src/librustc_typeck/check/op.rs

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