src/librustc_typeck/expr_use_visitor.rs

   1 //! A different sort of visitor for walking fn bodies. Unlike the
   2 //! normal visitor, which just walks the entire body in one shot, the
   3 //! `ExprUseVisitor` determines how expressions are being used.
   4
   5 pub use self::ConsumeMode::*;
   6 use self::OverloadedCallType::*;
   7
   8 // Export these here so that Clippy can use them.
   9 pub use mc::{Place, PlaceBase, Projection};
  10
  11 use rustc::infer::InferCtxt;
  12 use rustc::ty::{self, adjustment, TyCtxt};
  13 use rustc_hir as hir;
  14 use rustc_hir::def::Res;
  15 use rustc_hir::def_id::DefId;
  16 use rustc_hir::PatKind;
  17
  18 use crate::mem_categorization as mc;
  19 use rustc_span::Span;
  20
  21 ///////////////////////////////////////////////////////////////////////////
  22 // The Delegate trait
  23
  24 /// This trait defines the callbacks you can expect to receive when
  25 /// employing the ExprUseVisitor.
  26 pub trait Delegate<'tcx> {
  27     // The value found at `place` is either copied or moved, depending
  28     // on mode.
  29     fn consume(&mut self, place: &mc::Place<'tcx>, mode: ConsumeMode);
  30
  31     // The value found at `place` is being borrowed with kind `bk`.
  32     fn borrow(&mut self, place: &mc::Place<'tcx>, bk: ty::BorrowKind);
  33
  34     // The path at `place` is being assigned to.
  35     fn mutate(&mut self, assignee_place: &mc::Place<'tcx>);
  36 }
  37
  38 #[derive(Copy, Clone, PartialEq, Debug)]
  39 pub enum ConsumeMode {
  40     Copy, // reference to x where x has a type that copies
  41     Move, // reference to x where x has a type that moves
  42 }
  43
  44 #[derive(Copy, Clone, PartialEq, Debug)]
  45 pub enum MutateMode {
  46     Init,
  47     JustWrite,    // x = y
  48     WriteAndRead, // x += y
  49 }
  50
  51 #[derive(Copy, Clone)]
  52 enum OverloadedCallType {
  53     FnOverloadedCall,
  54     FnMutOverloadedCall,
  55     FnOnceOverloadedCall,
  56 }
  57
  58 impl OverloadedCallType {
  59     fn from_trait_id(tcx: TyCtxt<'_>, trait_id: DefId) -> OverloadedCallType {
  60         for &(maybe_function_trait, overloaded_call_type) in &[
  61             (tcx.lang_items().fn_once_trait(), FnOnceOverloadedCall),
  62             (tcx.lang_items().fn_mut_trait(), FnMutOverloadedCall),
  63             (tcx.lang_items().fn_trait(), FnOverloadedCall),
  64         ] {
  65             match maybe_function_trait {
  66                 Some(function_trait) if function_trait == trait_id => return overloaded_call_type,
  67                 _ => continue,
  68             }
  69         }
  70
  71         bug!("overloaded call didn't map to known function trait")
  72     }
  73
  74     fn from_method_id(tcx: TyCtxt<'_>, method_id: DefId) -> OverloadedCallType {
  75         let method = tcx.associated_item(method_id);
  76         OverloadedCallType::from_trait_id(tcx, method.container.id())
  77     }
  78 }
  79
  80 ///////////////////////////////////////////////////////////////////////////
  81 // The ExprUseVisitor type
  82 //
  83 // This is the code that actually walks the tree.
  84 pub struct ExprUseVisitor<'a, 'tcx> {
  85     mc: mc::MemCategorizationContext<'a, 'tcx>,
  86     delegate: &'a mut dyn Delegate<'tcx>,
  87 }
  88
  89 // If the MC results in an error, it's because the type check
  90 // failed (or will fail, when the error is uncovered and reported
  91 // during writeback). In this case, we just ignore this part of the
  92 // code.
  93 //
  94 // Note that this macro appears similar to try!(), but, unlike try!(),
  95 // it does not propagate the error.
  96 macro_rules! return_if_err {
  97     ($inp: expr) => {
  98         match $inp {
  99             Ok(v) => v,
 100             Err(()) => {
 101                 debug!("mc reported err");
 102                 return;
 103             }
 104         }
 105     };
 106 }
 107
 108 impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
 109     /// Creates the ExprUseVisitor, configuring it with the various options provided:
 110     ///
 111     /// - `delegate` -- who receives the callbacks
 112     /// - `param_env` --- parameter environment for trait lookups (esp. pertaining to `Copy`)
 113     /// - `tables` --- typeck results for the code being analyzed
 114     pub fn new(
 115         delegate: &'a mut (dyn Delegate<'tcx> + 'a),
 116         infcx: &'a InferCtxt<'a, 'tcx>,
 117         body_owner: DefId,
 118         param_env: ty::ParamEnv<'tcx>,
 119         tables: &'a ty::TypeckTables<'tcx>,
 120     ) -> Self {
 121         ExprUseVisitor {
 122             mc: mc::MemCategorizationContext::new(infcx, param_env, body_owner, tables),
 123             delegate,
 124         }
 125     }
 126
 127     pub fn consume_body(&mut self, body: &hir::Body<'_>) {
 128         debug!("consume_body(body={:?})", body);
 129
 130         for param in body.params {
 131             let param_ty = return_if_err!(self.mc.pat_ty_adjusted(&param.pat));
 132             debug!("consume_body: param_ty = {:?}", param_ty);
 133
 134             let param_place = self.mc.cat_rvalue(param.hir_id, param.pat.span, param_ty);
 135
 136             self.walk_irrefutable_pat(&param_place, &param.pat);
 137         }
 138
 139         self.consume_expr(&body.value);
 140     }
 141
 142     fn tcx(&self) -> TyCtxt<'tcx> {
 143         self.mc.tcx()
 144     }
 145
 146     fn delegate_consume(&mut self, place: &Place<'tcx>) {
 147         debug!("delegate_consume(place={:?})", place);
 148
 149         let mode = copy_or_move(&self.mc, place);
 150         self.delegate.consume(place, mode);
 151     }
 152
 153     fn consume_exprs(&mut self, exprs: &[hir::Expr<'_>]) {
 154         for expr in exprs {
 155             self.consume_expr(&expr);
 156         }
 157     }
 158
 159     pub fn consume_expr(&mut self, expr: &hir::Expr<'_>) {
 160         debug!("consume_expr(expr={:?})", expr);
 161
 162         let place = return_if_err!(self.mc.cat_expr(expr));
 163         self.delegate_consume(&place);
 164         self.walk_expr(expr);
 165     }
 166
 167     fn mutate_expr(&mut self, expr: &hir::Expr<'_>) {
 168         let place = return_if_err!(self.mc.cat_expr(expr));
 169         self.delegate.mutate(&place);
 170         self.walk_expr(expr);
 171     }
 172
 173     fn borrow_expr(&mut self, expr: &hir::Expr<'_>, bk: ty::BorrowKind) {
 174         debug!("borrow_expr(expr={:?}, bk={:?})", expr, bk);
 175
 176         let place = return_if_err!(self.mc.cat_expr(expr));
 177         self.delegate.borrow(&place, bk);
 178
 179         self.walk_expr(expr)
 180     }
 181
 182     fn select_from_expr(&mut self, expr: &hir::Expr<'_>) {
 183         self.walk_expr(expr)
 184     }
 185
 186     pub fn walk_expr(&mut self, expr: &hir::Expr<'_>) {
 187         debug!("walk_expr(expr={:?})", expr);
 188
 189         self.walk_adjustment(expr);
 190
 191         match expr.kind {
 192             hir::ExprKind::Path(_) => {}
 193
 194             hir::ExprKind::Type(ref subexpr, _) => self.walk_expr(subexpr),
 195
 196             hir::ExprKind::Unary(hir::UnOp::UnDeref, ref base) => {
 197                 // *base
 198                 self.select_from_expr(base);
 199             }
 200
 201             hir::ExprKind::Field(ref base, _) => {
 202                 // base.f
 203                 self.select_from_expr(base);
 204             }
 205
 206             hir::ExprKind::Index(ref lhs, ref rhs) => {
 207                 // lhs[rhs]
 208                 self.select_from_expr(lhs);
 209                 self.consume_expr(rhs);
 210             }
 211
 212             hir::ExprKind::Call(ref callee, ref args) => {
 213                 // callee(args)
 214                 self.walk_callee(expr, callee);
 215                 self.consume_exprs(args);
 216             }
 217
 218             hir::ExprKind::MethodCall(.., ref args) => {
 219                 // callee.m(args)
 220                 self.consume_exprs(args);
 221             }
 222
 223             hir::ExprKind::Struct(_, ref fields, ref opt_with) => {
 224                 self.walk_struct_expr(fields, opt_with);
 225             }
 226
 227             hir::ExprKind::Tup(ref exprs) => {
 228                 self.consume_exprs(exprs);
 229             }
 230
 231             hir::ExprKind::Match(ref discr, arms, _) => {
 232                 let discr_place = return_if_err!(self.mc.cat_expr(&discr));
 233                 self.borrow_expr(&discr, ty::ImmBorrow);
 234
 235                 // treatment of the discriminant is handled while walking the arms.
 236                 for arm in arms {
 237                     self.walk_arm(&discr_place, arm);
 238                 }
 239             }
 240
 241             hir::ExprKind::Array(ref exprs) => {
 242                 self.consume_exprs(exprs);
 243             }
 244
 245             hir::ExprKind::AddrOf(_, m, ref base) => {
 246                 // &base
 247                 // make sure that the thing we are pointing out stays valid
 248                 // for the lifetime `scope_r` of the resulting ptr:
 249                 let bk = ty::BorrowKind::from_mutbl(m);
 250                 self.borrow_expr(&base, bk);
 251             }
 252
 253             hir::ExprKind::InlineAsm(ref ia) => {
 254                 for (o, output) in ia.inner.outputs.iter().zip(ia.outputs_exprs) {
 255                     if o.is_indirect {
 256                         self.consume_expr(output);
 257                     } else {
 258                         self.mutate_expr(output);
 259                     }
 260                 }
 261                 self.consume_exprs(&ia.inputs_exprs);
 262             }
 263
 264             hir::ExprKind::Continue(..) | hir::ExprKind::Lit(..) | hir::ExprKind::Err => {}
 265
 266             hir::ExprKind::Loop(ref blk, _, _) => {
 267                 self.walk_block(blk);
 268             }
 269
 270             hir::ExprKind::Unary(_, ref lhs) => {
 271                 self.consume_expr(lhs);
 272             }
 273
 274             hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
 275                 self.consume_expr(lhs);
 276                 self.consume_expr(rhs);
 277             }
 278
 279             hir::ExprKind::Block(ref blk, _) => {
 280                 self.walk_block(blk);
 281             }
 282
 283             hir::ExprKind::Break(_, ref opt_expr) | hir::ExprKind::Ret(ref opt_expr) => {
 284                 if let Some(ref expr) = *opt_expr {
 285                     self.consume_expr(expr);
 286                 }
 287             }
 288
 289             hir::ExprKind::Assign(ref lhs, ref rhs, _) => {
 290                 self.mutate_expr(lhs);
 291                 self.consume_expr(rhs);
 292             }
 293
 294             hir::ExprKind::Cast(ref base, _) => {
 295                 self.consume_expr(base);
 296             }
 297
 298             hir::ExprKind::DropTemps(ref expr) => {
 299                 self.consume_expr(expr);
 300             }
 301
 302             hir::ExprKind::AssignOp(_, ref lhs, ref rhs) => {
 303                 if self.mc.tables.is_method_call(expr) {
 304                     self.consume_expr(lhs);
 305                 } else {
 306                     self.mutate_expr(lhs);
 307                 }
 308                 self.consume_expr(rhs);
 309             }
 310
 311             hir::ExprKind::Repeat(ref base, _) => {
 312                 self.consume_expr(base);
 313             }
 314
 315             hir::ExprKind::Closure(_, _, _, fn_decl_span, _) => {
 316                 self.walk_captures(expr, fn_decl_span);
 317             }
 318
 319             hir::ExprKind::Box(ref base) => {
 320                 self.consume_expr(base);
 321             }
 322
 323             hir::ExprKind::Yield(ref value, _) => {
 324                 self.consume_expr(value);
 325             }
 326         }
 327     }
 328
 329     fn walk_callee(&mut self, call: &hir::Expr<'_>, callee: &hir::Expr<'_>) {
 330         let callee_ty = return_if_err!(self.mc.expr_ty_adjusted(callee));
 331         debug!("walk_callee: callee={:?} callee_ty={:?}", callee, callee_ty);
 332         match callee_ty.kind {
 333             ty::FnDef(..) | ty::FnPtr(_) => {
 334                 self.consume_expr(callee);
 335             }
 336             ty::Error => {}
 337             _ => {
 338                 if let Some(def_id) = self.mc.tables.type_dependent_def_id(call.hir_id) {
 339                     match OverloadedCallType::from_method_id(self.tcx(), def_id) {
 340                         FnMutOverloadedCall => {
 341                             self.borrow_expr(callee, ty::MutBorrow);
 342                         }
 343                         FnOverloadedCall => {
 344                             self.borrow_expr(callee, ty::ImmBorrow);
 345                         }
 346                         FnOnceOverloadedCall => self.consume_expr(callee),
 347                     }
 348                 } else {
 349                     self.tcx()
 350                         .sess
 351                         .delay_span_bug(call.span, "no type-dependent def for overloaded call");
 352                 }
 353             }
 354         }
 355     }
 356
 357     fn walk_stmt(&mut self, stmt: &hir::Stmt<'_>) {
 358         match stmt.kind {
 359             hir::StmtKind::Local(ref local) => {
 360                 self.walk_local(&local);
 361             }
 362
 363             hir::StmtKind::Item(_) => {
 364                 // We don't visit nested items in this visitor,
 365                 // only the fn body we were given.
 366             }
 367
 368             hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
 369                 self.consume_expr(&expr);
 370             }
 371         }
 372     }
 373
 374     fn walk_local(&mut self, local: &hir::Local<'_>) {
 375         if let Some(ref expr) = local.init {
 376             // Variable declarations with
 377             // initializers are considered
 378             // "assigns", which is handled by
 379             // `walk_pat`:
 380             self.walk_expr(&expr);
 381             let init_place = return_if_err!(self.mc.cat_expr(&expr));
 382             self.walk_irrefutable_pat(&init_place, &local.pat);
 383         }
 384     }
 385
 386     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 387     /// depending on its type.
 388     fn walk_block(&mut self, blk: &hir::Block<'_>) {
 389         debug!("walk_block(blk.hir_id={})", blk.hir_id);
 390
 391         for stmt in blk.stmts {
 392             self.walk_stmt(stmt);
 393         }
 394
 395         if let Some(ref tail_expr) = blk.expr {
 396             self.consume_expr(&tail_expr);
 397         }
 398     }
 399
 400     fn walk_struct_expr(
 401         &mut self,
 402         fields: &[hir::Field<'_>],
 403         opt_with: &Option<&'hir hir::Expr<'_>>,
 404     ) {
 405         // Consume the expressions supplying values for each field.
 406         for field in fields {
 407             self.consume_expr(&field.expr);
 408         }
 409
 410         let with_expr = match *opt_with {
 411             Some(ref w) => &**w,
 412             None => {
 413                 return;
 414             }
 415         };
 416
 417         let with_place = return_if_err!(self.mc.cat_expr(&with_expr));
 418
 419         // Select just those fields of the `with`
 420         // expression that will actually be used
 421         match with_place.ty.kind {
 422             ty::Adt(adt, substs) if adt.is_struct() => {
 423                 // Consume those fields of the with expression that are needed.
 424                 for (f_index, with_field) in adt.non_enum_variant().fields.iter().enumerate() {
 425                     let is_mentioned = fields
 426                         .iter()
 427                         .any(|f| self.tcx().field_index(f.hir_id, self.mc.tables) == f_index);
 428                     if !is_mentioned {
 429                         let field_place = self.mc.cat_projection(
 430                             &*with_expr,
 431                             with_place.clone(),
 432                             with_field.ty(self.tcx(), substs),
 433                         );
 434                         self.delegate_consume(&field_place);
 435                     }
 436                 }
 437             }
 438             _ => {
 439                 // the base expression should always evaluate to a
 440                 // struct; however, when EUV is run during typeck, it
 441                 // may not. This will generate an error earlier in typeck,
 442                 // so we can just ignore it.
 443                 if !self.tcx().sess.has_errors() {
 444                     span_bug!(with_expr.span, "with expression doesn't evaluate to a struct");
 445                 }
 446             }
 447         }
 448
 449         // walk the with expression so that complex expressions
 450         // are properly handled.
 451         self.walk_expr(with_expr);
 452     }
 453
 454     // Invoke the appropriate delegate calls for anything that gets
 455     // consumed or borrowed as part of the automatic adjustment
 456     // process.
 457     fn walk_adjustment(&mut self, expr: &hir::Expr<'_>) {
 458         let adjustments = self.mc.tables.expr_adjustments(expr);
 459         let mut place = return_if_err!(self.mc.cat_expr_unadjusted(expr));
 460         for adjustment in adjustments {
 461             debug!("walk_adjustment expr={:?} adj={:?}", expr, adjustment);
 462             match adjustment.kind {
 463                 adjustment::Adjust::NeverToAny | adjustment::Adjust::Pointer(_) => {
 464                     // Creating a closure/fn-pointer or unsizing consumes
 465                     // the input and stores it into the resulting rvalue.
 466                     self.delegate_consume(&place);
 467                 }
 468
 469                 adjustment::Adjust::Deref(None) => {}
 470
 471                 // Autoderefs for overloaded Deref calls in fact reference
 472                 // their receiver. That is, if we have `(*x)` where `x`
 473                 // is of type `Rc<T>`, then this in fact is equivalent to
 474                 // `x.deref()`. Since `deref()` is declared with `&self`,
 475                 // this is an autoref of `x`.
 476                 adjustment::Adjust::Deref(Some(ref deref)) => {
 477                     let bk = ty::BorrowKind::from_mutbl(deref.mutbl);
 478                     self.delegate.borrow(&place, bk);
 479                 }
 480
 481                 adjustment::Adjust::Borrow(ref autoref) => {
 482                     self.walk_autoref(expr, &place, autoref);
 483                 }
 484             }
 485             place = return_if_err!(self.mc.cat_expr_adjusted(expr, place, &adjustment));
 486         }
 487     }
 488
 489     /// Walks the autoref `autoref` applied to the autoderef'd
 490     /// `expr`. `base_place` is the mem-categorized form of `expr`
 491     /// after all relevant autoderefs have occurred.
 492     fn walk_autoref(
 493         &mut self,
 494         expr: &hir::Expr<'_>,
 495         base_place: &mc::Place<'tcx>,
 496         autoref: &adjustment::AutoBorrow<'tcx>,
 497     ) {
 498         debug!(
 499             "walk_autoref(expr.hir_id={} base_place={:?} autoref={:?})",
 500             expr.hir_id, base_place, autoref
 501         );
 502
 503         match *autoref {
 504             adjustment::AutoBorrow::Ref(_, m) => {
 505                 self.delegate.borrow(base_place, ty::BorrowKind::from_mutbl(m.into()));
 506             }
 507
 508             adjustment::AutoBorrow::RawPtr(m) => {
 509                 debug!("walk_autoref: expr.hir_id={} base_place={:?}", expr.hir_id, base_place);
 510
 511                 self.delegate.borrow(base_place, ty::BorrowKind::from_mutbl(m));
 512             }
 513         }
 514     }
 515
 516     fn walk_arm(&mut self, discr_place: &Place<'tcx>, arm: &hir::Arm<'_>) {
 517         self.walk_pat(discr_place, &arm.pat);
 518
 519         if let Some(hir::Guard::If(ref e)) = arm.guard {
 520             self.consume_expr(e)
 521         }
 522
 523         self.consume_expr(&arm.body);
 524     }
 525
 526     /// Walks a pat that occurs in isolation (i.e., top-level of fn argument or
 527     /// let binding, and *not* a match arm or nested pat.)
 528     fn walk_irrefutable_pat(&mut self, discr_place: &Place<'tcx>, pat: &hir::Pat<'_>) {
 529         self.walk_pat(discr_place, pat);
 530     }
 531
 532     /// The core driver for walking a pattern
 533     fn walk_pat(&mut self, discr_place: &Place<'tcx>, pat: &hir::Pat<'_>) {
 534         debug!("walk_pat(discr_place={:?}, pat={:?})", discr_place, pat);
 535
 536         let tcx = self.tcx();
 537         let ExprUseVisitor { ref mc, ref mut delegate } = *self;
 538         return_if_err!(mc.cat_pattern(discr_place.clone(), pat, |place, pat| {
 539             if let PatKind::Binding(_, canonical_id, ..) = pat.kind {
 540                 debug!("walk_pat: binding place={:?} pat={:?}", place, pat,);
 541                 if let Some(bm) = mc.tables.extract_binding_mode(tcx.sess, pat.hir_id, pat.span) {
 542                     debug!("walk_pat: pat.hir_id={:?} bm={:?}", pat.hir_id, bm);
 543
 544                     // pat_ty: the type of the binding being produced.
 545                     let pat_ty = return_if_err!(mc.node_ty(pat.hir_id));
 546                     debug!("walk_pat: pat_ty={:?}", pat_ty);
 547
 548                     // Each match binding is effectively an assignment to the
 549                     // binding being produced.
 550                     let def = Res::Local(canonical_id);
 551                     if let Ok(ref binding_place) = mc.cat_res(pat.hir_id, pat.span, pat_ty, def) {
 552                         delegate.mutate(binding_place);
 553                     }
 554
 555                     // It is also a borrow or copy/move of the value being matched.
 556                     match bm {
 557                         ty::BindByReference(m) => {
 558                             let bk = ty::BorrowKind::from_mutbl(m);
 559                             delegate.borrow(place, bk);
 560                         }
 561                         ty::BindByValue(..) => {
 562                             let mode = copy_or_move(mc, place);
 563                             debug!("walk_pat binding consuming pat");
 564                             delegate.consume(place, mode);
 565                         }
 566                     }
 567                 }
 568             }
 569         }));
 570     }
 571
 572     fn walk_captures(&mut self, closure_expr: &hir::Expr<'_>, fn_decl_span: Span) {
 573         debug!("walk_captures({:?})", closure_expr);
 574
 575         let closure_def_id = self.tcx().hir().local_def_id(closure_expr.hir_id);
 576         if let Some(upvars) = self.tcx().upvars(closure_def_id) {
 577             for &var_id in upvars.keys() {
 578                 let upvar_id = ty::UpvarId {
 579                     var_path: ty::UpvarPath { hir_id: var_id },
 580                     closure_expr_id: closure_def_id.to_local(),
 581                 };
 582                 let upvar_capture = self.mc.tables.upvar_capture(upvar_id);
 583                 let captured_place = return_if_err!(self.cat_captured_var(
 584                     closure_expr.hir_id,
 585                     fn_decl_span,
 586                     var_id,
 587                 ));
 588                 match upvar_capture {
 589                     ty::UpvarCapture::ByValue => {
 590                         let mode = copy_or_move(&self.mc, &captured_place);
 591                         self.delegate.consume(&captured_place, mode);
 592                     }
 593                     ty::UpvarCapture::ByRef(upvar_borrow) => {
 594                         self.delegate.borrow(&captured_place, upvar_borrow.kind);
 595                     }
 596                 }
 597             }
 598         }
 599     }
 600
 601     fn cat_captured_var(
 602         &mut self,
 603         closure_hir_id: hir::HirId,
 604         closure_span: Span,
 605         var_id: hir::HirId,
 606     ) -> mc::McResult<mc::Place<'tcx>> {
 607         // Create the place for the variable being borrowed, from the
 608         // perspective of the creator (parent) of the closure.
 609         let var_ty = self.mc.node_ty(var_id)?;
 610         self.mc.cat_res(closure_hir_id, closure_span, var_ty, Res::Local(var_id))
 611     }
 612 }
 613
 614 fn copy_or_move<'a, 'tcx>(
 615     mc: &mc::MemCategorizationContext<'a, 'tcx>,
 616     place: &Place<'tcx>,
 617 ) -> ConsumeMode {
 618     if !mc.type_is_copy_modulo_regions(place.ty, place.span) { Move } else { Copy }
 619 }