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