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