src/librustc_typeck/check/op.rs

   1 //! Code related to processing overloaded binary and unary operators.
   2
   3 use super::method::MethodCallee;
   4 use super::{FnCtxt, Needs};
   5 use rustc_ast::ast::Ident;
   6 use rustc_errors::{self, struct_span_err, Applicability, DiagnosticBuilder};
   7 use rustc_hir as hir;
   8 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
   9 use rustc_middle::ty::adjustment::{
  10     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
  11 };
  12 use rustc_middle::ty::TyKind::{Adt, Array, Char, FnDef, Never, Ref, Str, Tuple, Uint};
  13 use rustc_middle::ty::{self, suggest_constraining_type_param, Ty, TyCtxt, TypeFoldable};
  14 use rustc_span::Span;
  15 use rustc_trait_selection::infer::InferCtxtExt;
  16
  17 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  18     /// Checks a `a <op>= b`
  19     pub fn check_binop_assign(
  20         &self,
  21         expr: &'tcx hir::Expr<'tcx>,
  22         op: hir::BinOp,
  23         lhs: &'tcx hir::Expr<'tcx>,
  24         rhs: &'tcx hir::Expr<'tcx>,
  25     ) -> Ty<'tcx> {
  26         let (lhs_ty, rhs_ty, return_ty) =
  27             self.check_overloaded_binop(expr, lhs, rhs, op, IsAssign::Yes);
  28
  29         let ty =
  30             if !lhs_ty.is_ty_var() && !rhs_ty.is_ty_var() && is_builtin_binop(lhs_ty, rhs_ty, op) {
  31                 self.enforce_builtin_binop_types(&lhs.span, lhs_ty, &rhs.span, rhs_ty, op);
  32                 self.tcx.mk_unit()
  33             } else {
  34                 return_ty
  35             };
  36
  37         self.check_lhs_assignable(lhs, "E0067", &op.span);
  38
  39         ty
  40     }
  41
  42     /// Checks a potentially overloaded binary operator.
  43     pub fn check_binop(
  44         &self,
  45         expr: &'tcx hir::Expr<'tcx>,
  46         op: hir::BinOp,
  47         lhs_expr: &'tcx hir::Expr<'tcx>,
  48         rhs_expr: &'tcx hir::Expr<'tcx>,
  49     ) -> Ty<'tcx> {
  50         let tcx = self.tcx;
  51
  52         debug!(
  53             "check_binop(expr.hir_id={}, expr={:?}, op={:?}, lhs_expr={:?}, rhs_expr={:?})",
  54             expr.hir_id, expr, op, lhs_expr, rhs_expr
  55         );
  56
  57         match BinOpCategory::from(op) {
  58             BinOpCategory::Shortcircuit => {
  59                 // && and || are a simple case.
  60                 self.check_expr_coercable_to_type(lhs_expr, tcx.types.bool);
  61                 let lhs_diverges = self.diverges.get();
  62                 self.check_expr_coercable_to_type(rhs_expr, tcx.types.bool);
  63
  64                 // Depending on the LHS' value, the RHS can never execute.
  65                 self.diverges.set(lhs_diverges);
  66
  67                 tcx.types.bool
  68             }
  69             _ => {
  70                 // Otherwise, we always treat operators as if they are
  71                 // overloaded. This is the way to be most flexible w/r/t
  72                 // types that get inferred.
  73                 let (lhs_ty, rhs_ty, return_ty) =
  74                     self.check_overloaded_binop(expr, lhs_expr, rhs_expr, op, IsAssign::No);
  75
  76                 // Supply type inference hints if relevant. Probably these
  77                 // hints should be enforced during select as part of the
  78                 // `consider_unification_despite_ambiguity` routine, but this
  79                 // more convenient for now.
  80                 //
  81                 // The basic idea is to help type inference by taking
  82                 // advantage of things we know about how the impls for
  83                 // scalar types are arranged. This is important in a
  84                 // scenario like `1_u32 << 2`, because it lets us quickly
  85                 // deduce that the result type should be `u32`, even
  86                 // though we don't know yet what type 2 has and hence
  87                 // can't pin this down to a specific impl.
  88                 if !lhs_ty.is_ty_var()
  89                     && !rhs_ty.is_ty_var()
  90                     && is_builtin_binop(lhs_ty, rhs_ty, op)
  91                 {
  92                     let builtin_return_ty = self.enforce_builtin_binop_types(
  93                         &lhs_expr.span,
  94                         lhs_ty,
  95                         &rhs_expr.span,
  96                         rhs_ty,
  97                         op,
  98                     );
  99                     self.demand_suptype(expr.span, builtin_return_ty, return_ty);
 100                 }
 101
 102                 return_ty
 103             }
 104         }
 105     }
 106
 107     fn enforce_builtin_binop_types(
 108         &self,
 109         lhs_span: &Span,
 110         lhs_ty: Ty<'tcx>,
 111         rhs_span: &Span,
 112         rhs_ty: Ty<'tcx>,
 113         op: hir::BinOp,
 114     ) -> Ty<'tcx> {
 115         debug_assert!(is_builtin_binop(lhs_ty, rhs_ty, op));
 116
 117         // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
 118         // (See https://github.com/rust-lang/rust/issues/57447.)
 119         let (lhs_ty, rhs_ty) = (deref_ty_if_possible(lhs_ty), deref_ty_if_possible(rhs_ty));
 120
 121         let tcx = self.tcx;
 122         match BinOpCategory::from(op) {
 123             BinOpCategory::Shortcircuit => {
 124                 self.demand_suptype(*lhs_span, tcx.mk_bool(), lhs_ty);
 125                 self.demand_suptype(*rhs_span, tcx.mk_bool(), rhs_ty);
 126                 tcx.mk_bool()
 127             }
 128
 129             BinOpCategory::Shift => {
 130                 // result type is same as LHS always
 131                 lhs_ty
 132             }
 133
 134             BinOpCategory::Math | BinOpCategory::Bitwise => {
 135                 // both LHS and RHS and result will have the same type
 136                 self.demand_suptype(*rhs_span, lhs_ty, rhs_ty);
 137                 lhs_ty
 138             }
 139
 140             BinOpCategory::Comparison => {
 141                 // both LHS and RHS and result will have the same type
 142                 self.demand_suptype(*rhs_span, lhs_ty, rhs_ty);
 143                 tcx.mk_bool()
 144             }
 145         }
 146     }
 147
 148     fn check_overloaded_binop(
 149         &self,
 150         expr: &'tcx hir::Expr<'tcx>,
 151         lhs_expr: &'tcx hir::Expr<'tcx>,
 152         rhs_expr: &'tcx hir::Expr<'tcx>,
 153         op: hir::BinOp,
 154         is_assign: IsAssign,
 155     ) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) {
 156         debug!(
 157             "check_overloaded_binop(expr.hir_id={}, op={:?}, is_assign={:?})",
 158             expr.hir_id, op, is_assign
 159         );
 160
 161         let lhs_ty = match is_assign {
 162             IsAssign::No => {
 163                 // Find a suitable supertype of the LHS expression's type, by coercing to
 164                 // a type variable, to pass as the `Self` to the trait, avoiding invariant
 165                 // trait matching creating lifetime constraints that are too strict.
 166                 // e.g., adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
 167                 // in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
 168                 let lhs_ty = self.check_expr_with_needs(lhs_expr, Needs::None);
 169                 let fresh_var = self.next_ty_var(TypeVariableOrigin {
 170                     kind: TypeVariableOriginKind::MiscVariable,
 171                     span: lhs_expr.span,
 172                 });
 173                 self.demand_coerce(lhs_expr, lhs_ty, fresh_var, AllowTwoPhase::No)
 174             }
 175             IsAssign::Yes => {
 176                 // rust-lang/rust#52126: We have to use strict
 177                 // equivalence on the LHS of an assign-op like `+=`;
 178                 // overwritten or mutably-borrowed places cannot be
 179                 // coerced to a supertype.
 180                 self.check_expr_with_needs(lhs_expr, Needs::MutPlace)
 181             }
 182         };
 183         let lhs_ty = self.resolve_vars_with_obligations(lhs_ty);
 184
 185         // N.B., as we have not yet type-checked the RHS, we don't have the
 186         // type at hand. Make a variable to represent it. The whole reason
 187         // for this indirection is so that, below, we can check the expr
 188         // using this variable as the expected type, which sometimes lets
 189         // us do better coercions than we would be able to do otherwise,
 190         // particularly for things like `String + &String`.
 191         let rhs_ty_var = self.next_ty_var(TypeVariableOrigin {
 192             kind: TypeVariableOriginKind::MiscVariable,
 193             span: rhs_expr.span,
 194         });
 195
 196         let result = self.lookup_op_method(lhs_ty, &[rhs_ty_var], Op::Binary(op, is_assign));
 197
 198         // see `NB` above
 199         let rhs_ty = self.check_expr_coercable_to_type(rhs_expr, rhs_ty_var);
 200         let rhs_ty = self.resolve_vars_with_obligations(rhs_ty);
 201
 202         let return_ty = match result {
 203             Ok(method) => {
 204                 let by_ref_binop = !op.node.is_by_value();
 205                 if is_assign == IsAssign::Yes || by_ref_binop {
 206                     if let ty::Ref(region, _, mutbl) = method.sig.inputs()[0].kind {
 207                         let mutbl = match mutbl {
 208                             hir::Mutability::Not => AutoBorrowMutability::Not,
 209                             hir::Mutability::Mut => AutoBorrowMutability::Mut {
 210                                 // Allow two-phase borrows for binops in initial deployment
 211                                 // since they desugar to methods
 212                                 allow_two_phase_borrow: AllowTwoPhase::Yes,
 213                             },
 214                         };
 215                         let autoref = Adjustment {
 216                             kind: Adjust::Borrow(AutoBorrow::Ref(region, mutbl)),
 217                             target: method.sig.inputs()[0],
 218                         };
 219                         self.apply_adjustments(lhs_expr, vec![autoref]);
 220                     }
 221                 }
 222                 if by_ref_binop {
 223                     if let ty::Ref(region, _, mutbl) = method.sig.inputs()[1].kind {
 224                         let mutbl = match mutbl {
 225                             hir::Mutability::Not => AutoBorrowMutability::Not,
 226                             hir::Mutability::Mut => AutoBorrowMutability::Mut {
 227                                 // Allow two-phase borrows for binops in initial deployment
 228                                 // since they desugar to methods
 229                                 allow_two_phase_borrow: AllowTwoPhase::Yes,
 230                             },
 231                         };
 232                         let autoref = Adjustment {
 233                             kind: Adjust::Borrow(AutoBorrow::Ref(region, mutbl)),
 234                             target: method.sig.inputs()[1],
 235                         };
 236                         // HACK(eddyb) Bypass checks due to reborrows being in
 237                         // some cases applied on the RHS, on top of which we need
 238                         // to autoref, which is not allowed by apply_adjustments.
 239                         // self.apply_adjustments(rhs_expr, vec![autoref]);
 240                         self.tables
 241                             .borrow_mut()
 242                             .adjustments_mut()
 243                             .entry(rhs_expr.hir_id)
 244                             .or_default()
 245                             .push(autoref);
 246                     }
 247                 }
 248                 self.write_method_call(expr.hir_id, method);
 249
 250                 method.sig.output()
 251             }
 252             Err(()) => {
 253                 // error types are considered "builtin"
 254                 if !lhs_ty.references_error() && !rhs_ty.references_error() {
 255                     let source_map = self.tcx.sess.source_map();
 256
 257                     match is_assign {
 258                         IsAssign::Yes => {
 259                             let mut err = struct_span_err!(
 260                                 self.tcx.sess,
 261                                 expr.span,
 262                                 E0368,
 263                                 "binary assignment operation `{}=` cannot be applied to type `{}`",
 264                                 op.node.as_str(),
 265                                 lhs_ty,
 266                             );
 267                             err.span_label(
 268                                 lhs_expr.span,
 269                                 format!("cannot use `{}=` on type `{}`", op.node.as_str(), lhs_ty),
 270                             );
 271                             let mut suggested_deref = false;
 272                             if let Ref(_, rty, _) = lhs_ty.kind {
 273                                 if {
 274                                     self.infcx.type_is_copy_modulo_regions(
 275                                         self.param_env,
 276                                         rty,
 277                                         lhs_expr.span,
 278                                     ) && self
 279                                         .lookup_op_method(rty, &[rhs_ty], Op::Binary(op, is_assign))
 280                                         .is_ok()
 281                                 } {
 282                                     if let Ok(lstring) = source_map.span_to_snippet(lhs_expr.span) {
 283                                         let msg = &format!(
 284                                             "`{}=` can be used on '{}', you can dereference `{}`",
 285                                             op.node.as_str(),
 286                                             rty.peel_refs(),
 287                                             lstring,
 288                                         );
 289                                         err.span_suggestion(
 290                                             lhs_expr.span,
 291                                             msg,
 292                                             format!("*{}", lstring),
 293                                             rustc_errors::Applicability::MachineApplicable,
 294                                         );
 295                                         suggested_deref = true;
 296                                     }
 297                                 }
 298                             }
 299                             let missing_trait = match op.node {
 300                                 hir::BinOpKind::Add => Some("std::ops::AddAssign"),
 301                                 hir::BinOpKind::Sub => Some("std::ops::SubAssign"),
 302                                 hir::BinOpKind::Mul => Some("std::ops::MulAssign"),
 303                                 hir::BinOpKind::Div => Some("std::ops::DivAssign"),
 304                                 hir::BinOpKind::Rem => Some("std::ops::RemAssign"),
 305                                 hir::BinOpKind::BitAnd => Some("std::ops::BitAndAssign"),
 306                                 hir::BinOpKind::BitXor => Some("std::ops::BitXorAssign"),
 307                                 hir::BinOpKind::BitOr => Some("std::ops::BitOrAssign"),
 308                                 hir::BinOpKind::Shl => Some("std::ops::ShlAssign"),
 309                                 hir::BinOpKind::Shr => Some("std::ops::ShrAssign"),
 310                                 _ => None,
 311                             };
 312                             if let Some(missing_trait) = missing_trait {
 313                                 if op.node == hir::BinOpKind::Add
 314                                     && self.check_str_addition(
 315                                         lhs_expr, rhs_expr, lhs_ty, rhs_ty, &mut err, true, op,
 316                                     )
 317                                 {
 318                                     // This has nothing here because it means we did string
 319                                     // concatenation (e.g., "Hello " += "World!"). This means
 320                                     // we don't want the note in the else clause to be emitted
 321                                 } else if let ty::Param(p) = lhs_ty.kind {
 322                                     suggest_constraining_param(
 323                                         self.tcx,
 324                                         self.body_id,
 325                                         &mut err,
 326                                         lhs_ty,
 327                                         rhs_ty,
 328                                         missing_trait,
 329                                         p,
 330                                         false,
 331                                     );
 332                                 } else if !suggested_deref {
 333                                     suggest_impl_missing(&mut err, lhs_ty, &missing_trait);
 334                                 }
 335                             }
 336                             err.emit();
 337                         }
 338                         IsAssign::No => {
 339                             let (message, missing_trait, use_output) = match op.node {
 340                                 hir::BinOpKind::Add => (
 341                                     format!("cannot add `{}` to `{}`", rhs_ty, lhs_ty),
 342                                     Some("std::ops::Add"),
 343                                     true,
 344                                 ),
 345                                 hir::BinOpKind::Sub => (
 346                                     format!("cannot subtract `{}` from `{}`", rhs_ty, lhs_ty),
 347                                     Some("std::ops::Sub"),
 348                                     true,
 349                                 ),
 350                                 hir::BinOpKind::Mul => (
 351                                     format!("cannot multiply `{}` to `{}`", rhs_ty, lhs_ty),
 352                                     Some("std::ops::Mul"),
 353                                     true,
 354                                 ),
 355                                 hir::BinOpKind::Div => (
 356                                     format!("cannot divide `{}` by `{}`", lhs_ty, rhs_ty),
 357                                     Some("std::ops::Div"),
 358                                     true,
 359                                 ),
 360                                 hir::BinOpKind::Rem => (
 361                                     format!("cannot mod `{}` by `{}`", lhs_ty, rhs_ty),
 362                                     Some("std::ops::Rem"),
 363                                     true,
 364                                 ),
 365                                 hir::BinOpKind::BitAnd => (
 366                                     format!("no implementation for `{} & {}`", lhs_ty, rhs_ty),
 367                                     Some("std::ops::BitAnd"),
 368                                     true,
 369                                 ),
 370                                 hir::BinOpKind::BitXor => (
 371                                     format!("no implementation for `{} ^ {}`", lhs_ty, rhs_ty),
 372                                     Some("std::ops::BitXor"),
 373                                     true,
 374                                 ),
 375                                 hir::BinOpKind::BitOr => (
 376                                     format!("no implementation for `{} | {}`", lhs_ty, rhs_ty),
 377                                     Some("std::ops::BitOr"),
 378                                     true,
 379                                 ),
 380                                 hir::BinOpKind::Shl => (
 381                                     format!("no implementation for `{} << {}`", lhs_ty, rhs_ty),
 382                                     Some("std::ops::Shl"),
 383                                     true,
 384                                 ),
 385                                 hir::BinOpKind::Shr => (
 386                                     format!("no implementation for `{} >> {}`", lhs_ty, rhs_ty),
 387                                     Some("std::ops::Shr"),
 388                                     true,
 389                                 ),
 390                                 hir::BinOpKind::Eq | hir::BinOpKind::Ne => (
 391                                     format!(
 392                                         "binary operation `{}` cannot be applied to type `{}`",
 393                                         op.node.as_str(),
 394                                         lhs_ty
 395                                     ),
 396                                     Some("std::cmp::PartialEq"),
 397                                     false,
 398                                 ),
 399                                 hir::BinOpKind::Lt
 400                                 | hir::BinOpKind::Le
 401                                 | hir::BinOpKind::Gt
 402                                 | hir::BinOpKind::Ge => (
 403                                     format!(
 404                                         "binary operation `{}` cannot be applied to type `{}`",
 405                                         op.node.as_str(),
 406                                         lhs_ty
 407                                     ),
 408                                     Some("std::cmp::PartialOrd"),
 409                                     false,
 410                                 ),
 411                                 _ => (
 412                                     format!(
 413                                         "binary operation `{}` cannot be applied to type `{}`",
 414                                         op.node.as_str(),
 415                                         lhs_ty
 416                                     ),
 417                                     None,
 418                                     false,
 419                                 ),
 420                             };
 421                             let mut err = struct_span_err!(
 422                                 self.tcx.sess,
 423                                 op.span,
 424                                 E0369,
 425                                 "{}",
 426                                 message.as_str()
 427                             );
 428
 429                             let mut involves_fn = false;
 430                             if !lhs_expr.span.eq(&rhs_expr.span) {
 431                                 involves_fn |= self.add_type_neq_err_label(
 432                                     &mut err,
 433                                     lhs_expr.span,
 434                                     lhs_ty,
 435                                     rhs_ty,
 436                                     op,
 437                                     is_assign,
 438                                 );
 439                                 involves_fn |= self.add_type_neq_err_label(
 440                                     &mut err,
 441                                     rhs_expr.span,
 442                                     rhs_ty,
 443                                     lhs_ty,
 444                                     op,
 445                                     is_assign,
 446                                 );
 447                             }
 448
 449                             let mut suggested_deref = false;
 450                             if let Ref(_, rty, _) = lhs_ty.kind {
 451                                 if {
 452                                     self.infcx.type_is_copy_modulo_regions(
 453                                         self.param_env,
 454                                         rty,
 455                                         lhs_expr.span,
 456                                     ) && self
 457                                         .lookup_op_method(rty, &[rhs_ty], Op::Binary(op, is_assign))
 458                                         .is_ok()
 459                                 } {
 460                                     if let Ok(lstring) = source_map.span_to_snippet(lhs_expr.span) {
 461                                         err.help(&format!(
 462                                             "`{}` can be used on '{}', you can \
 463                                             dereference `{2}`: `*{2}`",
 464                                             op.node.as_str(),
 465                                             rty.peel_refs(),
 466                                             lstring
 467                                         ));
 468                                         suggested_deref = true;
 469                                     }
 470                                 }
 471                             }
 472                             if let Some(missing_trait) = missing_trait {
 473                                 if op.node == hir::BinOpKind::Add
 474                                     && self.check_str_addition(
 475                                         lhs_expr, rhs_expr, lhs_ty, rhs_ty, &mut err, false, op,
 476                                     )
 477                                 {
 478                                     // This has nothing here because it means we did string
 479                                     // concatenation (e.g., "Hello " + "World!"). This means
 480                                     // we don't want the note in the else clause to be emitted
 481                                 } else if let ty::Param(p) = lhs_ty.kind {
 482                                     suggest_constraining_param(
 483                                         self.tcx,
 484                                         self.body_id,
 485                                         &mut err,
 486                                         lhs_ty,
 487                                         rhs_ty,
 488                                         missing_trait,
 489                                         p,
 490                                         use_output,
 491                                     );
 492                                 } else if !suggested_deref && !involves_fn {
 493                                     suggest_impl_missing(&mut err, lhs_ty, &missing_trait);
 494                                 }
 495                             }
 496                             err.emit();
 497                         }
 498                     }
 499                 }
 500                 self.tcx.types.err
 501             }
 502         };
 503
 504         (lhs_ty, rhs_ty, return_ty)
 505     }
 506
 507     /// If one of the types is an uncalled function and calling it would yield the other type,
 508     /// suggest calling the function. Returns `true` if suggestion would apply (even if not given).
 509     fn add_type_neq_err_label(
 510         &self,
 511         err: &mut rustc_errors::DiagnosticBuilder<'_>,
 512         span: Span,
 513         ty: Ty<'tcx>,
 514         other_ty: Ty<'tcx>,
 515         op: hir::BinOp,
 516         is_assign: IsAssign,
 517     ) -> bool /* did we suggest to call a function because of missing parenthesis? */ {
 518         err.span_label(span, ty.to_string());
 519         if let FnDef(def_id, _) = ty.kind {
 520             let source_map = self.tcx.sess.source_map();
 521             if !self.tcx.has_typeck_tables(def_id) {
 522                 return false;
 523             }
 524             // We're emitting a suggestion, so we can just ignore regions
 525             let fn_sig = *self.tcx.fn_sig(def_id).skip_binder();
 526
 527             let other_ty = if let FnDef(def_id, _) = other_ty.kind {
 528                 if !self.tcx.has_typeck_tables(def_id) {
 529                     return false;
 530                 }
 531                 // We're emitting a suggestion, so we can just ignore regions
 532                 self.tcx.fn_sig(def_id).skip_binder().output()
 533             } else {
 534                 other_ty
 535             };
 536
 537             if self
 538                 .lookup_op_method(fn_sig.output(), &[other_ty], Op::Binary(op, is_assign))
 539                 .is_ok()
 540             {
 541                 if let Ok(snippet) = source_map.span_to_snippet(span) {
 542                     let (variable_snippet, applicability) = if !fn_sig.inputs().is_empty() {
 543                         (format!("{}( /* arguments */ )", snippet), Applicability::HasPlaceholders)
 544                     } else {
 545                         (format!("{}()", snippet), Applicability::MaybeIncorrect)
 546                     };
 547
 548                     err.span_suggestion(
 549                         span,
 550                         "you might have forgotten to call this function",
 551                         variable_snippet,
 552                         applicability,
 553                     );
 554                 }
 555                 return true;
 556             }
 557         }
 558         false
 559     }
 560
 561     /// Provide actionable suggestions when trying to add two strings with incorrect types,
 562     /// like `&str + &str`, `String + String` and `&str + &String`.
 563     ///
 564     /// If this function returns `true` it means a note was printed, so we don't need
 565     /// to print the normal "implementation of `std::ops::Add` might be missing" note
 566     fn check_str_addition(
 567         &self,
 568         lhs_expr: &'tcx hir::Expr<'tcx>,
 569         rhs_expr: &'tcx hir::Expr<'tcx>,
 570         lhs_ty: Ty<'tcx>,
 571         rhs_ty: Ty<'tcx>,
 572         err: &mut rustc_errors::DiagnosticBuilder<'_>,
 573         is_assign: bool,
 574         op: hir::BinOp,
 575     ) -> bool {
 576         let source_map = self.tcx.sess.source_map();
 577         let remove_borrow_msg = "String concatenation appends the string on the right to the \
 578                                  string on the left and may require reallocation. This \
 579                                  requires ownership of the string on the left";
 580
 581         let msg = "`to_owned()` can be used to create an owned `String` \
 582                    from a string reference. String concatenation \
 583                    appends the string on the right to the string \
 584                    on the left and may require reallocation. This \
 585                    requires ownership of the string on the left";
 586
 587         let is_std_string = |ty| &format!("{:?}", ty) == "std::string::String";
 588
 589         match (&lhs_ty.kind, &rhs_ty.kind) {
 590             (&Ref(_, l_ty, _), &Ref(_, r_ty, _)) // &str or &String + &str, &String or &&str
 591                 if (l_ty.kind == Str || is_std_string(l_ty)) && (
 592                         r_ty.kind == Str || is_std_string(r_ty) ||
 593                         &format!("{:?}", rhs_ty) == "&&str"
 594                     ) =>
 595             {
 596                 if !is_assign { // Do not supply this message if `&str += &str`
 597                     err.span_label(
 598                         op.span,
 599                         "`+` cannot be used to concatenate two `&str` strings",
 600                     );
 601                     match source_map.span_to_snippet(lhs_expr.span) {
 602                         Ok(lstring) => {
 603                             err.span_suggestion(
 604                                 lhs_expr.span,
 605                                 if lstring.starts_with('&') {
 606                                     remove_borrow_msg
 607                                 } else {
 608                                     msg
 609                                 },
 610                                 if lstring.starts_with('&') {
 611                                     // let a = String::new();
 612                                     // let _ = &a + "bar";
 613                                     lstring[1..].to_string()
 614                                 } else {
 615                                     format!("{}.to_owned()", lstring)
 616                                 },
 617                                 Applicability::MachineApplicable,
 618                             )
 619                         }
 620                         _ => err.help(msg),
 621                     };
 622                 }
 623                 true
 624             }
 625             (&Ref(_, l_ty, _), &Adt(..)) // Handle `&str` & `&String` + `String`
 626                 if (l_ty.kind == Str || is_std_string(l_ty)) && is_std_string(rhs_ty) =>
 627             {
 628                 err.span_label(
 629                     op.span,
 630                     "`+` cannot be used to concatenate a `&str` with a `String`",
 631                 );
 632                 match (
 633                     source_map.span_to_snippet(lhs_expr.span),
 634                     source_map.span_to_snippet(rhs_expr.span),
 635                     is_assign,
 636                 ) {
 637                     (Ok(l), Ok(r), false) => {
 638                         let to_string = if l.starts_with('&') {
 639                             // let a = String::new(); let b = String::new();
 640                             // let _ = &a + b;
 641                             l[1..].to_string()
 642                         } else {
 643                             format!("{}.to_owned()", l)
 644                         };
 645                         err.multipart_suggestion(
 646                             msg,
 647                             vec![
 648                                 (lhs_expr.span, to_string),
 649                                 (rhs_expr.span, format!("&{}", r)),
 650                             ],
 651                             Applicability::MachineApplicable,
 652                         );
 653                     }
 654                     _ => {
 655                         err.help(msg);
 656                     }
 657                 };
 658                 true
 659             }
 660             _ => false,
 661         }
 662     }
 663
 664     pub fn check_user_unop(
 665         &self,
 666         ex: &'tcx hir::Expr<'tcx>,
 667         operand_ty: Ty<'tcx>,
 668         op: hir::UnOp,
 669     ) -> Ty<'tcx> {
 670         assert!(op.is_by_value());
 671         match self.lookup_op_method(operand_ty, &[], Op::Unary(op, ex.span)) {
 672             Ok(method) => {
 673                 self.write_method_call(ex.hir_id, method);
 674                 method.sig.output()
 675             }
 676             Err(()) => {
 677                 let actual = self.resolve_vars_if_possible(&operand_ty);
 678                 if !actual.references_error() {
 679                     let mut err = struct_span_err!(
 680                         self.tcx.sess,
 681                         ex.span,
 682                         E0600,
 683                         "cannot apply unary operator `{}` to type `{}`",
 684                         op.as_str(),
 685                         actual
 686                     );
 687                     err.span_label(
 688                         ex.span,
 689                         format!(
 690                             "cannot apply unary \
 691                                                     operator `{}`",
 692                             op.as_str()
 693                         ),
 694                     );
 695                     match actual.kind {
 696                         Uint(_) if op == hir::UnOp::UnNeg => {
 697                             err.note("unsigned values cannot be negated");
 698                         }
 699                         Str | Never | Char | Tuple(_) | Array(_, _) => {}
 700                         Ref(_, ref lty, _) if lty.kind == Str => {}
 701                         _ => {
 702                             let missing_trait = match op {
 703                                 hir::UnOp::UnNeg => "std::ops::Neg",
 704                                 hir::UnOp::UnNot => "std::ops::Not",
 705                                 hir::UnOp::UnDeref => "std::ops::UnDerf",
 706                             };
 707                             suggest_impl_missing(&mut err, operand_ty, &missing_trait);
 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 /// Dereferences a single level of immutable referencing.
 868 fn deref_ty_if_possible(ty: Ty<'tcx>) -> Ty<'tcx> {
 869     match ty.kind {
 870         ty::Ref(_, ty, hir::Mutability::Not) => ty,
 871         _ => ty,
 872     }
 873 }
 874
 875 /// Returns `true` if this is a built-in arithmetic operation (e.g., u32
 876 /// + u32, i16x4 == i16x4) and false if these types would have to be
 877 /// overloaded to be legal. There are two reasons that we distinguish
 878 /// builtin operations from overloaded ones (vs trying to drive
 879 /// everything uniformly through the trait system and intrinsics or
 880 /// something like that):
 881 ///
 882 /// 1. Builtin operations can trivially be evaluated in constants.
 883 /// 2. For comparison operators applied to SIMD types the result is
 884 ///    not of type `bool`. For example, `i16x4 == i16x4` yields a
 885 ///    type like `i16x4`. This means that the overloaded trait
 886 ///    `PartialEq` is not applicable.
 887 ///
 888 /// Reason #2 is the killer. I tried for a while to always use
 889 /// overloaded logic and just check the types in constants/codegen after
 890 /// the fact, and it worked fine, except for SIMD types. -nmatsakis
 891 fn is_builtin_binop<'tcx>(lhs: Ty<'tcx>, rhs: Ty<'tcx>, op: hir::BinOp) -> bool {
 892     // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
 893     // (See https://github.com/rust-lang/rust/issues/57447.)
 894     let (lhs, rhs) = (deref_ty_if_possible(lhs), deref_ty_if_possible(rhs));
 895
 896     match BinOpCategory::from(op) {
 897         BinOpCategory::Shortcircuit => true,
 898
 899         BinOpCategory::Shift => {
 900             lhs.references_error()
 901                 || rhs.references_error()
 902                 || lhs.is_integral() && rhs.is_integral()
 903         }
 904
 905         BinOpCategory::Math => {
 906             lhs.references_error()
 907                 || rhs.references_error()
 908                 || lhs.is_integral() && rhs.is_integral()
 909                 || lhs.is_floating_point() && rhs.is_floating_point()
 910         }
 911
 912         BinOpCategory::Bitwise => {
 913             lhs.references_error()
 914                 || rhs.references_error()
 915                 || lhs.is_integral() && rhs.is_integral()
 916                 || lhs.is_floating_point() && rhs.is_floating_point()
 917                 || lhs.is_bool() && rhs.is_bool()
 918         }
 919
 920         BinOpCategory::Comparison => {
 921             lhs.references_error() || rhs.references_error() || lhs.is_scalar() && rhs.is_scalar()
 922         }
 923     }
 924 }
 925
 926 /// If applicable, note that an implementation of `trait` for `ty` may fix the error.
 927 fn suggest_impl_missing(err: &mut DiagnosticBuilder<'_>, ty: Ty<'_>, missing_trait: &str) {
 928     if let Adt(def, _) = ty.peel_refs().kind {
 929         if def.did.is_local() {
 930             err.note(&format!(
 931                 "an implementation of `{}` might \
 932                 be missing for `{}`",
 933                 missing_trait, ty
 934             ));
 935         }
 936     }
 937 }
 938
 939 fn suggest_constraining_param(
 940     tcx: TyCtxt<'_>,
 941     body_id: hir::HirId,
 942     mut err: &mut DiagnosticBuilder<'_>,
 943     lhs_ty: Ty<'_>,
 944     rhs_ty: Ty<'_>,
 945     missing_trait: &str,
 946     p: ty::ParamTy,
 947     set_output: bool,
 948 ) {
 949     let hir = tcx.hir();
 950     let msg = &format!("`{}` might need a bound for `{}`", lhs_ty, missing_trait);
 951     // Try to find the def-id and details for the parameter p. We have only the index,
 952     // so we have to find the enclosing function's def-id, then look through its declared
 953     // generic parameters to get the declaration.
 954     let def_id = hir.body_owner_def_id(hir::BodyId { hir_id: body_id });
 955     let generics = tcx.generics_of(def_id);
 956     let param_def_id = generics.type_param(&p, tcx).def_id;
 957     if let Some(generics) = param_def_id
 958         .as_local()
 959         .map(|id| hir.as_local_hir_id(id))
 960         .and_then(|id| hir.find(hir.get_parent_item(id)))
 961         .as_ref()
 962         .and_then(|node| node.generics())
 963     {
 964         let output = if set_output { format!("<Output = {}>", rhs_ty) } else { String::new() };
 965         suggest_constraining_type_param(
 966             tcx,
 967             generics,
 968             &mut err,
 969             &format!("{}", lhs_ty),
 970             &format!("{}{}", missing_trait, output),
 971             None,
 972         );
 973     } else {
 974         let span = tcx.def_span(param_def_id);
 975         err.span_label(span, msg);
 976     }
 977 }