src/librustc/middle/expr_use_visitor.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! A different sort of visitor for walking fn bodies.  Unlike the
  12 //! normal visitor, which just walks the entire body in one shot, the
  13 //! `ExprUseVisitor` determines how expressions are being used.
  14
  15 pub use self::LoanCause::*;
  16 pub use self::ConsumeMode::*;
  17 pub use self::MoveReason::*;
  18 pub use self::MatchMode::*;
  19 use self::TrackMatchMode::*;
  20 use self::OverloadedCallType::*;
  21
  22 use hir::pat_util;
  23 use hir::def::Def;
  24 use hir::def_id::{DefId};
  25 use infer::InferCtxt;
  26 use middle::mem_categorization as mc;
  27 use ty::{self, TyCtxt, adjustment};
  28
  29 use hir::{self, PatKind};
  30
  31 use syntax::ast;
  32 use syntax::ptr::P;
  33 use syntax_pos::Span;
  34
  35 ///////////////////////////////////////////////////////////////////////////
  36 // The Delegate trait
  37
  38 /// This trait defines the callbacks you can expect to receive when
  39 /// employing the ExprUseVisitor.
  40 pub trait Delegate<'tcx> {
  41     // The value found at `cmt` is either copied or moved, depending
  42     // on mode.
  43     fn consume(&mut self,
  44                consume_id: ast::NodeId,
  45                consume_span: Span,
  46                cmt: mc::cmt<'tcx>,
  47                mode: ConsumeMode);
  48
  49     // The value found at `cmt` has been determined to match the
  50     // pattern binding `matched_pat`, and its subparts are being
  51     // copied or moved depending on `mode`.  Note that `matched_pat`
  52     // is called on all variant/structs in the pattern (i.e., the
  53     // interior nodes of the pattern's tree structure) while
  54     // consume_pat is called on the binding identifiers in the pattern
  55     // (which are leaves of the pattern's tree structure).
  56     //
  57     // Note that variants/structs and identifiers are disjoint; thus
  58     // `matched_pat` and `consume_pat` are never both called on the
  59     // same input pattern structure (though of `consume_pat` can be
  60     // called on a subpart of an input passed to `matched_pat).
  61     fn matched_pat(&mut self,
  62                    matched_pat: &hir::Pat,
  63                    cmt: mc::cmt<'tcx>,
  64                    mode: MatchMode);
  65
  66     // The value found at `cmt` is either copied or moved via the
  67     // pattern binding `consume_pat`, depending on mode.
  68     fn consume_pat(&mut self,
  69                    consume_pat: &hir::Pat,
  70                    cmt: mc::cmt<'tcx>,
  71                    mode: ConsumeMode);
  72
  73     // The value found at `borrow` is being borrowed at the point
  74     // `borrow_id` for the region `loan_region` with kind `bk`.
  75     fn borrow(&mut self,
  76               borrow_id: ast::NodeId,
  77               borrow_span: Span,
  78               cmt: mc::cmt<'tcx>,
  79               loan_region: ty::Region,
  80               bk: ty::BorrowKind,
  81               loan_cause: LoanCause);
  82
  83     // The local variable `id` is declared but not initialized.
  84     fn decl_without_init(&mut self,
  85                          id: ast::NodeId,
  86                          span: Span);
  87
  88     // The path at `cmt` is being assigned to.
  89     fn mutate(&mut self,
  90               assignment_id: ast::NodeId,
  91               assignment_span: Span,
  92               assignee_cmt: mc::cmt<'tcx>,
  93               mode: MutateMode);
  94 }
  95
  96 #[derive(Copy, Clone, PartialEq, Debug)]
  97 pub enum LoanCause {
  98     ClosureCapture(Span),
  99     AddrOf,
 100     AutoRef,
 101     AutoUnsafe,
 102     RefBinding,
 103     OverloadedOperator,
 104     ClosureInvocation,
 105     ForLoop,
 106     MatchDiscriminant
 107 }
 108
 109 #[derive(Copy, Clone, PartialEq, Debug)]
 110 pub enum ConsumeMode {
 111     Copy,                // reference to x where x has a type that copies
 112     Move(MoveReason),    // reference to x where x has a type that moves
 113 }
 114
 115 #[derive(Copy, Clone, PartialEq, Debug)]
 116 pub enum MoveReason {
 117     DirectRefMove,
 118     PatBindingMove,
 119     CaptureMove,
 120 }
 121
 122 #[derive(Copy, Clone, PartialEq, Debug)]
 123 pub enum MatchMode {
 124     NonBindingMatch,
 125     BorrowingMatch,
 126     CopyingMatch,
 127     MovingMatch,
 128 }
 129
 130 #[derive(Copy, Clone, PartialEq, Debug)]
 131 enum TrackMatchMode {
 132     Unknown,
 133     Definite(MatchMode),
 134     Conflicting,
 135 }
 136
 137 impl TrackMatchMode {
 138     // Builds up the whole match mode for a pattern from its constituent
 139     // parts.  The lattice looks like this:
 140     //
 141     //          Conflicting
 142     //            /     \
 143     //           /       \
 144     //      Borrowing   Moving
 145     //           \       /
 146     //            \     /
 147     //            Copying
 148     //               |
 149     //          NonBinding
 150     //               |
 151     //            Unknown
 152     //
 153     // examples:
 154     //
 155     // * `(_, some_int)` pattern is Copying, since
 156     //   NonBinding + Copying => Copying
 157     //
 158     // * `(some_int, some_box)` pattern is Moving, since
 159     //   Copying + Moving => Moving
 160     //
 161     // * `(ref x, some_box)` pattern is Conflicting, since
 162     //   Borrowing + Moving => Conflicting
 163     //
 164     // Note that the `Unknown` and `Conflicting` states are
 165     // represented separately from the other more interesting
 166     // `Definite` states, which simplifies logic here somewhat.
 167     fn lub(&mut self, mode: MatchMode) {
 168         *self = match (*self, mode) {
 169             // Note that clause order below is very significant.
 170             (Unknown, new) => Definite(new),
 171             (Definite(old), new) if old == new => Definite(old),
 172
 173             (Definite(old), NonBindingMatch) => Definite(old),
 174             (Definite(NonBindingMatch), new) => Definite(new),
 175
 176             (Definite(old), CopyingMatch) => Definite(old),
 177             (Definite(CopyingMatch), new) => Definite(new),
 178
 179             (Definite(_), _) => Conflicting,
 180             (Conflicting, _) => *self,
 181         };
 182     }
 183
 184     fn match_mode(&self) -> MatchMode {
 185         match *self {
 186             Unknown => NonBindingMatch,
 187             Definite(mode) => mode,
 188             Conflicting => {
 189                 // Conservatively return MovingMatch to let the
 190                 // compiler continue to make progress.
 191                 MovingMatch
 192             }
 193         }
 194     }
 195 }
 196
 197 #[derive(Copy, Clone, PartialEq, Debug)]
 198 pub enum MutateMode {
 199     Init,
 200     JustWrite,    // x = y
 201     WriteAndRead, // x += y
 202 }
 203
 204 #[derive(Copy, Clone)]
 205 enum OverloadedCallType {
 206     FnOverloadedCall,
 207     FnMutOverloadedCall,
 208     FnOnceOverloadedCall,
 209 }
 210
 211 impl OverloadedCallType {
 212     fn from_trait_id(tcx: TyCtxt, trait_id: DefId) -> OverloadedCallType {
 213         for &(maybe_function_trait, overloaded_call_type) in &[
 214             (tcx.lang_items.fn_once_trait(), FnOnceOverloadedCall),
 215             (tcx.lang_items.fn_mut_trait(), FnMutOverloadedCall),
 216             (tcx.lang_items.fn_trait(), FnOverloadedCall)
 217         ] {
 218             match maybe_function_trait {
 219                 Some(function_trait) if function_trait == trait_id => {
 220                     return overloaded_call_type
 221                 }
 222                 _ => continue,
 223             }
 224         }
 225
 226         bug!("overloaded call didn't map to known function trait")
 227     }
 228
 229     fn from_method_id(tcx: TyCtxt, method_id: DefId) -> OverloadedCallType {
 230         let method = tcx.impl_or_trait_item(method_id);
 231         OverloadedCallType::from_trait_id(tcx, method.container().id())
 232     }
 233 }
 234
 235 ///////////////////////////////////////////////////////////////////////////
 236 // The ExprUseVisitor type
 237 //
 238 // This is the code that actually walks the tree. Like
 239 // mem_categorization, it requires a TYPER, which is a type that
 240 // supplies types from the tree. After type checking is complete, you
 241 // can just use the tcx as the typer.
 242 pub struct ExprUseVisitor<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 243     mc: mc::MemCategorizationContext<'a, 'gcx, 'tcx>,
 244     delegate: &'a mut Delegate<'tcx>,
 245 }
 246
 247 // If the TYPER results in an error, it's because the type check
 248 // failed (or will fail, when the error is uncovered and reported
 249 // during writeback). In this case, we just ignore this part of the
 250 // code.
 251 //
 252 // Note that this macro appears similar to try!(), but, unlike try!(),
 253 // it does not propagate the error.
 254 macro_rules! return_if_err {
 255     ($inp: expr) => (
 256         match $inp {
 257             Ok(v) => v,
 258             Err(()) => {
 259                 debug!("mc reported err");
 260                 return
 261             }
 262         }
 263     )
 264 }
 265
 266 /// Whether the elements of an overloaded operation are passed by value or by reference
 267 enum PassArgs {
 268     ByValue,
 269     ByRef,
 270 }
 271
 272 impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
 273     pub fn new(delegate: &'a mut (Delegate<'tcx>+'a),
 274                infcx: &'a InferCtxt<'a, 'gcx, 'tcx>)
 275                -> Self
 276     {
 277         ExprUseVisitor::with_options(delegate, infcx, mc::MemCategorizationOptions::default())
 278     }
 279
 280     pub fn with_options(delegate: &'a mut (Delegate<'tcx>+'a),
 281                         infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 282                         options: mc::MemCategorizationOptions)
 283                -> Self
 284     {
 285         ExprUseVisitor {
 286             mc: mc::MemCategorizationContext::with_options(infcx, options),
 287             delegate: delegate
 288         }
 289     }
 290
 291     pub fn walk_fn(&mut self,
 292                    decl: &hir::FnDecl,
 293                    body: &hir::Block) {
 294         self.walk_arg_patterns(decl, body);
 295         self.walk_block(body);
 296     }
 297
 298     fn walk_arg_patterns(&mut self,
 299                          decl: &hir::FnDecl,
 300                          body: &hir::Block) {
 301         for arg in &decl.inputs {
 302             let arg_ty = return_if_err!(self.mc.infcx.node_ty(arg.pat.id));
 303
 304             let fn_body_scope = self.tcx().region_maps.node_extent(body.id);
 305             let arg_cmt = self.mc.cat_rvalue(
 306                 arg.id,
 307                 arg.pat.span,
 308                 ty::ReScope(fn_body_scope), // Args live only as long as the fn body.
 309                 arg_ty);
 310
 311             self.walk_irrefutable_pat(arg_cmt, &arg.pat);
 312         }
 313     }
 314
 315     fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> {
 316         self.mc.infcx.tcx
 317     }
 318
 319     fn delegate_consume(&mut self,
 320                         consume_id: ast::NodeId,
 321                         consume_span: Span,
 322                         cmt: mc::cmt<'tcx>) {
 323         debug!("delegate_consume(consume_id={}, cmt={:?})",
 324                consume_id, cmt);
 325
 326         let mode = copy_or_move(self.mc.infcx, &cmt, DirectRefMove);
 327         self.delegate.consume(consume_id, consume_span, cmt, mode);
 328     }
 329
 330     fn consume_exprs(&mut self, exprs: &[P<hir::Expr>]) {
 331         for expr in exprs {
 332             self.consume_expr(&expr);
 333         }
 334     }
 335
 336     pub fn consume_expr(&mut self, expr: &hir::Expr) {
 337         debug!("consume_expr(expr={:?})", expr);
 338
 339         let cmt = return_if_err!(self.mc.cat_expr(expr));
 340         self.delegate_consume(expr.id, expr.span, cmt);
 341         self.walk_expr(expr);
 342     }
 343
 344     fn mutate_expr(&mut self,
 345                    assignment_expr: &hir::Expr,
 346                    expr: &hir::Expr,
 347                    mode: MutateMode) {
 348         let cmt = return_if_err!(self.mc.cat_expr(expr));
 349         self.delegate.mutate(assignment_expr.id, assignment_expr.span, cmt, mode);
 350         self.walk_expr(expr);
 351     }
 352
 353     fn borrow_expr(&mut self,
 354                    expr: &hir::Expr,
 355                    r: ty::Region,
 356                    bk: ty::BorrowKind,
 357                    cause: LoanCause) {
 358         debug!("borrow_expr(expr={:?}, r={:?}, bk={:?})",
 359                expr, r, bk);
 360
 361         let cmt = return_if_err!(self.mc.cat_expr(expr));
 362         self.delegate.borrow(expr.id, expr.span, cmt, r, bk, cause);
 363
 364         self.walk_expr(expr)
 365     }
 366
 367     fn select_from_expr(&mut self, expr: &hir::Expr) {
 368         self.walk_expr(expr)
 369     }
 370
 371     pub fn walk_expr(&mut self, expr: &hir::Expr) {
 372         debug!("walk_expr(expr={:?})", expr);
 373
 374         self.walk_adjustment(expr);
 375
 376         match expr.node {
 377             hir::ExprPath(..) => { }
 378
 379             hir::ExprType(ref subexpr, _) => {
 380                 self.walk_expr(&subexpr)
 381             }
 382
 383             hir::ExprUnary(hir::UnDeref, ref base) => {      // *base
 384                 if !self.walk_overloaded_operator(expr, &base, Vec::new(), PassArgs::ByRef) {
 385                     self.select_from_expr(&base);
 386                 }
 387             }
 388
 389             hir::ExprField(ref base, _) => {         // base.f
 390                 self.select_from_expr(&base);
 391             }
 392
 393             hir::ExprTupField(ref base, _) => {         // base.<n>
 394                 self.select_from_expr(&base);
 395             }
 396
 397             hir::ExprIndex(ref lhs, ref rhs) => {       // lhs[rhs]
 398                 if !self.walk_overloaded_operator(expr,
 399                                                   &lhs,
 400                                                   vec![&rhs],
 401                                                   PassArgs::ByValue) {
 402                     self.select_from_expr(&lhs);
 403                     self.consume_expr(&rhs);
 404                 }
 405             }
 406
 407             hir::ExprCall(ref callee, ref args) => {    // callee(args)
 408                 self.walk_callee(expr, &callee);
 409                 self.consume_exprs(args);
 410             }
 411
 412             hir::ExprMethodCall(_, _, ref args) => { // callee.m(args)
 413                 self.consume_exprs(args);
 414             }
 415
 416             hir::ExprStruct(_, ref fields, ref opt_with) => {
 417                 self.walk_struct_expr(expr, fields, opt_with);
 418             }
 419
 420             hir::ExprTup(ref exprs) => {
 421                 self.consume_exprs(exprs);
 422             }
 423
 424             hir::ExprIf(ref cond_expr, ref then_blk, ref opt_else_expr) => {
 425                 self.consume_expr(&cond_expr);
 426                 self.walk_block(&then_blk);
 427                 if let Some(ref else_expr) = *opt_else_expr {
 428                     self.consume_expr(&else_expr);
 429                 }
 430             }
 431
 432             hir::ExprMatch(ref discr, ref arms, _) => {
 433                 let discr_cmt = return_if_err!(self.mc.cat_expr(&discr));
 434                 self.borrow_expr(&discr, ty::ReEmpty, ty::ImmBorrow, MatchDiscriminant);
 435
 436                 // treatment of the discriminant is handled while walking the arms.
 437                 for arm in arms {
 438                     let mode = self.arm_move_mode(discr_cmt.clone(), arm);
 439                     let mode = mode.match_mode();
 440                     self.walk_arm(discr_cmt.clone(), arm, mode);
 441                 }
 442             }
 443
 444             hir::ExprVec(ref exprs) => {
 445                 self.consume_exprs(exprs);
 446             }
 447
 448             hir::ExprAddrOf(m, ref base) => {   // &base
 449                 // make sure that the thing we are pointing out stays valid
 450                 // for the lifetime `scope_r` of the resulting ptr:
 451                 let expr_ty = return_if_err!(self.mc.infcx.node_ty(expr.id));
 452                 if let ty::TyRef(&r, _) = expr_ty.sty {
 453                     let bk = ty::BorrowKind::from_mutbl(m);
 454                     self.borrow_expr(&base, r, bk, AddrOf);
 455                 }
 456             }
 457
 458             hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
 459                 for (o, output) in ia.outputs.iter().zip(outputs) {
 460                     if o.is_indirect {
 461                         self.consume_expr(output);
 462                     } else {
 463                         self.mutate_expr(expr, output,
 464                                          if o.is_rw {
 465                                              MutateMode::WriteAndRead
 466                                          } else {
 467                                              MutateMode::JustWrite
 468                                          });
 469                     }
 470                 }
 471                 self.consume_exprs(inputs);
 472             }
 473
 474             hir::ExprBreak(..) |
 475             hir::ExprAgain(..) |
 476             hir::ExprLit(..) => {}
 477
 478             hir::ExprLoop(ref blk, _) => {
 479                 self.walk_block(&blk);
 480             }
 481
 482             hir::ExprWhile(ref cond_expr, ref blk, _) => {
 483                 self.consume_expr(&cond_expr);
 484                 self.walk_block(&blk);
 485             }
 486
 487             hir::ExprUnary(op, ref lhs) => {
 488                 let pass_args = if op.is_by_value() {
 489                     PassArgs::ByValue
 490                 } else {
 491                     PassArgs::ByRef
 492                 };
 493
 494                 if !self.walk_overloaded_operator(expr, &lhs, Vec::new(), pass_args) {
 495                     self.consume_expr(&lhs);
 496                 }
 497             }
 498
 499             hir::ExprBinary(op, ref lhs, ref rhs) => {
 500                 let pass_args = if op.node.is_by_value() {
 501                     PassArgs::ByValue
 502                 } else {
 503                     PassArgs::ByRef
 504                 };
 505
 506                 if !self.walk_overloaded_operator(expr, &lhs, vec![&rhs], pass_args) {
 507                     self.consume_expr(&lhs);
 508                     self.consume_expr(&rhs);
 509                 }
 510             }
 511
 512             hir::ExprBlock(ref blk) => {
 513                 self.walk_block(&blk);
 514             }
 515
 516             hir::ExprRet(ref opt_expr) => {
 517                 if let Some(ref expr) = *opt_expr {
 518                     self.consume_expr(&expr);
 519                 }
 520             }
 521
 522             hir::ExprAssign(ref lhs, ref rhs) => {
 523                 self.mutate_expr(expr, &lhs, MutateMode::JustWrite);
 524                 self.consume_expr(&rhs);
 525             }
 526
 527             hir::ExprCast(ref base, _) => {
 528                 self.consume_expr(&base);
 529             }
 530
 531             hir::ExprAssignOp(op, ref lhs, ref rhs) => {
 532                 // NB All our assignment operations take the RHS by value
 533                 assert!(op.node.is_by_value());
 534
 535                 if !self.walk_overloaded_operator(expr, lhs, vec![rhs], PassArgs::ByValue) {
 536                     self.mutate_expr(expr, &lhs, MutateMode::WriteAndRead);
 537                     self.consume_expr(&rhs);
 538                 }
 539             }
 540
 541             hir::ExprRepeat(ref base, ref count) => {
 542                 self.consume_expr(&base);
 543                 self.consume_expr(&count);
 544             }
 545
 546             hir::ExprClosure(_, _, _, fn_decl_span) => {
 547                 self.walk_captures(expr, fn_decl_span)
 548             }
 549
 550             hir::ExprBox(ref base) => {
 551                 self.consume_expr(&base);
 552             }
 553         }
 554     }
 555
 556     fn walk_callee(&mut self, call: &hir::Expr, callee: &hir::Expr) {
 557         let callee_ty = return_if_err!(self.mc.infcx.expr_ty_adjusted(callee));
 558         debug!("walk_callee: callee={:?} callee_ty={:?}",
 559                callee, callee_ty);
 560         let call_scope = self.tcx().region_maps.node_extent(call.id);
 561         match callee_ty.sty {
 562             ty::TyFnDef(..) | ty::TyFnPtr(_) => {
 563                 self.consume_expr(callee);
 564             }
 565             ty::TyError => { }
 566             _ => {
 567                 let overloaded_call_type =
 568                     match self.mc.infcx.node_method_id(ty::MethodCall::expr(call.id)) {
 569                         Some(method_id) => {
 570                             OverloadedCallType::from_method_id(self.tcx(), method_id)
 571                         }
 572                         None => {
 573                             span_bug!(
 574                                 callee.span,
 575                                 "unexpected callee type {}",
 576                                 callee_ty)
 577                         }
 578                     };
 579                 match overloaded_call_type {
 580                     FnMutOverloadedCall => {
 581                         self.borrow_expr(callee,
 582                                          ty::ReScope(call_scope),
 583                                          ty::MutBorrow,
 584                                          ClosureInvocation);
 585                     }
 586                     FnOverloadedCall => {
 587                         self.borrow_expr(callee,
 588                                          ty::ReScope(call_scope),
 589                                          ty::ImmBorrow,
 590                                          ClosureInvocation);
 591                     }
 592                     FnOnceOverloadedCall => self.consume_expr(callee),
 593                 }
 594             }
 595         }
 596     }
 597
 598     fn walk_stmt(&mut self, stmt: &hir::Stmt) {
 599         match stmt.node {
 600             hir::StmtDecl(ref decl, _) => {
 601                 match decl.node {
 602                     hir::DeclLocal(ref local) => {
 603                         self.walk_local(&local);
 604                     }
 605
 606                     hir::DeclItem(_) => {
 607                         // we don't visit nested items in this visitor,
 608                         // only the fn body we were given.
 609                     }
 610                 }
 611             }
 612
 613             hir::StmtExpr(ref expr, _) |
 614             hir::StmtSemi(ref expr, _) => {
 615                 self.consume_expr(&expr);
 616             }
 617         }
 618     }
 619
 620     fn walk_local(&mut self, local: &hir::Local) {
 621         match local.init {
 622             None => {
 623                 let delegate = &mut self.delegate;
 624                 pat_util::pat_bindings(&local.pat, |_, id, span, _| {
 625                     delegate.decl_without_init(id, span);
 626                 })
 627             }
 628
 629             Some(ref expr) => {
 630                 // Variable declarations with
 631                 // initializers are considered
 632                 // "assigns", which is handled by
 633                 // `walk_pat`:
 634                 self.walk_expr(&expr);
 635                 let init_cmt = return_if_err!(self.mc.cat_expr(&expr));
 636                 self.walk_irrefutable_pat(init_cmt, &local.pat);
 637             }
 638         }
 639     }
 640
 641     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 642     /// depending on its type.
 643     fn walk_block(&mut self, blk: &hir::Block) {
 644         debug!("walk_block(blk.id={})", blk.id);
 645
 646         for stmt in &blk.stmts {
 647             self.walk_stmt(stmt);
 648         }
 649
 650         if let Some(ref tail_expr) = blk.expr {
 651             self.consume_expr(&tail_expr);
 652         }
 653     }
 654
 655     fn walk_struct_expr(&mut self,
 656                         _expr: &hir::Expr,
 657                         fields: &[hir::Field],
 658                         opt_with: &Option<P<hir::Expr>>) {
 659         // Consume the expressions supplying values for each field.
 660         for field in fields {
 661             self.consume_expr(&field.expr);
 662         }
 663
 664         let with_expr = match *opt_with {
 665             Some(ref w) => &**w,
 666             None => { return; }
 667         };
 668
 669         let with_cmt = return_if_err!(self.mc.cat_expr(&with_expr));
 670
 671         // Select just those fields of the `with`
 672         // expression that will actually be used
 673         if let ty::TyStruct(def, substs) = with_cmt.ty.sty {
 674             // Consume those fields of the with expression that are needed.
 675             for with_field in &def.struct_variant().fields {
 676                 if !contains_field_named(with_field, fields) {
 677                     let cmt_field = self.mc.cat_field(
 678                         &*with_expr,
 679                         with_cmt.clone(),
 680                         with_field.name,
 681                         with_field.ty(self.tcx(), substs)
 682                     );
 683                     self.delegate_consume(with_expr.id, with_expr.span, cmt_field);
 684                 }
 685             }
 686         } else {
 687             // the base expression should always evaluate to a
 688             // struct; however, when EUV is run during typeck, it
 689             // may not. This will generate an error earlier in typeck,
 690             // so we can just ignore it.
 691             if !self.tcx().sess.has_errors() {
 692                 span_bug!(
 693                     with_expr.span,
 694                     "with expression doesn't evaluate to a struct");
 695             }
 696         };
 697
 698         // walk the with expression so that complex expressions
 699         // are properly handled.
 700         self.walk_expr(with_expr);
 701
 702         fn contains_field_named(field: ty::FieldDef,
 703                                 fields: &[hir::Field])
 704                                 -> bool
 705         {
 706             fields.iter().any(
 707                 |f| f.name.node == field.name)
 708         }
 709     }
 710
 711     // Invoke the appropriate delegate calls for anything that gets
 712     // consumed or borrowed as part of the automatic adjustment
 713     // process.
 714     fn walk_adjustment(&mut self, expr: &hir::Expr) {
 715         let infcx = self.mc.infcx;
 716         //NOTE(@jroesch): mixed RefCell borrow causes crash
 717         let adj = infcx.adjustments().get(&expr.id).map(|x| x.clone());
 718         if let Some(adjustment) = adj {
 719             match adjustment {
 720                 adjustment::AdjustNeverToAny(..) |
 721                 adjustment::AdjustReifyFnPointer |
 722                 adjustment::AdjustUnsafeFnPointer |
 723                 adjustment::AdjustMutToConstPointer => {
 724                     // Creating a closure/fn-pointer or unsizing consumes
 725                     // the input and stores it into the resulting rvalue.
 726                     debug!("walk_adjustment: trivial adjustment");
 727                     let cmt_unadjusted =
 728                         return_if_err!(self.mc.cat_expr_unadjusted(expr));
 729                     self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
 730                 }
 731                 adjustment::AdjustDerefRef(ref adj) => {
 732                     self.walk_autoderefref(expr, adj);
 733                 }
 734             }
 735         }
 736     }
 737
 738     /// Autoderefs for overloaded Deref calls in fact reference their receiver. That is, if we have
 739     /// `(*x)` where `x` is of type `Rc<T>`, then this in fact is equivalent to `x.deref()`. Since
 740     /// `deref()` is declared with `&self`, this is an autoref of `x`.
 741     fn walk_autoderefs(&mut self,
 742                        expr: &hir::Expr,
 743                        autoderefs: usize) {
 744         debug!("walk_autoderefs expr={:?} autoderefs={}", expr, autoderefs);
 745
 746         for i in 0..autoderefs {
 747             let deref_id = ty::MethodCall::autoderef(expr.id, i as u32);
 748             if let Some(method_ty) = self.mc.infcx.node_method_ty(deref_id) {
 749                 let cmt = return_if_err!(self.mc.cat_expr_autoderefd(expr, i));
 750
 751                 // the method call infrastructure should have
 752                 // replaced all late-bound regions with variables:
 753                 let self_ty = method_ty.fn_sig().input(0);
 754                 let self_ty = self.tcx().no_late_bound_regions(&self_ty).unwrap();
 755
 756                 let (m, r) = match self_ty.sty {
 757                     ty::TyRef(r, ref m) => (m.mutbl, r),
 758                     _ => span_bug!(expr.span,
 759                                    "bad overloaded deref type {:?}",
 760                                    method_ty)
 761                 };
 762                 let bk = ty::BorrowKind::from_mutbl(m);
 763                 self.delegate.borrow(expr.id, expr.span, cmt,
 764                                      *r, bk, AutoRef);
 765             }
 766         }
 767     }
 768
 769     fn walk_autoderefref(&mut self,
 770                          expr: &hir::Expr,
 771                          adj: &adjustment::AutoDerefRef<'tcx>) {
 772         debug!("walk_autoderefref expr={:?} adj={:?}",
 773                expr,
 774                adj);
 775
 776         self.walk_autoderefs(expr, adj.autoderefs);
 777
 778         let cmt_derefd =
 779             return_if_err!(self.mc.cat_expr_autoderefd(expr, adj.autoderefs));
 780
 781         let cmt_refd =
 782             self.walk_autoref(expr, cmt_derefd, adj.autoref);
 783
 784         if adj.unsize.is_some() {
 785             // Unsizing consumes the thin pointer and produces a fat one.
 786             self.delegate_consume(expr.id, expr.span, cmt_refd);
 787         }
 788     }
 789
 790
 791     /// Walks the autoref `opt_autoref` applied to the autoderef'd
 792     /// `expr`. `cmt_derefd` is the mem-categorized form of `expr`
 793     /// after all relevant autoderefs have occurred. Because AutoRefs
 794     /// can be recursive, this function is recursive: it first walks
 795     /// deeply all the way down the autoref chain, and then processes
 796     /// the autorefs on the way out. At each point, it returns the
 797     /// `cmt` for the rvalue that will be produced by introduced an
 798     /// autoref.
 799     fn walk_autoref(&mut self,
 800                     expr: &hir::Expr,
 801                     cmt_base: mc::cmt<'tcx>,
 802                     opt_autoref: Option<adjustment::AutoRef<'tcx>>)
 803                     -> mc::cmt<'tcx>
 804     {
 805         debug!("walk_autoref(expr.id={} cmt_derefd={:?} opt_autoref={:?})",
 806                expr.id,
 807                cmt_base,
 808                opt_autoref);
 809
 810         let cmt_base_ty = cmt_base.ty;
 811
 812         let autoref = match opt_autoref {
 813             Some(ref autoref) => autoref,
 814             None => {
 815                 // No AutoRef.
 816                 return cmt_base;
 817             }
 818         };
 819
 820         match *autoref {
 821             adjustment::AutoPtr(r, m) => {
 822                 self.delegate.borrow(expr.id,
 823                                      expr.span,
 824                                      cmt_base,
 825                                      *r,
 826                                      ty::BorrowKind::from_mutbl(m),
 827                                      AutoRef);
 828             }
 829
 830             adjustment::AutoUnsafe(m) => {
 831                 debug!("walk_autoref: expr.id={} cmt_base={:?}",
 832                        expr.id,
 833                        cmt_base);
 834
 835                 // Converting from a &T to *T (or &mut T to *mut T) is
 836                 // treated as borrowing it for the enclosing temporary
 837                 // scope.
 838                 let r = ty::ReScope(self.tcx().region_maps.node_extent(expr.id));
 839
 840                 self.delegate.borrow(expr.id,
 841                                      expr.span,
 842                                      cmt_base,
 843                                      r,
 844                                      ty::BorrowKind::from_mutbl(m),
 845                                      AutoUnsafe);
 846             }
 847         }
 848
 849         // Construct the categorization for the result of the autoref.
 850         // This is always an rvalue, since we are producing a new
 851         // (temporary) indirection.
 852
 853         let adj_ty = cmt_base_ty.adjust_for_autoref(self.tcx(), opt_autoref);
 854
 855         self.mc.cat_rvalue_node(expr.id, expr.span, adj_ty)
 856     }
 857
 858
 859     // When this returns true, it means that the expression *is* a
 860     // method-call (i.e. via the operator-overload).  This true result
 861     // also implies that walk_overloaded_operator already took care of
 862     // recursively processing the input arguments, and thus the caller
 863     // should not do so.
 864     fn walk_overloaded_operator(&mut self,
 865                                 expr: &hir::Expr,
 866                                 receiver: &hir::Expr,
 867                                 rhs: Vec<&hir::Expr>,
 868                                 pass_args: PassArgs)
 869                                 -> bool
 870     {
 871         if !self.mc.infcx.is_method_call(expr.id) {
 872             return false;
 873         }
 874
 875         match pass_args {
 876             PassArgs::ByValue => {
 877                 self.consume_expr(receiver);
 878                 for &arg in &rhs {
 879                     self.consume_expr(arg);
 880                 }
 881
 882                 return true;
 883             },
 884             PassArgs::ByRef => {},
 885         }
 886
 887         self.walk_expr(receiver);
 888
 889         // Arguments (but not receivers) to overloaded operator
 890         // methods are implicitly autoref'd which sadly does not use
 891         // adjustments, so we must hardcode the borrow here.
 892
 893         let r = ty::ReScope(self.tcx().region_maps.node_extent(expr.id));
 894         let bk = ty::ImmBorrow;
 895
 896         for &arg in &rhs {
 897             self.borrow_expr(arg, r, bk, OverloadedOperator);
 898         }
 899         return true;
 900     }
 901
 902     fn arm_move_mode(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm) -> TrackMatchMode {
 903         let mut mode = Unknown;
 904         for pat in &arm.pats {
 905             self.determine_pat_move_mode(discr_cmt.clone(), &pat, &mut mode);
 906         }
 907         mode
 908     }
 909
 910     fn walk_arm(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm, mode: MatchMode) {
 911         for pat in &arm.pats {
 912             self.walk_pat(discr_cmt.clone(), &pat, mode);
 913         }
 914
 915         if let Some(ref guard) = arm.guard {
 916             self.consume_expr(&guard);
 917         }
 918
 919         self.consume_expr(&arm.body);
 920     }
 921
 922     /// Walks a pat that occurs in isolation (i.e. top-level of fn
 923     /// arg or let binding.  *Not* a match arm or nested pat.)
 924     fn walk_irrefutable_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat) {
 925         let mut mode = Unknown;
 926         self.determine_pat_move_mode(cmt_discr.clone(), pat, &mut mode);
 927         let mode = mode.match_mode();
 928         self.walk_pat(cmt_discr, pat, mode);
 929     }
 930
 931     /// Identifies any bindings within `pat` and accumulates within
 932     /// `mode` whether the overall pattern/match structure is a move,
 933     /// copy, or borrow.
 934     fn determine_pat_move_mode(&mut self,
 935                                cmt_discr: mc::cmt<'tcx>,
 936                                pat: &hir::Pat,
 937                                mode: &mut TrackMatchMode) {
 938         debug!("determine_pat_move_mode cmt_discr={:?} pat={:?}", cmt_discr,
 939                pat);
 940         return_if_err!(self.mc.cat_pattern(cmt_discr, pat, |_mc, cmt_pat, pat| {
 941             match pat.node {
 942                 PatKind::Binding(hir::BindByRef(..), _, _) =>
 943                     mode.lub(BorrowingMatch),
 944                 PatKind::Binding(hir::BindByValue(..), _, _) => {
 945                     match copy_or_move(self.mc.infcx, &cmt_pat, PatBindingMove) {
 946                         Copy => mode.lub(CopyingMatch),
 947                         Move(..) => mode.lub(MovingMatch),
 948                     }
 949                 }
 950                 _ => {}
 951             }
 952         }));
 953     }
 954
 955     /// The core driver for walking a pattern; `match_mode` must be
 956     /// established up front, e.g. via `determine_pat_move_mode` (see
 957     /// also `walk_irrefutable_pat` for patterns that stand alone).
 958     fn walk_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat, match_mode: MatchMode) {
 959         debug!("walk_pat cmt_discr={:?} pat={:?}", cmt_discr, pat);
 960
 961         let tcx = &self.tcx();
 962         let mc = &self.mc;
 963         let infcx = self.mc.infcx;
 964         let delegate = &mut self.delegate;
 965         return_if_err!(mc.cat_pattern(cmt_discr.clone(), pat, |mc, cmt_pat, pat| {
 966             if let PatKind::Binding(bmode, _, _) = pat.node {
 967                 debug!("binding cmt_pat={:?} pat={:?} match_mode={:?}", cmt_pat, pat, match_mode);
 968
 969                 // pat_ty: the type of the binding being produced.
 970                 let pat_ty = return_if_err!(infcx.node_ty(pat.id));
 971
 972                 // Each match binding is effectively an assignment to the
 973                 // binding being produced.
 974                 if let Ok(binding_cmt) = mc.cat_def(pat.id, pat.span, pat_ty,
 975                                                     tcx.expect_def(pat.id)) {
 976                     delegate.mutate(pat.id, pat.span, binding_cmt, MutateMode::Init);
 977                 }
 978
 979                 // It is also a borrow or copy/move of the value being matched.
 980                 match bmode {
 981                     hir::BindByRef(m) => {
 982                         if let ty::TyRef(&r, _) = pat_ty.sty {
 983                             let bk = ty::BorrowKind::from_mutbl(m);
 984                             delegate.borrow(pat.id, pat.span, cmt_pat, r, bk, RefBinding);
 985                         }
 986                     }
 987                     hir::BindByValue(..) => {
 988                         let mode = copy_or_move(infcx, &cmt_pat, PatBindingMove);
 989                         debug!("walk_pat binding consuming pat");
 990                         delegate.consume_pat(pat, cmt_pat, mode);
 991                     }
 992                 }
 993             }
 994         }));
 995
 996         // Do a second pass over the pattern, calling `matched_pat` on
 997         // the interior nodes (enum variants and structs), as opposed
 998         // to the above loop's visit of than the bindings that form
 999         // the leaves of the pattern tree structure.
1000         return_if_err!(mc.cat_pattern(cmt_discr, pat, |mc, cmt_pat, pat| {
1001             match tcx.expect_def_or_none(pat.id) {
1002                 Some(Def::Variant(enum_did, variant_did)) => {
1003                     let downcast_cmt = if tcx.lookup_adt_def(enum_did).is_univariant() {
1004                         cmt_pat
1005                     } else {
1006                         let cmt_pat_ty = cmt_pat.ty;
1007                         mc.cat_downcast(pat, cmt_pat, cmt_pat_ty, variant_did)
1008                     };
1009
1010                     debug!("variant downcast_cmt={:?} pat={:?}", downcast_cmt, pat);
1011                     delegate.matched_pat(pat, downcast_cmt, match_mode);
1012                 }
1013                 Some(Def::Struct(..)) | Some(Def::TyAlias(..)) | Some(Def::AssociatedTy(..)) => {
1014                     debug!("struct cmt_pat={:?} pat={:?}", cmt_pat, pat);
1015                     delegate.matched_pat(pat, cmt_pat, match_mode);
1016                 }
1017                 _ => {}
1018             }
1019         }));
1020     }
1021
1022     fn walk_captures(&mut self, closure_expr: &hir::Expr, fn_decl_span: Span) {
1023         debug!("walk_captures({:?})", closure_expr);
1024
1025         self.tcx().with_freevars(closure_expr.id, |freevars| {
1026             for freevar in freevars {
1027                 let id_var = freevar.def.var_id();
1028                 let upvar_id = ty::UpvarId { var_id: id_var,
1029                                              closure_expr_id: closure_expr.id };
1030                 let upvar_capture = self.mc.infcx.upvar_capture(upvar_id).unwrap();
1031                 let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.id,
1032                                                                    fn_decl_span,
1033                                                                    freevar.def));
1034                 match upvar_capture {
1035                     ty::UpvarCapture::ByValue => {
1036                         let mode = copy_or_move(self.mc.infcx, &cmt_var, CaptureMove);
1037                         self.delegate.consume(closure_expr.id, freevar.span, cmt_var, mode);
1038                     }
1039                     ty::UpvarCapture::ByRef(upvar_borrow) => {
1040                         self.delegate.borrow(closure_expr.id,
1041                                              fn_decl_span,
1042                                              cmt_var,
1043                                              upvar_borrow.region,
1044                                              upvar_borrow.kind,
1045                                              ClosureCapture(freevar.span));
1046                     }
1047                 }
1048             }
1049         });
1050     }
1051
1052     fn cat_captured_var(&mut self,
1053                         closure_id: ast::NodeId,
1054                         closure_span: Span,
1055                         upvar_def: Def)
1056                         -> mc::McResult<mc::cmt<'tcx>> {
1057         // Create the cmt for the variable being borrowed, from the
1058         // caller's perspective
1059         let var_id = upvar_def.var_id();
1060         let var_ty = self.mc.infcx.node_ty(var_id)?;
1061         self.mc.cat_def(closure_id, closure_span, var_ty, upvar_def)
1062     }
1063 }
1064
1065 fn copy_or_move<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
1066                                 cmt: &mc::cmt<'tcx>,
1067                                 move_reason: MoveReason)
1068                                 -> ConsumeMode
1069 {
1070     if infcx.type_moves_by_default(cmt.ty, cmt.span) {
1071         Move(move_reason)
1072     } else {
1073         Copy
1074     }
1075 }