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::{PlaceBase, Place, Projection};
  10
  11 use rustc::hir::{self, PatKind};
  12 use rustc::hir::def::Res;
  13 use rustc::hir::def_id::DefId;
  14 use rustc::hir::ptr::P;
  15 use rustc::infer::InferCtxt;
  16 use rustc::ty::{self, TyCtxt, adjustment};
  17
  18 use crate::mem_categorization as mc;
  19 use syntax_pos::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 => {
  67                     return overloaded_call_type
  68                 }
  69                 _ => continue,
  70             }
  71         }
  72
  73         bug!("overloaded call didn't map to known function trait")
  74     }
  75
  76     fn from_method_id(tcx: TyCtxt<'_>, method_id: DefId) -> OverloadedCallType {
  77         let method = tcx.associated_item(method_id);
  78         OverloadedCallType::from_trait_id(tcx, method.container.id())
  79     }
  80 }
  81
  82 ///////////////////////////////////////////////////////////////////////////
  83 // The ExprUseVisitor type
  84 //
  85 // This is the code that actually walks the tree.
  86 pub struct ExprUseVisitor<'a, 'tcx> {
  87     mc: mc::MemCategorizationContext<'a, 'tcx>,
  88     delegate: &'a mut dyn Delegate<'tcx>,
  89 }
  90
  91 // If the MC results in an error, it's because the type check
  92 // failed (or will fail, when the error is uncovered and reported
  93 // during writeback). In this case, we just ignore this part of the
  94 // code.
  95 //
  96 // Note that this macro appears similar to try!(), but, unlike try!(),
  97 // it does not propagate the error.
  98 macro_rules! return_if_err {
  99     ($inp: expr) => (
 100         match $inp {
 101             Ok(v) => v,
 102             Err(()) => {
 103                 debug!("mc reported err");
 104                 return
 105             }
 106         }
 107     )
 108 }
 109
 110 impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
 111     /// Creates the ExprUseVisitor, configuring it with the various options provided:
 112     ///
 113     /// - `delegate` -- who receives the callbacks
 114     /// - `param_env` --- parameter environment for trait lookups (esp. pertaining to `Copy`)
 115     /// - `tables` --- typeck results for the code being analyzed
 116     pub fn new(
 117         delegate: &'a mut (dyn Delegate<'tcx> + 'a),
 118         infcx: &'a InferCtxt<'a, 'tcx>,
 119         body_owner: DefId,
 120         param_env: ty::ParamEnv<'tcx>,
 121         tables: &'a ty::TypeckTables<'tcx>,
 122     ) -> Self {
 123         ExprUseVisitor {
 124             mc: mc::MemCategorizationContext::new(
 125                 infcx,
 126                 param_env,
 127                 body_owner,
 128                 tables,
 129             ),
 130             delegate,
 131         }
 132     }
 133
 134     pub fn consume_body(&mut self, body: &hir::Body<'_>) {
 135         debug!("consume_body(body={:?})", body);
 136
 137         for param in body.params {
 138             let param_ty = return_if_err!(self.mc.pat_ty_adjusted(&param.pat));
 139             debug!("consume_body: param_ty = {:?}", param_ty);
 140
 141             let param_place = self.mc.cat_rvalue(param.hir_id, param.pat.span, param_ty);
 142
 143             self.walk_irrefutable_pat(&param_place, &param.pat);
 144         }
 145
 146         self.consume_expr(&body.value);
 147     }
 148
 149     fn tcx(&self) -> TyCtxt<'tcx> {
 150         self.mc.tcx()
 151     }
 152
 153     fn delegate_consume(&mut self, place: &Place<'tcx>) {
 154         debug!("delegate_consume(place={:?})", place);
 155
 156         let mode = copy_or_move(&self.mc, place);
 157         self.delegate.consume(place, mode);
 158     }
 159
 160     fn consume_exprs(&mut self, exprs: &[hir::Expr]) {
 161         for expr in exprs {
 162             self.consume_expr(&expr);
 163         }
 164     }
 165
 166     pub fn consume_expr(&mut self, expr: &hir::Expr) {
 167         debug!("consume_expr(expr={:?})", expr);
 168
 169         let place = return_if_err!(self.mc.cat_expr(expr));
 170         self.delegate_consume(&place);
 171         self.walk_expr(expr);
 172     }
 173
 174     fn mutate_expr(&mut self, expr: &hir::Expr) {
 175         let place = return_if_err!(self.mc.cat_expr(expr));
 176         self.delegate.mutate(&place);
 177         self.walk_expr(expr);
 178     }
 179
 180     fn borrow_expr(&mut self, expr: &hir::Expr, bk: ty::BorrowKind) {
 181         debug!("borrow_expr(expr={:?}, bk={:?})", expr, bk);
 182
 183         let place = return_if_err!(self.mc.cat_expr(expr));
 184         self.delegate.borrow(&place, bk);
 185
 186         self.walk_expr(expr)
 187     }
 188
 189     fn select_from_expr(&mut self, expr: &hir::Expr) {
 190         self.walk_expr(expr)
 191     }
 192
 193     pub fn walk_expr(&mut self, expr: &hir::Expr) {
 194         debug!("walk_expr(expr={:?})", expr);
 195
 196         self.walk_adjustment(expr);
 197
 198         match expr.kind {
 199             hir::ExprKind::Path(_) => { }
 200
 201             hir::ExprKind::Type(ref subexpr, _) => {
 202                 self.walk_expr(subexpr)
 203             }
 204
 205             hir::ExprKind::Unary(hir::UnDeref, ref base) => { // *base
 206                 self.select_from_expr(base);
 207             }
 208
 209             hir::ExprKind::Field(ref base, _) => { // base.f
 210                 self.select_from_expr(base);
 211             }
 212
 213             hir::ExprKind::Index(ref lhs, ref rhs) => { // lhs[rhs]
 214                 self.select_from_expr(lhs);
 215                 self.consume_expr(rhs);
 216             }
 217
 218             hir::ExprKind::Call(ref callee, ref args) => { // callee(args)
 219                 self.walk_callee(expr, callee);
 220                 self.consume_exprs(args);
 221             }
 222
 223             hir::ExprKind::MethodCall(.., ref args) => { // callee.m(args)
 224                 self.consume_exprs(args);
 225             }
 226
 227             hir::ExprKind::Struct(_, ref fields, ref opt_with) => {
 228                 self.walk_struct_expr(fields, opt_with);
 229             }
 230
 231             hir::ExprKind::Tup(ref exprs) => {
 232                 self.consume_exprs(exprs);
 233             }
 234
 235             hir::ExprKind::Match(ref discr, ref arms, _) => {
 236                 let discr_place = return_if_err!(self.mc.cat_expr(&discr));
 237                 self.borrow_expr(&discr, ty::ImmBorrow);
 238
 239                 // treatment of the discriminant is handled while walking the arms.
 240                 for arm in arms {
 241                     self.walk_arm(&discr_place, arm);
 242                 }
 243             }
 244
 245             hir::ExprKind::Array(ref exprs) => {
 246                 self.consume_exprs(exprs);
 247             }
 248
 249             hir::ExprKind::AddrOf(_, m, ref base) => {   // &base
 250                 // make sure that the thing we are pointing out stays valid
 251                 // for the lifetime `scope_r` of the resulting ptr:
 252                 let bk = ty::BorrowKind::from_mutbl(m);
 253                 self.borrow_expr(&base, bk);
 254             }
 255
 256             hir::ExprKind::InlineAsm(ref ia) => {
 257                 for (o, output) in ia.inner.outputs.iter().zip(&ia.outputs_exprs) {
 258                     if o.is_indirect {
 259                         self.consume_expr(output);
 260                     } else {
 261                         self.mutate_expr(output);
 262                     }
 263                 }
 264                 self.consume_exprs(&ia.inputs_exprs);
 265             }
 266
 267             hir::ExprKind::Continue(..) |
 268             hir::ExprKind::Lit(..) |
 269             hir::ExprKind::Err => {}
 270
 271             hir::ExprKind::Loop(ref blk, _, _) => {
 272                 self.walk_block(blk);
 273             }
 274
 275             hir::ExprKind::Unary(_, ref lhs) => {
 276                 self.consume_expr(lhs);
 277             }
 278
 279             hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
 280                 self.consume_expr(lhs);
 281                 self.consume_expr(rhs);
 282             }
 283
 284             hir::ExprKind::Block(ref blk, _) => {
 285                 self.walk_block(blk);
 286             }
 287
 288             hir::ExprKind::Break(_, ref opt_expr) | hir::ExprKind::Ret(ref opt_expr) => {
 289                 if let Some(ref expr) = *opt_expr {
 290                     self.consume_expr(expr);
 291                 }
 292             }
 293
 294             hir::ExprKind::Assign(ref lhs, ref rhs) => {
 295                 self.mutate_expr(lhs);
 296                 self.consume_expr(rhs);
 297             }
 298
 299             hir::ExprKind::Cast(ref base, _) => {
 300                 self.consume_expr(base);
 301             }
 302
 303             hir::ExprKind::DropTemps(ref expr) => {
 304                 self.consume_expr(expr);
 305             }
 306
 307             hir::ExprKind::AssignOp(_, ref lhs, ref rhs) => {
 308                 if self.mc.tables.is_method_call(expr) {
 309                     self.consume_expr(lhs);
 310                 } else {
 311                     self.mutate_expr(lhs);
 312                 }
 313                 self.consume_expr(rhs);
 314             }
 315
 316             hir::ExprKind::Repeat(ref base, _) => {
 317                 self.consume_expr(base);
 318             }
 319
 320             hir::ExprKind::Closure(_, _, _, fn_decl_span, _) => {
 321                 self.walk_captures(expr, fn_decl_span);
 322             }
 323
 324             hir::ExprKind::Box(ref base) => {
 325                 self.consume_expr(base);
 326             }
 327
 328             hir::ExprKind::Yield(ref value, _) => {
 329                 self.consume_expr(value);
 330             }
 331         }
 332     }
 333
 334     fn walk_callee(&mut self, call: &hir::Expr, callee: &hir::Expr) {
 335         let callee_ty = return_if_err!(self.mc.expr_ty_adjusted(callee));
 336         debug!("walk_callee: callee={:?} callee_ty={:?}",
 337                callee, callee_ty);
 338         match callee_ty.kind {
 339             ty::FnDef(..) | ty::FnPtr(_) => {
 340                 self.consume_expr(callee);
 341             }
 342             ty::Error => { }
 343             _ => {
 344                 if let Some(def_id) = self.mc.tables.type_dependent_def_id(call.hir_id) {
 345                     match OverloadedCallType::from_method_id(self.tcx(), def_id) {
 346                         FnMutOverloadedCall => {
 347                             self.borrow_expr(callee, ty::MutBorrow);
 348                         }
 349                         FnOverloadedCall => {
 350                             self.borrow_expr(callee, ty::ImmBorrow);
 351                         }
 352                         FnOnceOverloadedCall => self.consume_expr(callee),
 353                     }
 354                 } else {
 355                     self.tcx().sess.delay_span_bug(call.span,
 356                                                    "no type-dependent def for overloaded call");
 357                 }
 358             }
 359         }
 360     }
 361
 362     fn walk_stmt(&mut self, stmt: &hir::Stmt) {
 363         match stmt.kind {
 364             hir::StmtKind::Local(ref local) => {
 365                 self.walk_local(&local);
 366             }
 367
 368             hir::StmtKind::Item(_) => {
 369                 // We don't visit nested items in this visitor,
 370                 // only the fn body we were given.
 371             }
 372
 373             hir::StmtKind::Expr(ref expr) |
 374             hir::StmtKind::Semi(ref expr) => {
 375                 self.consume_expr(&expr);
 376             }
 377         }
 378     }
 379
 380     fn walk_local(&mut self, local: &hir::Local) {
 381         if let Some(ref expr) = local.init {
 382             // Variable declarations with
 383             // initializers are considered
 384             // "assigns", which is handled by
 385             // `walk_pat`:
 386             self.walk_expr(&expr);
 387             let init_place = return_if_err!(self.mc.cat_expr(&expr));
 388             self.walk_irrefutable_pat(&init_place, &local.pat);
 389         }
 390     }
 391
 392     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 393     /// depending on its type.
 394     fn walk_block(&mut self, blk: &hir::Block) {
 395         debug!("walk_block(blk.hir_id={})", blk.hir_id);
 396
 397         for stmt in &blk.stmts {
 398             self.walk_stmt(stmt);
 399         }
 400
 401         if let Some(ref tail_expr) = blk.expr {
 402             self.consume_expr(&tail_expr);
 403         }
 404     }
 405
 406     fn walk_struct_expr(&mut self,
 407                         fields: &[hir::Field],
 408                         opt_with: &Option<P<hir::Expr>>) {
 409         // Consume the expressions supplying values for each field.
 410         for field in fields {
 411             self.consume_expr(&field.expr);
 412         }
 413
 414         let with_expr = match *opt_with {
 415             Some(ref w) => &**w,
 416             None => { return; }
 417         };
 418
 419         let with_place = return_if_err!(self.mc.cat_expr(&with_expr));
 420
 421         // Select just those fields of the `with`
 422         // expression that will actually be used
 423         match with_place.ty.kind {
 424             ty::Adt(adt, substs) if adt.is_struct() => {
 425                 // Consume those fields of the with expression that are needed.
 426                 for (f_index, with_field) in adt.non_enum_variant().fields.iter().enumerate() {
 427                     let is_mentioned = fields.iter().any(|f| {
 428                         self.tcx().field_index(f.hir_id, self.mc.tables) == f_index
 429                     });
 430                     if !is_mentioned {
 431                         let field_place = self.mc.cat_projection(
 432                             &*with_expr,
 433                             with_place.clone(),
 434                             with_field.ty(self.tcx(), substs),
 435                         );
 436                         self.delegate_consume(&field_place);
 437                     }
 438                 }
 439             }
 440             _ => {
 441                 // the base expression should always evaluate to a
 442                 // struct; however, when EUV is run during typeck, it
 443                 // may not. This will generate an error earlier in typeck,
 444                 // so we can just ignore it.
 445                 if !self.tcx().sess.has_errors() {
 446                     span_bug!(
 447                         with_expr.span,
 448                         "with expression doesn't evaluate to a struct");
 449                 }
 450             }
 451         }
 452
 453         // walk the with expression so that complex expressions
 454         // are properly handled.
 455         self.walk_expr(with_expr);
 456     }
 457
 458     // Invoke the appropriate delegate calls for anything that gets
 459     // consumed or borrowed as part of the automatic adjustment
 460     // process.
 461     fn walk_adjustment(&mut self, expr: &hir::Expr) {
 462         let adjustments = self.mc.tables.expr_adjustments(expr);
 463         let mut place = return_if_err!(self.mc.cat_expr_unadjusted(expr));
 464         for adjustment in adjustments {
 465             debug!("walk_adjustment expr={:?} adj={:?}", expr, adjustment);
 466             match adjustment.kind {
 467                 adjustment::Adjust::NeverToAny |
 468                 adjustment::Adjust::Pointer(_)  => {
 469                     // Creating a closure/fn-pointer or unsizing consumes
 470                     // the input and stores it into the resulting rvalue.
 471                     self.delegate_consume(&place);
 472                 }
 473
 474                 adjustment::Adjust::Deref(None) => {}
 475
 476                 // Autoderefs for overloaded Deref calls in fact reference
 477                 // their receiver. That is, if we have `(*x)` where `x`
 478                 // is of type `Rc<T>`, then this in fact is equivalent to
 479                 // `x.deref()`. Since `deref()` is declared with `&self`,
 480                 // this is an autoref of `x`.
 481                 adjustment::Adjust::Deref(Some(ref deref)) => {
 482                     let bk = ty::BorrowKind::from_mutbl(deref.mutbl);
 483                     self.delegate.borrow(&place, bk);
 484                 }
 485
 486                 adjustment::Adjust::Borrow(ref autoref) => {
 487                     self.walk_autoref(expr, &place, autoref);
 488                 }
 489             }
 490             place = return_if_err!(self.mc.cat_expr_adjusted(expr, place, &adjustment));
 491         }
 492     }
 493
 494     /// Walks the autoref `autoref` applied to the autoderef'd
 495     /// `expr`. `base_place` is the mem-categorized form of `expr`
 496     /// after all relevant autoderefs have occurred.
 497     fn walk_autoref(&mut self,
 498                     expr: &hir::Expr,
 499                     base_place: &mc::Place<'tcx>,
 500                     autoref: &adjustment::AutoBorrow<'tcx>) {
 501         debug!("walk_autoref(expr.hir_id={} base_place={:?} autoref={:?})",
 502                expr.hir_id,
 503                base_place,
 504                autoref);
 505
 506         match *autoref {
 507             adjustment::AutoBorrow::Ref(_, m) => {
 508                 self.delegate.borrow(base_place, ty::BorrowKind::from_mutbl(m.into()));
 509             }
 510
 511             adjustment::AutoBorrow::RawPtr(m) => {
 512                 debug!("walk_autoref: expr.hir_id={} base_place={:?}",
 513                        expr.hir_id,
 514                        base_place);
 515
 516
 517                 self.delegate.borrow(base_place, ty::BorrowKind::from_mutbl(m));
 518             }
 519         }
 520     }
 521
 522     fn walk_arm(&mut self, discr_place: &Place<'tcx>, arm: &hir::Arm) {
 523         self.walk_pat(discr_place, &arm.pat);
 524
 525         if let Some(hir::Guard::If(ref e)) = arm.guard {
 526             self.consume_expr(e)
 527         }
 528
 529         self.consume_expr(&arm.body);
 530     }
 531
 532     /// Walks a pat that occurs in isolation (i.e., top-level of fn argument or
 533     /// let binding, and *not* a match arm or nested pat.)
 534     fn walk_irrefutable_pat(&mut self, discr_place: &Place<'tcx>, pat: &hir::Pat) {
 535         self.walk_pat(discr_place, pat);
 536     }
 537
 538
 539     /// The core driver for walking a pattern
 540     fn walk_pat(&mut self, discr_place: &Place<'tcx>, pat: &hir::Pat) {
 541         debug!("walk_pat(discr_place={:?}, pat={:?})", discr_place, pat);
 542
 543         let tcx = self.tcx();
 544         let ExprUseVisitor { ref mc, ref mut delegate } = *self;
 545         return_if_err!(mc.cat_pattern(discr_place.clone(), pat, |place, pat| {
 546             if let PatKind::Binding(_, canonical_id, ..) = pat.kind {
 547                 debug!(
 548                     "walk_pat: binding place={:?} pat={:?}",
 549                     place,
 550                     pat,
 551                 );
 552                 if let Some(&bm) = mc.tables.pat_binding_modes().get(pat.hir_id) {
 553                     debug!("walk_pat: pat.hir_id={:?} bm={:?}", pat.hir_id, bm);
 554
 555                     // pat_ty: the type of the binding being produced.
 556                     let pat_ty = return_if_err!(mc.node_ty(pat.hir_id));
 557                     debug!("walk_pat: pat_ty={:?}", pat_ty);
 558
 559                     // Each match binding is effectively an assignment to the
 560                     // binding being produced.
 561                     let def = Res::Local(canonical_id);
 562                     if let Ok(ref binding_place) = mc.cat_res(pat.hir_id, pat.span, pat_ty, def) {
 563                         delegate.mutate(binding_place);
 564                     }
 565
 566                     // It is also a borrow or copy/move of the value being matched.
 567                     match bm {
 568                         ty::BindByReference(m) => {
 569                             let bk = ty::BorrowKind::from_mutbl(m);
 570                             delegate.borrow(place, bk);
 571                         }
 572                         ty::BindByValue(..) => {
 573                             let mode = copy_or_move(mc, place);
 574                             debug!("walk_pat binding consuming pat");
 575                             delegate.consume(place, mode);
 576                         }
 577                     }
 578                 } else {
 579                     tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 580                 }
 581             }
 582         }));
 583     }
 584
 585     fn walk_captures(&mut self, closure_expr: &hir::Expr, fn_decl_span: Span) {
 586         debug!("walk_captures({:?})", closure_expr);
 587
 588         let closure_def_id = self.tcx().hir().local_def_id(closure_expr.hir_id);
 589         if let Some(upvars) = self.tcx().upvars(closure_def_id) {
 590             for &var_id in upvars.keys() {
 591                 let upvar_id = ty::UpvarId {
 592                     var_path: ty::UpvarPath { hir_id: var_id },
 593                     closure_expr_id: closure_def_id.to_local(),
 594                 };
 595                 let upvar_capture = self.mc.tables.upvar_capture(upvar_id);
 596                 let captured_place = return_if_err!(self.cat_captured_var(
 597                     closure_expr.hir_id,
 598                     fn_decl_span,
 599                     var_id,
 600                 ));
 601                 match upvar_capture {
 602                     ty::UpvarCapture::ByValue => {
 603                         let mode = copy_or_move(&self.mc, &captured_place);
 604                         self.delegate.consume(&captured_place, mode);
 605                     }
 606                     ty::UpvarCapture::ByRef(upvar_borrow) => {
 607                         self.delegate.borrow(&captured_place, upvar_borrow.kind);
 608                     }
 609                 }
 610             }
 611         }
 612     }
 613
 614     fn cat_captured_var(&mut self,
 615                         closure_hir_id: hir::HirId,
 616                         closure_span: Span,
 617                         var_id: hir::HirId)
 618                         -> mc::McResult<mc::Place<'tcx>> {
 619         // Create the place for the variable being borrowed, from the
 620         // perspective of the creator (parent) of the closure.
 621         let var_ty = self.mc.node_ty(var_id)?;
 622         self.mc.cat_res(closure_hir_id, closure_span, var_ty, Res::Local(var_id))
 623     }
 624 }
 625
 626 fn copy_or_move<'a, 'tcx>(
 627     mc: &mc::MemCategorizationContext<'a, 'tcx>,
 628     place: &Place<'tcx>,
 629 ) -> ConsumeMode {
 630     if !mc.type_is_copy_modulo_regions(place.ty, place.span) {
 631         Move
 632     } else {
 633         Copy
 634     }
 635 }