compiler/rustc_typeck/src/check/place_op.rs

   1 use crate::check::method::MethodCallee;
   2 use crate::check::{FnCtxt, PlaceOp};
   3 use rustc_hir as hir;
   4 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
   5 use rustc_infer::infer::InferOk;
   6 use rustc_middle::ty::adjustment::{Adjust, Adjustment, OverloadedDeref, PointerCast};
   7 use rustc_middle::ty::adjustment::{AllowTwoPhase, AutoBorrow, AutoBorrowMutability};
   8 use rustc_middle::ty::{self, Ty};
   9 use rustc_span::symbol::{sym, Ident};
  10 use rustc_span::Span;
  11 use rustc_trait_selection::autoderef::Autoderef;
  12 use std::slice;
  13
  14 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  15     /// Type-check `*oprnd_expr` with `oprnd_expr` type-checked already.
  16     pub(super) fn lookup_derefing(
  17         &self,
  18         expr: &hir::Expr<'_>,
  19         oprnd_expr: &'tcx hir::Expr<'tcx>,
  20         oprnd_ty: Ty<'tcx>,
  21     ) -> Option<Ty<'tcx>> {
  22         if let Some(mt) = oprnd_ty.builtin_deref(true) {
  23             return Some(mt.ty);
  24         }
  25
  26         let ok = self.try_overloaded_deref(expr.span, oprnd_ty)?;
  27         let method = self.register_infer_ok_obligations(ok);
  28         if let ty::Ref(region, _, hir::Mutability::Not) = method.sig.inputs()[0].kind() {
  29             self.apply_adjustments(
  30                 oprnd_expr,
  31                 vec![Adjustment {
  32                     kind: Adjust::Borrow(AutoBorrow::Ref(region, AutoBorrowMutability::Not)),
  33                     target: method.sig.inputs()[0],
  34                 }],
  35             );
  36         } else {
  37             span_bug!(expr.span, "input to deref is not a ref?");
  38         }
  39         let ty = self.make_overloaded_place_return_type(method).ty;
  40         self.write_method_call(expr.hir_id, method);
  41         Some(ty)
  42     }
  43
  44     /// Type-check `*base_expr[index_expr]` with `base_expr` and `index_expr` type-checked already.
  45     pub(super) fn lookup_indexing(
  46         &self,
  47         expr: &hir::Expr<'_>,
  48         base_expr: &'tcx hir::Expr<'tcx>,
  49         base_ty: Ty<'tcx>,
  50         idx_ty: Ty<'tcx>,
  51     ) -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)> {
  52         // FIXME(#18741) -- this is almost but not quite the same as the
  53         // autoderef that normal method probing does. They could likely be
  54         // consolidated.
  55
  56         let mut autoderef = self.autoderef(base_expr.span, base_ty);
  57         let mut result = None;
  58         while result.is_none() && autoderef.next().is_some() {
  59             result = self.try_index_step(expr, base_expr, &autoderef, idx_ty);
  60         }
  61         self.register_predicates(autoderef.into_obligations());
  62         result
  63     }
  64
  65     /// To type-check `base_expr[index_expr]`, we progressively autoderef
  66     /// (and otherwise adjust) `base_expr`, looking for a type which either
  67     /// supports builtin indexing or overloaded indexing.
  68     /// This loop implements one step in that search; the autoderef loop
  69     /// is implemented by `lookup_indexing`.
  70     fn try_index_step(
  71         &self,
  72         expr: &hir::Expr<'_>,
  73         base_expr: &hir::Expr<'_>,
  74         autoderef: &Autoderef<'a, 'tcx>,
  75         index_ty: Ty<'tcx>,
  76     ) -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)> {
  77         let adjusted_ty =
  78             self.structurally_resolved_type(autoderef.span(), autoderef.final_ty(false));
  79         debug!(
  80             "try_index_step(expr={:?}, base_expr={:?}, adjusted_ty={:?}, \
  81              index_ty={:?})",
  82             expr, base_expr, adjusted_ty, index_ty
  83         );
  84
  85         for &unsize in &[false, true] {
  86             let mut self_ty = adjusted_ty;
  87             if unsize {
  88                 // We only unsize arrays here.
  89                 if let ty::Array(element_ty, _) = adjusted_ty.kind() {
  90                     self_ty = self.tcx.mk_slice(element_ty);
  91                 } else {
  92                     continue;
  93                 }
  94             }
  95
  96             // If some lookup succeeds, write callee into table and extract index/element
  97             // type from the method signature.
  98             // If some lookup succeeded, install method in table
  99             let input_ty = self.next_ty_var(TypeVariableOrigin {
 100                 kind: TypeVariableOriginKind::AutoDeref,
 101                 span: base_expr.span,
 102             });
 103             let method =
 104                 self.try_overloaded_place_op(expr.span, self_ty, &[input_ty], PlaceOp::Index);
 105
 106             if let Some(result) = method {
 107                 debug!("try_index_step: success, using overloaded indexing");
 108                 let method = self.register_infer_ok_obligations(result);
 109
 110                 let mut adjustments = self.adjust_steps(autoderef);
 111                 if let ty::Ref(region, _, hir::Mutability::Not) = method.sig.inputs()[0].kind() {
 112                     adjustments.push(Adjustment {
 113                         kind: Adjust::Borrow(AutoBorrow::Ref(region, AutoBorrowMutability::Not)),
 114                         target: self.tcx.mk_ref(
 115                             region,
 116                             ty::TypeAndMut { mutbl: hir::Mutability::Not, ty: adjusted_ty },
 117                         ),
 118                     });
 119                 } else {
 120                     span_bug!(expr.span, "input to index is not a ref?");
 121                 }
 122                 if unsize {
 123                     adjustments.push(Adjustment {
 124                         kind: Adjust::Pointer(PointerCast::Unsize),
 125                         target: method.sig.inputs()[0],
 126                     });
 127                 }
 128                 self.apply_adjustments(base_expr, adjustments);
 129
 130                 self.write_method_call(expr.hir_id, method);
 131
 132                 return Some((input_ty, self.make_overloaded_place_return_type(method).ty));
 133             }
 134         }
 135
 136         None
 137     }
 138
 139     /// Try to resolve an overloaded place op. We only deal with the immutable
 140     /// variant here (Deref/Index). In some contexts we would need the mutable
 141     /// variant (DerefMut/IndexMut); those would be later converted by
 142     /// `convert_place_derefs_to_mutable`.
 143     pub(super) fn try_overloaded_place_op(
 144         &self,
 145         span: Span,
 146         base_ty: Ty<'tcx>,
 147         arg_tys: &[Ty<'tcx>],
 148         op: PlaceOp,
 149     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 150         debug!("try_overloaded_place_op({:?},{:?},{:?})", span, base_ty, op);
 151
 152         let (imm_tr, imm_op) = match op {
 153             PlaceOp::Deref => (self.tcx.lang_items().deref_trait(), sym::deref),
 154             PlaceOp::Index => (self.tcx.lang_items().index_trait(), sym::index),
 155         };
 156         imm_tr.and_then(|trait_did| {
 157             self.lookup_method_in_trait(
 158                 span,
 159                 Ident::with_dummy_span(imm_op),
 160                 trait_did,
 161                 base_ty,
 162                 Some(arg_tys),
 163             )
 164         })
 165     }
 166
 167     fn try_mutable_overloaded_place_op(
 168         &self,
 169         span: Span,
 170         base_ty: Ty<'tcx>,
 171         arg_tys: &[Ty<'tcx>],
 172         op: PlaceOp,
 173     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 174         debug!("try_mutable_overloaded_place_op({:?},{:?},{:?})", span, base_ty, op);
 175
 176         let (mut_tr, mut_op) = match op {
 177             PlaceOp::Deref => (self.tcx.lang_items().deref_mut_trait(), sym::deref_mut),
 178             PlaceOp::Index => (self.tcx.lang_items().index_mut_trait(), sym::index_mut),
 179         };
 180         mut_tr.and_then(|trait_did| {
 181             self.lookup_method_in_trait(
 182                 span,
 183                 Ident::with_dummy_span(mut_op),
 184                 trait_did,
 185                 base_ty,
 186                 Some(arg_tys),
 187             )
 188         })
 189     }
 190
 191     /// Convert auto-derefs, indices, etc of an expression from `Deref` and `Index`
 192     /// into `DerefMut` and `IndexMut` respectively.
 193     ///
 194     /// This is a second pass of typechecking derefs/indices. We need this because we do not
 195     /// always know whether a place needs to be mutable or not in the first pass.
 196     /// This happens whether there is an implicit mutable reborrow, e.g. when the type
 197     /// is used as the receiver of a method call.
 198     pub fn convert_place_derefs_to_mutable(&self, expr: &hir::Expr<'_>) {
 199         // Gather up expressions we want to munge.
 200         let mut exprs = vec![expr];
 201
 202         while let hir::ExprKind::Field(ref expr, _)
 203         | hir::ExprKind::Index(ref expr, _)
 204         | hir::ExprKind::Unary(hir::UnOp::Deref, ref expr) = exprs.last().unwrap().kind
 205         {
 206             exprs.push(&expr);
 207         }
 208
 209         debug!("convert_place_derefs_to_mutable: exprs={:?}", exprs);
 210
 211         // Fix up autoderefs and derefs.
 212         let mut inside_union = false;
 213         for (i, &expr) in exprs.iter().rev().enumerate() {
 214             debug!("convert_place_derefs_to_mutable: i={} expr={:?}", i, expr);
 215
 216             let mut source = self.node_ty(expr.hir_id);
 217             if matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Deref, _)) {
 218                 // Clear previous flag; after a pointer indirection it does not apply any more.
 219                 inside_union = false;
 220             }
 221             if source.ty_adt_def().map_or(false, |adt| adt.is_union()) {
 222                 inside_union = true;
 223             }
 224             // Fix up the autoderefs. Autorefs can only occur immediately preceding
 225             // overloaded place ops, and will be fixed by them in order to get
 226             // the correct region.
 227             // Do not mutate adjustments in place, but rather take them,
 228             // and replace them after mutating them, to avoid having the
 229             // typeck results borrowed during (`deref_mut`) method resolution.
 230             let previous_adjustments =
 231                 self.typeck_results.borrow_mut().adjustments_mut().remove(expr.hir_id);
 232             if let Some(mut adjustments) = previous_adjustments {
 233                 for adjustment in &mut adjustments {
 234                     if let Adjust::Deref(Some(ref mut deref)) = adjustment.kind {
 235                         if let Some(ok) = self.try_mutable_overloaded_place_op(
 236                             expr.span,
 237                             source,
 238                             &[],
 239                             PlaceOp::Deref,
 240                         ) {
 241                             let method = self.register_infer_ok_obligations(ok);
 242                             if let ty::Ref(region, _, mutbl) = *method.sig.output().kind() {
 243                                 *deref = OverloadedDeref { region, mutbl, span: deref.span };
 244                             }
 245                             // If this is a union field, also throw an error for `DerefMut` of `ManuallyDrop` (see RFC 2514).
 246                             // This helps avoid accidental drops.
 247                             if inside_union
 248                                 && source.ty_adt_def().map_or(false, |adt| adt.is_manually_drop())
 249                             {
 250                                 let mut err = self.tcx.sess.struct_span_err(
 251                                     expr.span,
 252                                     "not automatically applying `DerefMut` on `ManuallyDrop` union field",
 253                                 );
 254                                 err.help(
 255                                     "writing to this reference calls the destructor for the old value",
 256                                 );
 257                                 err.help("add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor");
 258                                 err.emit();
 259                             }
 260                         }
 261                     }
 262                     source = adjustment.target;
 263                 }
 264                 self.typeck_results.borrow_mut().adjustments_mut().insert(expr.hir_id, adjustments);
 265             }
 266
 267             match expr.kind {
 268                 hir::ExprKind::Index(ref base_expr, ..) => {
 269                     self.convert_place_op_to_mutable(PlaceOp::Index, expr, base_expr);
 270                 }
 271                 hir::ExprKind::Unary(hir::UnOp::Deref, ref base_expr) => {
 272                     self.convert_place_op_to_mutable(PlaceOp::Deref, expr, base_expr);
 273                 }
 274                 _ => {}
 275             }
 276         }
 277     }
 278
 279     fn convert_place_op_to_mutable(
 280         &self,
 281         op: PlaceOp,
 282         expr: &hir::Expr<'_>,
 283         base_expr: &hir::Expr<'_>,
 284     ) {
 285         debug!("convert_place_op_to_mutable({:?}, {:?}, {:?})", op, expr, base_expr);
 286         if !self.typeck_results.borrow().is_method_call(expr) {
 287             debug!("convert_place_op_to_mutable - builtin, nothing to do");
 288             return;
 289         }
 290
 291         // Need to deref because overloaded place ops take self by-reference.
 292         let base_ty = self
 293             .typeck_results
 294             .borrow()
 295             .expr_ty_adjusted(base_expr)
 296             .builtin_deref(false)
 297             .expect("place op takes something that is not a ref")
 298             .ty;
 299
 300         let arg_ty = match op {
 301             PlaceOp::Deref => None,
 302             PlaceOp::Index => {
 303                 // We would need to recover the `T` used when we resolve `<_ as Index<T>>::index`
 304                 // in try_index_step. This is the subst at index 1.
 305                 //
 306                 // Note: we should *not* use `expr_ty` of index_expr here because autoderef
 307                 // during coercions can cause type of index_expr to differ from `T` (#72002).
 308                 // We also could not use `expr_ty_adjusted` of index_expr because reborrowing
 309                 // during coercions can also cause type of index_expr to differ from `T`,
 310                 // which can potentially cause regionck failure (#74933).
 311                 Some(self.typeck_results.borrow().node_substs(expr.hir_id).type_at(1))
 312             }
 313         };
 314         let arg_tys = match arg_ty {
 315             None => &[],
 316             Some(ref ty) => slice::from_ref(ty),
 317         };
 318
 319         let method = self.try_mutable_overloaded_place_op(expr.span, base_ty, arg_tys, op);
 320         let method = match method {
 321             Some(ok) => self.register_infer_ok_obligations(ok),
 322             // Couldn't find the mutable variant of the place op, keep the
 323             // current, immutable version.
 324             None => return,
 325         };
 326         debug!("convert_place_op_to_mutable: method={:?}", method);
 327         self.write_method_call(expr.hir_id, method);
 328
 329         let region = if let ty::Ref(r, _, hir::Mutability::Mut) = method.sig.inputs()[0].kind() {
 330             r
 331         } else {
 332             span_bug!(expr.span, "input to mutable place op is not a mut ref?");
 333         };
 334
 335         // Convert the autoref in the base expr to mutable with the correct
 336         // region and mutability.
 337         let base_expr_ty = self.node_ty(base_expr.hir_id);
 338         if let Some(adjustments) =
 339             self.typeck_results.borrow_mut().adjustments_mut().get_mut(base_expr.hir_id)
 340         {
 341             let mut source = base_expr_ty;
 342             for adjustment in &mut adjustments[..] {
 343                 if let Adjust::Borrow(AutoBorrow::Ref(..)) = adjustment.kind {
 344                     debug!("convert_place_op_to_mutable: converting autoref {:?}", adjustment);
 345                     let mutbl = AutoBorrowMutability::Mut {
 346                         // Deref/indexing can be desugared to a method call,
 347                         // so maybe we could use two-phase here.
 348                         // See the documentation of AllowTwoPhase for why that's
 349                         // not the case today.
 350                         allow_two_phase_borrow: AllowTwoPhase::No,
 351                     };
 352                     adjustment.kind = Adjust::Borrow(AutoBorrow::Ref(region, mutbl));
 353                     adjustment.target =
 354                         self.tcx.mk_ref(region, ty::TypeAndMut { ty: source, mutbl: mutbl.into() });
 355                 }
 356                 source = adjustment.target;
 357             }
 358
 359             // If we have an autoref followed by unsizing at the end, fix the unsize target.
 360             if let [.., Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(..)), .. }, Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), ref mut target }] =
 361                 adjustments[..]
 362             {
 363                 *target = method.sig.inputs()[0];
 364             }
 365         }
 366     }
 367 }