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