary(op, IsAssign::No) = op {
 826         match op.node {
 827             hir::BinOpKind::Add => (sym::add, lang.add_trait()),
 828             hir::BinOpKind::Sub => (sym::sub, lang.sub_trait()),
 829             hir::BinOpKind::Mul => (sym::mul, lang.mul_trait()),
 830             hir::BinOpKind::Div => (sym::div, lang.div_trait()),
 831             hir::BinOpKind::Rem => (sym::rem, lang.rem_trait()),
 832             hir::BinOpKind::BitXor => (sym::bitxor, lang.bitxor_trait()),
 833             hir::BinOpKind::BitAnd => (sym::bitand, lang.bitand_trait()),
 834             hir::BinOpKind::BitOr => (sym::bitor, lang.bitor_trait()),
 835             hir::BinOpKind::Shl => (sym::shl, lang.shl_trait()),
 836             hir::BinOpKind::Shr => (sym::shr, lang.shr_trait()),
 837             hir::BinOpKind::Lt => (sym::lt, lang.partial_ord_trait()),
 838             hir::BinOpKind::Le => (sym::le, lang.partial_ord_trait()),
 839             hir::BinOpKind::Ge => (sym::ge, lang.partial_ord_trait()),
 840             hir::BinOpKind::Gt => (sym::gt, lang.partial_ord_trait()),
 841             hir::BinOpKind::Eq => (sym::eq, lang.eq_trait()),
 842             hir::BinOpKind::Ne => (sym::ne, lang.eq_trait()),
 843             hir::BinOpKind::And | hir::BinOpKind::Or => {
 844                 span_bug!(span, "&& and || are not overloadable")
 845             }
 846         }
 847     } else if let Op::Unary(hir::UnOp::Not, _) = op {
 848         (sym::not, lang.not_trait())
 849     } else if let Op::Unary(hir::UnOp::Neg, _) = op {
 850         (sym::neg, lang.neg_trait())
 851     } else {
 852         bug!("lookup_op_method: op not supported: {:?}", op)
 853     }
 854 }
 855
 856 // Binary operator categories. These categories summarize the behavior
 857 // with respect to the builtin operations supported.
 858 enum BinOpCategory {
 859     /// &&, || -- cannot be overridden
 860     Shortcircuit,
 861
 862     /// <<, >> -- when shifting a single integer, rhs can be any
 863     /// integer type. For simd, types must match.
 864     Shift,
 865
 866     /// +, -, etc -- takes equal types, produces same type as input,
 867     /// applicable to ints/floats/simd
 868     Math,
 869
 870     /// &, |, ^ -- takes equal types, produces same type as input,
 871     /// applicable to ints/floats/simd/bool
 872     Bitwise,
 873
 874     /// ==, !=, etc -- takes equal types, produces bools, except for simd,
 875     /// which produce the input type
 876     Comparison,
 877 }
 878
 879 impl BinOpCategory {
 880     fn from(op: hir::BinOp) -> BinOpCategory {
 881         match op.node {
 882             hir::BinOpKind::Shl | hir::BinOpKind::Shr => BinOpCategory::Shift,
 883
 884             hir::BinOpKind::Add
 885             | hir::BinOpKind::Sub
 886             | hir::BinOpKind::Mul
 887             | hir::BinOpKind::Div
 888             | hir::BinOpKind::Rem => BinOpCategory::Math,
 889
 890             hir::BinOpKind::BitXor | hir::BinOpKind::BitAnd | hir::BinOpKind::BitOr => {
 891                 BinOpCategory::Bitwise
 892             }
 893
 894             hir::BinOpKind::Eq
 895             | hir::BinOpKind::Ne
 896             | hir::BinOpKind::Lt
 897             | hir::BinOpKind::Le
 898             | hir::BinOpKind::Ge
 899             | hir::BinOpKind::Gt => BinOpCategory::Comparison,
 900
 901             hir::BinOpKind::And | hir::BinOpKind::Or => BinOpCategory::Shortcircuit,
 902         }
 903     }
 904 }
 905
 906 /// Whether the binary operation is an assignment (`a += b`), or not (`a + b`)
 907 #[derive(Clone, Copy, Debug, PartialEq)]
 908 enum IsAssign {
 909     No,
 910     Yes,
 911 }
 912
 913 #[derive(Clone, Copy, Debug)]
 914 enum Op {
 915     Binary(hir::BinOp, IsAssign),
 916     Unary(hir::UnOp, Span),
 917 }
 918
 919 /// Dereferences a single level of immutable referencing.
 920 fn deref_ty_if_possible<'tcx>(ty: Ty<'tcx>) -> Ty<'tcx> {
 921     match ty.kind() {
 922         ty::Ref(_, ty, hir::Mutability::Not) => *ty,
 923         _ => ty,
 924     }
 925 }
 926
 927 /// Returns `true` if this is a built-in arithmetic operation (e.g., u32
 928 /// + u32, i16x4 == i16x4) and false if these types would have to be
 929 /// overloaded to be legal. There are two reasons that we distinguish
 930 /// builtin operations from overloaded ones (vs trying to drive
 931 /// everything uniformly through the trait system and intrinsics or
 932 /// something like that):
 933 ///
 934 /// 1. Builtin operations can trivially be evaluated in constants.
 935 /// 2. For comparison operators applied to SIMD types the result is
 936 ///    not of type `bool`. For example, `i16x4 == i16x4` yields a
 937 ///    type like `i16x4`. This means that the overloaded trait
 938 ///    `PartialEq` is not applicable.
 939 ///
 940 /// Reason #2 is the killer. I tried for a while to always use
 941 /// overloaded logic and just check the types in constants/codegen after
 942 /// the fact, and it worked fine, except for SIMD types. -nmatsakis
 943 fn is_builtin_binop<'tcx>(lhs: Ty<'tcx>, rhs: Ty<'tcx>, op: hir::BinOp) -> bool {
 944     // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
 945     // (See https://github.com/rust-lang/rust/issues/57447.)
 946     let (lhs, rhs) = (deref_ty_if_possible(lhs), deref_ty_if_possible(rhs));
 947
 948     match BinOpCategory::from(op) {
 949         BinOpCategory::Shortcircuit => true,
 950
 951         BinOpCategory::Shift => {
 952             lhs.references_error()
 953                 || rhs.references_error()
 954                 || lhs.is_integral() && rhs.is_integral()
 955         }
 956
 957         BinOpCategory::Math => {
 958             lhs.references_error()
 959                 || rhs.references_error()
 960                 || lhs.is_integral() && rhs.is_integral()
 961                 || lhs.is_floating_point() && rhs.is_floating_point()
 962         }
 963
 964         BinOpCategory::Bitwise => {
 965             lhs.references_error()
 966                 || rhs.references_error()
 967                 || lhs.is_integral() && rhs.is_integral()
 968                 || lhs.is_floating_point() && rhs.is_floating_point()
 969                 || lhs.is_bool() && rhs.is_bool()
 970         }
 971
 972         BinOpCategory::Comparison => {
 973             lhs.references_error() || rhs.references_error() || lhs.is_scalar() && rhs.is_scalar()
 974         }
 975     }
 976 }
 977
 978 struct TypeParamEraser<'a, 'tcx>(&'a FnCtxt<'a, 'tcx>, Span);
 979
 980 impl<'tcx> TypeFolder<'tcx> for TypeParamEraser<'_, 'tcx> {
 981     fn tcx(&self) -> TyCtxt<'tcx> {
 982         self.0.tcx
 983     }
 984
 985     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 986         match ty.kind() {
 987             ty::Param(_) => self.0.next_ty_var(TypeVariableOrigin {
 988                 kind: TypeVariableOriginKind::MiscVariable,
 989                 span: self.1,
 990             }),
 991             _ => ty.super_fold_with(self),
 992         }
 993     }
 994 }