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