compiler/rustc_typeck/src/check/op.rs

   1 //! Code related to processing overloaded binary and unary operators.
   2
   3 use super::method::MethodCallee;
   4 use super::{has_expected_num_generic_args, FnCtxt};
   5 use rustc_ast as ast;
   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::fold::TypeFolder;
  13 use rustc_middle::ty::TyKind::{Adt, Array, Char, FnDef, Never, Ref, Str, Tuple, Uint};
  14 use rustc_middle::ty::{
  15     self, suggest_constraining_type_param, Ty, TyCtxt, TypeFoldable, TypeVisitor,
  16 };
  17 use rustc_span::source_map::Spanned;
  18 use rustc_span::symbol::{sym, Ident};
  19 use rustc_span::Span;
  20 use rustc_trait_selection::infer::InferCtxtExt;
  21 use rustc_trait_selection::traits::{FulfillmentError, TraitEngine, TraitEngineExt};
  22
  23 use std::ops::ControlFlow;
  24
  25 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  26     /// Checks a `a <op>= b`
  27     pub fn check_binop_assign(
  28         &self,
  29         expr: &'tcx hir::Expr<'tcx>,
  30         op: hir::BinOp,
  31         lhs: &'tcx hir::Expr<'tcx>,
  32         rhs: &'tcx hir::Expr<'tcx>,
  33     ) -> Ty<'tcx> {
  34         let (lhs_ty, rhs_ty, return_ty) =
  35             self.check_overloaded_binop(expr, lhs, rhs, op, IsAssign::Yes);
  36
  37         let ty =
  38             if !lhs_ty.is_ty_var() && !rhs_ty.is_ty_var() && is_builtin_binop(lhs_ty, rhs_ty, op) {
  39                 self.enforce_builtin_binop_types(&lhs.span, lhs_ty, &rhs.span, rhs_ty, op);
  40                 self.tcx.mk_unit()
  41             } else {
  42                 return_ty
  43             };
  44
  45         self.check_lhs_assignable(lhs, "E0067", &op.span);
  46
  47         ty
  48     }
  49
  50     /// Checks a potentially overloaded binary operator.
  51     pub fn check_binop(
  52         &self,
  53         expr: &'tcx hir::Expr<'tcx>,
  54         op: hir::BinOp,
  55         lhs_expr: &'tcx hir::Expr<'tcx>,
  56         rhs_expr: &'tcx hir::Expr<'tcx>,
  57     ) -> Ty<'tcx> {
  58         let tcx = self.tcx;
  59
  60         debug!(
  61             "check_binop(expr.hir_id={}, expr={:?}, op={:?}, lhs_expr={:?}, rhs_expr={:?})",
  62             expr.hir_id, expr, op, lhs_expr, rhs_expr
  63         );
  64
  65         match BinOpCategory::from(op) {
  66             BinOpCategory::Shortcircuit => {
  67                 // && and || are a simple case.
  68                 self.check_expr_coercable_to_type(lhs_expr, tcx.types.bool, None);
  69                 let lhs_diverges = self.diverges.get();
  70                 self.check_expr_coercable_to_type(rhs_expr, tcx.types.bool, None);
  71
  72                 // Depending on the LHS' value, the RHS can never execute.
  73                 self.diverges.set(lhs_diverges);
  74
  75                 tcx.types.bool
  76             }
  77             _ => {
  78                 // Otherwise, we always treat operators as if they are
  79                 // overloaded. This is the way to be most flexible w/r/t
  80                 // types that get inferred.
  81                 let (lhs_ty, rhs_ty, return_ty) =
  82                     self.check_overloaded_binop(expr, lhs_expr, rhs_expr, op, IsAssign::No);
  83
  84                 // Supply type inference hints if relevant. Probably these
  85                 // hints should be enforced during select as part of the
  86                 // `consider_unification_despite_ambiguity` routine, but this
  87                 // more convenient for now.
  88                 //
  89                 // The basic idea is to help type inference by taking
  90                 // advantage of things we know about how the impls for
  91                 // scalar types are arranged. This is important in a
  92                 // scenario like `1_u32 << 2`, because it lets us quickly
  93                 // deduce that the result type should be `u32`, even
  94                 // though we don't know yet what type 2 has and hence
  95                 // can't pin this down to a specific impl.
  96                 if !lhs_ty.is_ty_var()
  97                     && !rhs_ty.is_ty_var()
  98                     && is_builtin_binop(lhs_ty, rhs_ty, op)
  99                 {
 100                     let builtin_return_ty = self.enforce_builtin_binop_types(
 101                         &lhs_expr.span,
 102                         lhs_ty,
 103                         &rhs_expr.span,
 104                         rhs_ty,
 105                         op,
 106                     );
 107                     self.demand_suptype(expr.span, builtin_return_ty, return_ty);
 108                 }
 109
 110                 return_ty
 111             }
 112         }
 113     }
 114
 115     fn enforce_builtin_binop_types(
 116         &self,
 117         lhs_span: &Span,
 118         lhs_ty: Ty<'tcx>,
 119         rhs_span: &Span,
 120         rhs_ty: Ty<'tcx>,
 121         op: hir::BinOp,
 122     ) -> Ty<'tcx> {
 123         debug_assert!(is_builtin_binop(lhs_ty, rhs_ty, op));
 124
 125         // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
 126         // (See https://github.com/rust-lang/rust/issues/57447.)
 127         let (lhs_ty, rhs_ty) = (deref_ty_if_possible(lhs_ty), deref_ty_if_possible(rhs_ty));
 128
 129         let tcx = self.tcx;
 130         match BinOpCategory::from(op) {
 131             BinOpCategory::Shortcircuit => {
 132                 self.demand_suptype(*lhs_span, tcx.types.bool, lhs_ty);
 133                 self.demand_suptype(*rhs_span, tcx.types.bool, rhs_ty);
 134                 tcx.types.bool
 135             }
 136
 137             BinOpCategory::Shift => {
 138                 // result type is same as LHS always
 139                 lhs_ty
 140             }
 141
 142             BinOpCategory::Math | BinOpCategory::Bitwise => {
 143                 // both LHS and RHS and result will have the same type
 144                 self.demand_suptype(*rhs_span, lhs_ty, rhs_ty);
 145                 lhs_ty
 146             }
 147
 148             BinOpCategory::Comparison => {
 149                 // both LHS and RHS and result will have the same type
 150                 self.demand_suptype(*rhs_span, lhs_ty, rhs_ty);
 151                 tcx.types.bool
 152             }
 153         }
 154     }
 155
 156     fn check_overloaded_binop(
 157         &self,
 158         expr: &'tcx hir::Expr<'tcx>,
 159         lhs_expr: &'tcx hir::Expr<'tcx>,
 160         rhs_expr: &'tcx hir::Expr<'tcx>,
 161         op: hir::BinOp,
 162         is_assign: IsAssign,
 163     ) -> (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>) {
 164         debug!(
 165             "check_overloaded_binop(expr.hir_id={}, op={:?}, is_assign={:?})",
 166             expr.hir_id, op, is_assign
 167         );
 168
 169         let lhs_ty = match is_assign {
 170             IsAssign::No => {
 171                 // Find a suitable supertype of the LHS expression's type, by coercing to
 172                 // a type variable, to pass as the `Self` to the trait, avoiding invariant
 173                 // trait matching creating lifetime constraints that are too strict.
 174                 // e.g., adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
 175                 // in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
 176                 let lhs_ty = self.check_expr(lhs_expr);
 177                 let fresh_var = self.next_ty_var(TypeVariableOrigin {
 178                     kind: TypeVariableOriginKind::MiscVariable,
 179                     span: lhs_expr.span,
 180                 });
 181                 self.demand_coerce(lhs_expr, lhs_ty, fresh_var, Some(rhs_expr), AllowTwoPhase::No)
 182             }
 183             IsAssign::Yes => {
 184                 // rust-lang/rust#52126: We have to use strict
 185                 // equivalence on the LHS of an assign-op like `+=`;
 186                 // overwritten or mutably-borrowed places cannot be
 187                 // coerced to a supertype.
 188                 self.check_expr(lhs_expr)
 189             }
 190         };
 191         let lhs_ty = self.resolve_vars_with_obligations(lhs_ty);
 192
 193         // N.B., as we have not yet type-checked the RHS, we don't have the
 194         // type at hand. Make a variable to represent it. The whole reason
 195         // for this indirection is so that, below, we can check the expr
 196         // using this variable as the expected type, which sometimes lets
 197         // us do better coercions than we would be able to do otherwise,
 198         // particularly for things like `String + &String`.
 199         let rhs_ty_var = self.next_ty_var(TypeVariableOrigin {
 200             kind: TypeVariableOriginKind::MiscVariable,
 201             span: rhs_expr.span,
 202         });
 203
 204         let result = self.lookup_op_method(lhs_ty, &[rhs_ty_var], Op::Binary(op, is_assign));
 205
 206         // see `NB` above
 207         let rhs_ty = self.check_expr_coercable_to_type(rhs_expr, rhs_ty_var, Some(lhs_expr));
 208         let rhs_ty = self.resolve_vars_with_obligations(rhs_ty);
 209
 210         let return_ty = match result {
 211             Ok(method) => {
 212                 let by_ref_binop = !op.node.is_by_value();
 213                 if is_assign == IsAssign::Yes || by_ref_binop {
 214                     if let ty::Ref(region, _, mutbl) = method.sig.inputs()[0].kind() {
 215                         let mutbl = match mutbl {
 216                             hir::Mutability::Not => AutoBorrowMutability::Not,
 217                             hir::Mutability::Mut => AutoBorrowMutability::Mut {
 218                                 // Allow two-phase borrows for binops in initial deployment
 219                                 // since they desugar to methods
 220                                 allow_two_phase_borrow: AllowTwoPhase::Yes,
 221                             },
 222                         };
 223                         let autoref = Adjustment {
 224                             kind: Adjust::Borrow(AutoBorrow::Ref(region, mutbl)),
 225                             target: method.sig.inputs()[0],
 226                         };
 227                         self.apply_adjustments(lhs_expr, vec![autoref]);
 228                     }
 229                 }
 230                 if by_ref_binop {
 231                     if let ty::Ref(region, _, mutbl) = method.sig.inputs()[1].kind() {
 232                         let mutbl = match mutbl {
 233                             hir::Mutability::Not => AutoBorrowMutability::Not,
 234                             hir::Mutability::Mut => AutoBorrowMutability::Mut {
 235                                 // Allow two-phase borrows for binops in initial deployment
 236                                 // since they desugar to methods
 237                                 allow_two_phase_borrow: AllowTwoPhase::Yes,
 238                             },
 239                         };
 240                         let autoref = Adjustment {
 241                             kind: Adjust::Borrow(AutoBorrow::Ref(region, mutbl)),
 242                             target: method.sig.inputs()[1],
 243                         };
 244                         // HACK(eddyb) Bypass checks due to reborrows being in
 245                         // some cases applied on the RHS, on top of which we need
 246                         // to autoref, which is not allowed by apply_adjustments.
 247                         // self.apply_adjustments(rhs_expr, vec![autoref]);
 248                         self.typeck_results
 249                             .borrow_mut()
 250                             .adjustments_mut()
 251                             .entry(rhs_expr.hir_id)
 252                             .or_default()
 253                             .push(autoref);
 254                     }
 255                 }
 256                 self.write_method_call(expr.hir_id, method);
 257
 258                 method.sig.output()
 259             }
 260             // error types are considered "builtin"
 261             Err(_) if lhs_ty.references_error() || rhs_ty.references_error() => self.tcx.ty_error(),
 262             Err(errors) => {
 263                 let source_map = self.tcx.sess.source_map();
 264                 let (mut err, missing_trait, use_output) = match is_assign {
 265                     IsAssign::Yes => {
 266                         let mut err = struct_span_err!(
 267                             self.tcx.sess,
 268                             expr.span,
 269                             E0368,
 270                             "binary assignment operation `{}=` cannot be applied to type `{}`",
 271                             op.node.as_str(),
 272                             lhs_ty,
 273                         );
 274                         err.span_label(
 275                             lhs_expr.span,
 276                             format!("cannot use `{}=` on type `{}`", op.node.as_str(), lhs_ty),
 277                         );
 278                         let missing_trait = match op.node {
 279                             hir::BinOpKind::Add => Some("std::ops::AddAssign"),
 280                             hir::BinOpKind::Sub => Some("std::ops::SubAssign"),
 281                             hir::BinOpKind::Mul => Some("std::ops::MulAssign"),
 282                             hir::BinOpKind::Div => Some("std::ops::DivAssign"),
 283                             hir::BinOpKind::Rem => Some("std::ops::RemAssign"),
 284                             hir::BinOpKind::BitAnd => Some("std::ops::BitAndAssign"),
 285                             hir::BinOpKind::BitXor => Some("std::ops::BitXorAssign"),
 286                             hir::BinOpKind::BitOr => Some("std::ops::BitOrAssign"),
 287                             hir::BinOpKind::Shl => Some("std::ops::ShlAssign"),
 288                             hir::BinOpKind::Shr => Some("std::ops::ShrAssign"),
 289                             _ => None,
 290                         };
 291                         self.note_unmet_impls_on_type(&mut err, errors);
 292                         (err, missing_trait, false)
 293                     }
 294                     IsAssign::No => {
 295                         let (message, missing_trait, use_output) = match op.node {
 296                             hir::BinOpKind::Add => (
 297                                 format!("cannot add `{}` to `{}`", rhs_ty, lhs_ty),
 298                                 Some("std::ops::Add"),
 299                                 true,
 300                             ),
 301                             hir::BinOpKind::Sub => (
 302                                 format!("cannot subtract `{}` from `{}`", rhs_ty, lhs_ty),
 303                                 Some("std::ops::Sub"),
 304                                 true,
 305                             ),
 306                             hir::BinOpKind::Mul => (
 307                                 format!("cannot multiply `{}` by `{}`", lhs_ty, rhs_ty),
 308                                 Some("std::ops::Mul"),
 309                                 true,
 310                             ),
 311                             hir::BinOpKind::Div => (
 312                                 format!("cannot divide `{}` by `{}`", lhs_ty, rhs_ty),
 313                                 Some("std::ops::Div"),
 314                                 true,
 315                             ),
 316                             hir::BinOpKind::Rem => (
 317                                 format!("cannot mod `{}` by `{}`", lhs_ty, rhs_ty),
 318                                 Some("std::ops::Rem"),
 319                                 true,
 320                             ),
 321                             hir::BinOpKind::BitAnd => (
 322                                 format!("no implementation for `{} & {}`", lhs_ty, rhs_ty),
 323                                 Some("std::ops::BitAnd"),
 324                                 true,
 325                             ),
 326                             hir::BinOpKind::BitXor => (
 327                                 format!("no implementation for `{} ^ {}`", lhs_ty, rhs_ty),
 328                                 Some("std::ops::BitXor"),
 329                                 true,
 330                             ),
 331                             hir::BinOpKind::BitOr => (
 332                                 format!("no implementation for `{} | {}`", lhs_ty, rhs_ty),
 333                                 Some("std::ops::BitOr"),
 334                                 true,
 335                             ),
 336                             hir::BinOpKind::Shl => (
 337                                 format!("no implementation for `{} << {}`", lhs_ty, rhs_ty),
 338                                 Some("std::ops::Shl"),
 339                                 true,
 340                             ),
 341                             hir::BinOpKind::Shr => (
 342                                 format!("no implementation for `{} >> {}`", lhs_ty, rhs_ty),
 343                                 Some("std::ops::Shr"),
 344                                 true,
 345                             ),
 346                             hir::BinOpKind::Eq | hir::BinOpKind::Ne => (
 347                                 format!(
 348                                     "binary operation `{}` cannot be applied to type `{}`",
 349                                     op.node.as_str(),
 350                                     lhs_ty
 351                                 ),
 352                                 Some("std::cmp::PartialEq"),
 353                                 false,
 354                             ),
 355                             hir::BinOpKind::Lt
 356                             | hir::BinOpKind::Le
 357                             | hir::BinOpKind::Gt
 358                             | hir::BinOpKind::Ge => (
 359                                 format!(
 360                                     "binary operation `{}` cannot be applied to type `{}`",
 361                                     op.node.as_str(),
 362                                     lhs_ty
 363                                 ),
 364                                 Some("std::cmp::PartialOrd"),
 365                                 false,
 366                             ),
 367                             _ => (
 368                                 format!(
 369                                     "binary operation `{}` cannot be applied to type `{}`",
 370                                     op.node.as_str(),
 371                                     lhs_ty
 372                                 ),
 373                                 None,
 374                                 false,
 375                             ),
 376                         };
 377                         let mut err =
 378                             struct_span_err!(self.tcx.sess, op.span, E0369, "{}", message.as_str());
 379                         if !lhs_expr.span.eq(&rhs_expr.span) {
 380                             self.add_type_neq_err_label(
 381                                 &mut err,
 382                                 lhs_expr.span,
 383                                 lhs_ty,
 384                                 rhs_ty,
 385                                 op,
 386                                 is_assign,
 387                             );
 388                             self.add_type_neq_err_label(
 389                                 &mut err,
 390                                 rhs_expr.span,
 391                                 rhs_ty,
 392                                 lhs_ty,
 393                                 op,
 394                                 is_assign,
 395                             );
 396                         }
 397                         self.note_unmet_impls_on_type(&mut err, errors);
 398                         (err, missing_trait, use_output)
 399                     }
 400                 };
 401                 if let Ref(_, rty, _) = lhs_ty.kind() {
 402                     if self.infcx.type_is_copy_modulo_regions(self.param_env, rty, lhs_expr.span)
 403                         && self.lookup_op_method(rty, &[rhs_ty], Op::Binary(op, is_assign)).is_ok()
 404                     {
 405                         if let Ok(lstring) = source_map.span_to_snippet(lhs_expr.span) {
 406                             let msg = &format!(
 407                                 "`{}{}` can be used on `{}`, you can dereference `{}`",
 408                                 op.node.as_str(),
 409                                 match is_assign {
 410                                     IsAssign::Yes => "=",
 411                                     IsAssign::No => "",
 412                                 },
 413                                 rty.peel_refs(),
 414                                 lstring,
 415                             );
 416                             err.span_suggestion_verbose(
 417                                 lhs_expr.span.shrink_to_lo(),
 418                                 msg,
 419                                 "*".to_string(),
 420                                 rustc_errors::Applicability::MachineApplicable,
 421                             );
 422                         }
 423                     }
 424                 }
 425                 if let Some(missing_trait) = missing_trait {
 426                     let mut visitor = TypeParamVisitor(self.tcx, vec![]);
 427                     visitor.visit_ty(lhs_ty);
 428
 429                     if op.node == hir::BinOpKind::Add
 430                         && self.check_str_addition(
 431                             lhs_expr, rhs_expr, lhs_ty, rhs_ty, &mut err, is_assign, op,
 432                         )
 433                     {
 434                         // This has nothing here because it means we did string
 435                         // concatenation (e.g., "Hello " + "World!"). This means
 436                         // we don't want the note in the else clause to be emitted
 437                     } else if let [ty] = &visitor.1[..] {
 438                         if let ty::Param(p) = *ty.kind() {
 439                             // Check if the method would be found if the type param wasn't
 440                             // involved. If so, it means that adding a trait bound to the param is
 441                             // enough. Otherwise we do not give the suggestion.
 442                             let mut eraser = TypeParamEraser(self, expr.span);
 443                             let needs_bound = self
 444                                 .lookup_op_method(
 445                                     eraser.fold_ty(lhs_ty),
 446                                     &[eraser.fold_ty(rhs_ty)],
 447                                     Op::Binary(op, is_assign),
 448                                 )
 449                                 .is_ok();
 450                             if needs_bound {
 451                                 suggest_constraining_param(
 452                                     self.tcx,
 453                                     self.body_id,
 454                                     &mut err,
 455                                     ty,
 456                                     rhs_ty,
 457                                     missing_trait,
 458                                     p,
 459                                     use_output,
 460                                 );
 461                             } else if *ty != lhs_ty {
 462                                 // When we know that a missing bound is responsible, we don't show
 463                                 // this note as it is redundant.
 464                                 err.note(&format!(
 465                                     "the trait `{}` is not implemented for `{}`",
 466                                     missing_trait, lhs_ty
 467                                 ));
 468                             }
 469                         } else {
 470                             bug!("type param visitor stored a non type param: {:?}", ty.kind());
 471                         }
 472                     }
 473                 }
 474                 err.emit();
 475                 self.tcx.ty_error()
 476             }
 477         };
 478
 479         (lhs_ty, rhs_ty, return_ty)
 480     }
 481
 482     /// If one of the types is an uncalled function and calling it would yield the other type,
 483     /// suggest calling the function. Returns `true` if suggestion would apply (even if not given).
 484     fn add_type_neq_err_label(
 485         &self,
 486         err: &mut rustc_errors::DiagnosticBuilder<'_>,
 487         span: Span,
 488         ty: Ty<'tcx>,
 489         other_ty: Ty<'tcx>,
 490         op: hir::BinOp,
 491         is_assign: IsAssign,
 492     ) -> bool /* did we suggest to call a function because of missing parentheses? */ {
 493         err.span_label(span, ty.to_string());
 494         if let FnDef(def_id, _) = *ty.kind() {
 495             let source_map = self.tcx.sess.source_map();
 496             if !self.tcx.has_typeck_results(def_id) {
 497                 return false;
 498             }
 499             // FIXME: Instead of exiting early when encountering bound vars in
 500             // the function signature, consider keeping the binder here and
 501             // propagating it downwards.
 502             let Some(fn_sig) = self.tcx.fn_sig(def_id).no_bound_vars() else {
 503                 return false;
 504             };
 505
 506             let other_ty = if let FnDef(def_id, _) = *other_ty.kind() {
 507                 if !self.tcx.has_typeck_results(def_id) {
 508                     return false;
 509                 }
 510                 // We're emitting a suggestion, so we can just ignore regions
 511                 self.tcx.fn_sig(def_id).skip_binder().output()
 512             } else {
 513                 other_ty
 514             };
 515
 516             if self
 517                 .lookup_op_method(fn_sig.output(), &[other_ty], Op::Binary(op, is_assign))
 518                 .is_ok()
 519             {
 520                 if let Ok(snippet) = source_map.span_to_snippet(span) {
 521                     let (variable_snippet, applicability) = if !fn_sig.inputs().is_empty() {
 522                         (format!("{}( /* arguments */ )", snippet), Applicability::HasPlaceholders)
 523                     } else {
 524                         (format!("{}()", snippet), Applicability::MaybeIncorrect)
 525                     };
 526
 527                     err.span_suggestion(
 528                         span,
 529                         "you might have forgotten to call this function",
 530                         variable_snippet,
 531                         applicability,
 532                     );
 533                 }
 534                 return true;
 535             }
 536         }
 537         false
 538     }
 539
 540     /// Provide actionable suggestions when trying to add two strings with incorrect types,
 541     /// like `&str + &str`, `String + String` and `&str + &String`.
 542     ///
 543     /// If this function returns `true` it means a note was printed, so we don't need
 544     /// to print the normal "implementation of `std::ops::Add` might be missing" note
 545     fn check_str_addition(
 546         &self,
 547         lhs_expr: &'tcx hir::Expr<'tcx>,
 548         rhs_expr: &'tcx hir::Expr<'tcx>,
 549         lhs_ty: Ty<'tcx>,
 550         rhs_ty: Ty<'tcx>,
 551         err: &mut rustc_errors::DiagnosticBuilder<'_>,
 552         is_assign: IsAssign,
 553         op: hir::BinOp,
 554     ) -> bool {
 555         let source_map = self.tcx.sess.source_map();
 556         let remove_borrow_msg = "String concatenation appends the string on the right to the \
 557                                  string on the left and may require reallocation. This \
 558                                  requires ownership of the string on the left";
 559
 560         let msg = "`to_owned()` can be used to create an owned `String` \
 561                    from a string reference. String concatenation \
 562                    appends the string on the right to the string \
 563                    on the left and may require reallocation. This \
 564                    requires ownership of the string on the left";
 565
 566         let string_type = self.tcx.get_diagnostic_item(sym::String);
 567         let is_std_string = |ty: Ty<'tcx>| match ty.ty_adt_def() {
 568             Some(ty_def) => Some(ty_def.did) == string_type,
 569             None => false,
 570         };
 571
 572         match (lhs_ty.kind(), rhs_ty.kind()) {
 573             (&Ref(_, l_ty, _), &Ref(_, r_ty, _)) // &str or &String + &str, &String or &&str
 574                 if (*l_ty.kind() == Str || is_std_string(l_ty)) && (
 575                         *r_ty.kind() == Str || is_std_string(r_ty) ||
 576                         &format!("{:?}", rhs_ty) == "&&str"
 577                     ) =>
 578             {
 579                 if let IsAssign::No = is_assign { // Do not supply this message if `&str += &str`
 580                     err.span_label(
 581                         op.span,
 582                         "`+` cannot be used to concatenate two `&str` strings",
 583                     );
 584                     match source_map.span_to_snippet(lhs_expr.span) {
 585                         Ok(lstring) => {
 586                             err.span_suggestion(
 587                                 lhs_expr.span,
 588                                 if lstring.starts_with('&') {
 589                                     remove_borrow_msg
 590                                 } else {
 591                                     msg
 592                                 },
 593                                 if let Some(stripped) = lstring.strip_prefix('&') {
 594                                     // let a = String::new();
 595                                     // let _ = &a + "bar";
 596                                     stripped.to_string()
 597                                 } else {
 598                                     format!("{}.to_owned()", lstring)
 599                                 },
 600                                 Applicability::MachineApplicable,
 601                             )
 602                         }
 603                         _ => err.help(msg),
 604                     };
 605                 }
 606                 true
 607             }
 608             (&Ref(_, l_ty, _), &Adt(..)) // Handle `&str` & `&String` + `String`
 609                 if (*l_ty.kind() == Str || is_std_string(l_ty)) && is_std_string(rhs_ty) =>
 610             {
 611                 err.span_label(
 612                     op.span,
 613                     "`+` cannot be used to concatenate a `&str` with a `String`",
 614                 );
 615                 match (
 616                     source_map.span_to_snippet(lhs_expr.span),
 617                     source_map.span_to_snippet(rhs_expr.span),
 618                     is_assign,
 619                 ) {
 620                     (Ok(l), Ok(r), IsAssign::No) => {
 621                         let to_string = if let Some(stripped) = l.strip_prefix('&') {
 622                             // let a = String::new(); let b = String::new();
 623                             // let _ = &a + b;
 624                             stripped.to_string()
 625                         } else {
 626                             format!("{}.to_owned()", l)
 627                         };
 628                         err.multipart_suggestion(
 629                             msg,
 630                             vec![
 631                                 (lhs_expr.span, to_string),
 632                                 (rhs_expr.span, format!("&{}", r)),
 633                             ],
 634                             Applicability::MachineApplicable,
 635                         );
 636                     }
 637                     _ => {
 638                         err.help(msg);
 639                     }
 640                 };
 641                 true
 642             }
 643             _ => false,
 644         }
 645     }
 646
 647     pub fn check_user_unop(
 648         &self,
 649         ex: &'tcx hir::Expr<'tcx>,
 650         operand_ty: Ty<'tcx>,
 651         op: hir::UnOp,
 652     ) -> Ty<'tcx> {
 653         assert!(op.is_by_value());
 654         match self.lookup_op_method(operand_ty, &[], Op::Unary(op, ex.span)) {
 655             Ok(method) => {
 656                 self.write_method_call(ex.hir_id, method);
 657                 method.sig.output()
 658             }
 659             Err(errors) => {
 660                 let actual = self.resolve_vars_if_possible(operand_ty);
 661                 if !actual.references_error() {
 662                     let mut err = struct_span_err!(
 663                         self.tcx.sess,
 664                         ex.span,
 665                         E0600,
 666                         "cannot apply unary operator `{}` to type `{}`",
 667                         op.as_str(),
 668                         actual
 669                     );
 670                     err.span_label(
 671                         ex.span,
 672                         format!("cannot apply unary operator `{}`", op.as_str()),
 673                     );
 674
 675                     let sp = self.tcx.sess.source_map().start_point(ex.span);
 676                     if let Some(sp) =
 677                         self.tcx.sess.parse_sess.ambiguous_block_expr_parse.borrow().get(&sp)
 678                     {
 679                         // If the previous expression was a block expression, suggest parentheses
 680                         // (turning this into a binary subtraction operation instead.)
 681                         // for example, `{2} - 2` -> `({2}) - 2` (see src\test\ui\parser\expr-as-stmt.rs)
 682                         self.tcx.sess.parse_sess.expr_parentheses_needed(&mut err, *sp);
 683                     } else {
 684                         match actual.kind() {
 685                             Uint(_) if op == hir::UnOp::Neg => {
 686                                 err.note("unsigned values cannot be negated");
 687
 688                                 if let hir::ExprKind::Unary(
 689                                     _,
 690                                     hir::Expr {
 691                                         kind:
 692                                             hir::ExprKind::Lit(Spanned {
 693                                                 node: ast::LitKind::Int(1, _),
 694                                                 ..
 695                                             }),
 696                                         ..
 697                                     },
 698                                 ) = ex.kind
 699                                 {
 700                                     err.span_suggestion(
 701                                         ex.span,
 702                                         &format!(
 703                                             "you may have meant the maximum value of `{}`",
 704                                             actual
 705                                         ),
 706                                         format!("{}::MAX", actual),
 707                                         Applicability::MaybeIncorrect,
 708                                     );
 709                                 }
 710                             }
 711                             Str | Never | Char | Tuple(_) | Array(_, _) => {}
 712                             Ref(_, lty, _) if *lty.kind() == Str => {}
 713                             _ => {
 714                                 self.note_unmet_impls_on_type(&mut err, errors);
 715                             }
 716                         }
 717                     }
 718                     err.emit();
 719                 }
 720                 self.tcx.ty_error()
 721             }
 722         }
 723     }
 724
 725     fn lookup_op_method(
 726         &self,
 727         lhs_ty: Ty<'tcx>,
 728         other_tys: &[Ty<'tcx>],
 729         op: Op,
 730     ) -> Result<MethodCallee<'tcx>, Vec<FulfillmentError<'tcx>>> {
 731         let lang = self.tcx.lang_items();
 732
 733         let span = match op {
 734             Op::Binary(op, _) => op.span,
 735             Op::Unary(_, span) => span,
 736         };
 737         let (opname, trait_did) = if let Op::Binary(op, IsAssign::Yes) = op {
 738             match op.node {
 739                 hir::BinOpKind::Add => (sym::add_assign, lang.add_assign_trait()),
 740                 hir::BinOpKind::Sub => (sym::sub_assign, lang.sub_assign_trait()),
 741                 hir::BinOpKind::Mul => (sym::mul_assign, lang.mul_assign_trait()),
 742                 hir::BinOpKind::Div => (sym::div_assign, lang.div_assign_trait()),
 743                 hir::BinOpKind::Rem => (sym::rem_assign, lang.rem_assign_trait()),
 744                 hir::BinOpKind::BitXor => (sym::bitxor_assign, lang.bitxor_assign_trait()),
 745                 hir::BinOpKind::BitAnd => (sym::bitand_assign, lang.bitand_assign_trait()),
 746                 hir::BinOpKind::BitOr => (sym::bitor_assign, lang.bitor_assign_trait()),
 747                 hir::BinOpKind::Shl => (sym::shl_assign, lang.shl_assign_trait()),
 748                 hir::BinOpKind::Shr => (sym::shr_assign, lang.shr_assign_trait()),
 749                 hir::BinOpKind::Lt
 750                 | hir::BinOpKind::Le
 751                 | hir::BinOpKind::Ge
 752                 | hir::BinOpKind::Gt
 753                 | hir::BinOpKind::Eq
 754                 | hir::BinOpKind::Ne
 755                 | hir::BinOpKind::And
 756                 | hir::BinOpKind::Or => {
 757                     span_bug!(span, "impossible assignment operation: {}=", op.node.as_str())
 758                 }
 759             }
 760         } else if let Op::Binary(op, IsAssign::No) = op {
 761             match op.node {
 762                 hir::BinOpKind::Add => (sym::add, lang.add_trait()),
 763                 hir::BinOpKind::Sub => (sym::sub, lang.sub_trait()),
 764                 hir::BinOpKind::Mul => (sym::mul, lang.mul_trait()),
 765                 hir::BinOpKind::Div => (sym::div, lang.div_trait()),
 766                 hir::BinOpKind::Rem => (sym::rem, lang.rem_trait()),
 767                 hir::BinOpKind::BitXor => (sym::bitxor, lang.bitxor_trait()),
 768                 hir::BinOpKind::BitAnd => (sym::bitand, lang.bitand_trait()),
 769                 hir::BinOpKind::BitOr => (sym::bitor, lang.bitor_trait()),
 770                 hir::BinOpKind::Shl => (sym::shl, lang.shl_trait()),
 771                 hir::BinOpKind::Shr => (sym::shr, lang.shr_trait()),
 772                 hir::BinOpKind::Lt => (sym::lt, lang.partial_ord_trait()),
 773                 hir::BinOpKind::Le => (sym::le, lang.partial_ord_trait()),
 774                 hir::BinOpKind::Ge => (sym::ge, lang.partial_ord_trait()),
 775                 hir::BinOpKind::Gt => (sym::gt, lang.partial_ord_trait()),
 776                 hir::BinOpKind::Eq => (sym::eq, lang.eq_trait()),
 777                 hir::BinOpKind::Ne => (sym::ne, lang.eq_trait()),
 778                 hir::BinOpKind::And | hir::BinOpKind::Or => {
 779                     span_bug!(span, "&& and || are not overloadable")
 780                 }
 781             }
 782         } else if let Op::Unary(hir::UnOp::Not, _) = op {
 783             (sym::not, lang.not_trait())
 784         } else if let Op::Unary(hir::UnOp::Neg, _) = op {
 785             (sym::neg, lang.neg_trait())
 786         } else {
 787             bug!("lookup_op_method: op not supported: {:?}", op)
 788         };
 789
 790         debug!(
 791             "lookup_op_method(lhs_ty={:?}, op={:?}, opname={:?}, trait_did={:?})",
 792             lhs_ty, op, opname, trait_did
 793         );
 794
 795         // Catches cases like #83893, where a lang item is declared with the
 796         // wrong number of generic arguments. Should have yielded an error
 797         // elsewhere by now, but we have to catch it here so that we do not
 798         // index `other_tys` out of bounds (if the lang item has too many
 799         // generic arguments, `other_tys` is too short).
 800         if !has_expected_num_generic_args(
 801             self.tcx,
 802             trait_did,
 803             match op {
 804                 // Binary ops have a generic right-hand side, unary ops don't
 805                 Op::Binary(..) => 1,
 806                 Op::Unary(..) => 0,
 807             },
 808         ) {
 809             return Err(vec![]);
 810         }
 811
 812         let opname = Ident::with_dummy_span(opname);
 813         let method = trait_did.and_then(|trait_did| {
 814             self.lookup_method_in_trait(span, opname, trait_did, lhs_ty, Some(other_tys))
 815         });
 816
 817         match (method, trait_did) {
 818             (Some(ok), _) => {
 819                 let method = self.register_infer_ok_obligations(ok);
 820                 self.select_obligations_where_possible(false, |_| {});
 821                 Ok(method)
 822             }
 823             (None, None) => Err(vec![]),
 824             (None, Some(trait_did)) => {
 825                 let (obligation, _) =
 826                     self.obligation_for_method(span, trait_did, lhs_ty, Some(other_tys));
 827                 let mut fulfill = <dyn TraitEngine<'_>>::new(self.tcx);
 828                 fulfill.register_predicate_obligation(self, obligation);
 829                 Err(fulfill.select_where_possible(&self.infcx))
 830             }
 831         }
 832     }
 833 }
 834
 835 // Binary operator categories. These categories summarize the behavior
 836 // with respect to the builtin operationrs supported.
 837 enum BinOpCategory {
 838     /// &&, || -- cannot be overridden
 839     Shortcircuit,
 840
 841     /// <<, >> -- when shifting a single integer, rhs can be any
 842     /// integer type. For simd, types must match.
 843     Shift,
 844
 845     /// +, -, etc -- takes equal types, produces same type as input,
 846     /// applicable to ints/floats/simd
 847     Math,
 848
 849     /// &, |, ^ -- takes equal types, produces same type as input,
 850     /// applicable to ints/floats/simd/bool
 851     Bitwise,
 852
 853     /// ==, !=, etc -- takes equal types, produces bools, except for simd,
 854     /// which produce the input type
 855     Comparison,
 856 }
 857
 858 impl BinOpCategory {
 859     fn from(op: hir::BinOp) -> BinOpCategory {
 860         match op.node {
 861             hir::BinOpKind::Shl | hir::BinOpKind::Shr => BinOpCategory::Shift,
 862
 863             hir::BinOpKind::Add
 864             | hir::BinOpKind::Sub
 865             | hir::BinOpKind::Mul
 866             | hir::BinOpKind::Div
 867             | hir::BinOpKind::Rem => BinOpCategory::Math,
 868
 869             hir::BinOpKind::BitXor | hir::BinOpKind::BitAnd | hir::BinOpKind::BitOr => {
 870                 BinOpCategory::Bitwise
 871             }
 872
 873             hir::BinOpKind::Eq
 874             | hir::BinOpKind::Ne
 875             | hir::BinOpKind::Lt
 876             | hir::BinOpKind::Le
 877             | hir::BinOpKind::Ge
 878             | hir::BinOpKind::Gt => BinOpCategory::Comparison,
 879
 880             hir::BinOpKind::And | hir::BinOpKind::Or => BinOpCategory::Shortcircuit,
 881         }
 882     }
 883 }
 884
 885 /// Whether the binary operation is an assignment (`a += b`), or not (`a + b`)
 886 #[derive(Clone, Copy, Debug, PartialEq)]
 887 enum IsAssign {
 888     No,
 889     Yes,
 890 }
 891
 892 #[derive(Clone, Copy, Debug)]
 893 enum Op {
 894     Binary(hir::BinOp, IsAssign),
 895     Unary(hir::UnOp, Span),
 896 }
 897
 898 /// Dereferences a single level of immutable referencing.
 899 fn deref_ty_if_possible(ty: Ty<'tcx>) -> Ty<'tcx> {
 900     match ty.kind() {
 901         ty::Ref(_, ty, hir::Mutability::Not) => ty,
 902         _ => ty,
 903     }
 904 }
 905
 906 /// Returns `true` if this is a built-in arithmetic operation (e.g., u32
 907 /// + u32, i16x4 == i16x4) and false if these types would have to be
 908 /// overloaded to be legal. There are two reasons that we distinguish
 909 /// builtin operations from overloaded ones (vs trying to drive
 910 /// everything uniformly through the trait system and intrinsics or
 911 /// something like that):
 912 ///
 913 /// 1. Builtin operations can trivially be evaluated in constants.
 914 /// 2. For comparison operators applied to SIMD types the result is
 915 ///    not of type `bool`. For example, `i16x4 == i16x4` yields a
 916 ///    type like `i16x4`. This means that the overloaded trait
 917 ///    `PartialEq` is not applicable.
 918 ///
 919 /// Reason #2 is the killer. I tried for a while to always use
 920 /// overloaded logic and just check the types in constants/codegen after
 921 /// the fact, and it worked fine, except for SIMD types. -nmatsakis
 922 fn is_builtin_binop<'tcx>(lhs: Ty<'tcx>, rhs: Ty<'tcx>, op: hir::BinOp) -> bool {
 923     // Special-case a single layer of referencing, so that things like `5.0 + &6.0f32` work.
 924     // (See https://github.com/rust-lang/rust/issues/57447.)
 925     let (lhs, rhs) = (deref_ty_if_possible(lhs), deref_ty_if_possible(rhs));
 926
 927     match BinOpCategory::from(op) {
 928         BinOpCategory::Shortcircuit => true,
 929
 930         BinOpCategory::Shift => {
 931             lhs.references_error()
 932                 || rhs.references_error()
 933                 || lhs.is_integral() && rhs.is_integral()
 934         }
 935
 936         BinOpCategory::Math => {
 937             lhs.references_error()
 938                 || rhs.references_error()
 939                 || lhs.is_integral() && rhs.is_integral()
 940                 || lhs.is_floating_point() && rhs.is_floating_point()
 941         }
 942
 943         BinOpCategory::Bitwise => {
 944             lhs.references_error()
 945                 || rhs.references_error()
 946                 || lhs.is_integral() && rhs.is_integral()
 947                 || lhs.is_floating_point() && rhs.is_floating_point()
 948                 || lhs.is_bool() && rhs.is_bool()
 949         }
 950
 951         BinOpCategory::Comparison => {
 952             lhs.references_error() || rhs.references_error() || lhs.is_scalar() && rhs.is_scalar()
 953         }
 954     }
 955 }
 956
 957 fn suggest_constraining_param(
 958     tcx: TyCtxt<'_>,
 959     body_id: hir::HirId,
 960     mut err: &mut DiagnosticBuilder<'_>,
 961     lhs_ty: Ty<'_>,
 962     rhs_ty: Ty<'_>,
 963     missing_trait: &str,
 964     p: ty::ParamTy,
 965     set_output: bool,
 966 ) {
 967     let hir = tcx.hir();
 968     let msg = &format!("`{}` might need a bound for `{}`", lhs_ty, missing_trait);
 969     // Try to find the def-id and details for the parameter p. We have only the index,
 970     // so we have to find the enclosing function's def-id, then look through its declared
 971     // generic parameters to get the declaration.
 972     let def_id = hir.body_owner_def_id(hir::BodyId { hir_id: body_id });
 973     let generics = tcx.generics_of(def_id);
 974     let param_def_id = generics.type_param(&p, tcx).def_id;
 975     if let Some(generics) = param_def_id
 976         .as_local()
 977         .map(|id| hir.local_def_id_to_hir_id(id))
 978         .and_then(|id| hir.find(hir.get_parent_item(id)))
 979         .as_ref()
 980         .and_then(|node| node.generics())
 981     {
 982         let output = if set_output { format!("<Output = {}>", rhs_ty) } else { String::new() };
 983         suggest_constraining_type_param(
 984             tcx,
 985             generics,
 986             &mut err,
 987             &format!("{}", lhs_ty),
 988             &format!("{}{}", missing_trait, output),
 989             None,
 990         );
 991     } else {
 992         let span = tcx.def_span(param_def_id);
 993         err.span_label(span, msg);
 994     }
 995 }
 996
 997 struct TypeParamVisitor<'tcx>(TyCtxt<'tcx>, Vec<Ty<'tcx>>);
 998
 999 impl<'tcx> TypeVisitor<'tcx> for TypeParamVisitor<'tcx> {
1000     fn tcx_for_anon_const_substs(&self) -> Option<TyCtxt<'tcx>> {
1001         Some(self.0)
1002     }
1003     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1004         if let ty::Param(_) = ty.kind() {
1005             self.1.push(ty);
1006         }
1007         ty.super_visit_with(self)
1008     }
1009 }
1010
1011 struct TypeParamEraser<'a, 'tcx>(&'a FnCtxt<'a, 'tcx>, Span);
1012
1013 impl TypeFolder<'tcx> for TypeParamEraser<'_, 'tcx> {
1014     fn tcx(&self) -> TyCtxt<'tcx> {
1015         self.0.tcx
1016     }
1017
1018     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1019         match ty.kind() {
1020             ty::Param(_) => self.0.next_ty_var(TypeVariableOrigin {
1021                 kind: TypeVariableOriginKind::MiscVariable,
1022                 span: self.1,
1023             }),
1024             _ => ty.super_fold_with(self),
1025         }
1026     }
1027 }