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::MutateMode::*;
  16 pub use self::LoanCause::*;
  17 pub use self::ConsumeMode::*;
  18 pub use self::MoveReason::*;
  19 pub use self::MatchMode::*;
  20 use self::TrackMatchMode::*;
  21 use self::OverloadedCallType::*;
  22
  23 use middle::{def, region, pat_util};
  24 use middle::mem_categorization as mc;
  25 use middle::mem_categorization::Typer;
  26 use middle::ty::{mod};
  27 use middle::ty::{MethodCall, MethodObject, MethodTraitObject};
  28 use middle::ty::{MethodOrigin, MethodParam, MethodTypeParam};
  29 use middle::ty::{MethodStatic, MethodStaticUnboxedClosure};
  30 use util::ppaux::Repr;
  31
  32 use std::kinds;
  33 use syntax::{ast, ast_util};
  34 use syntax::ptr::P;
  35 use syntax::codemap::Span;
  36
  37 ///////////////////////////////////////////////////////////////////////////
  38 // The Delegate trait
  39
  40 /// This trait defines the callbacks you can expect to receive when
  41 /// employing the ExprUseVisitor.
  42 pub trait Delegate<'tcx> {
  43     // The value found at `cmt` is either copied or moved, depending
  44     // on mode.
  45     fn consume(&mut self,
  46                consume_id: ast::NodeId,
  47                consume_span: Span,
  48                cmt: mc::cmt<'tcx>,
  49                mode: ConsumeMode);
  50
  51     // The value found at `cmt` has been determined to match the
  52     // pattern binding `matched_pat`, and its subparts are being
  53     // copied or moved depending on `mode`.  Note that `matched_pat`
  54     // is called on all variant/structs in the pattern (i.e., the
  55     // interior nodes of the pattern's tree structure) while
  56     // consume_pat is called on the binding identifiers in the pattern
  57     // (which are leaves of the pattern's tree structure).
  58     //
  59     // Note that variants/structs and identifiers are disjoint; thus
  60     // `matched_pat` and `consume_pat` are never both called on the
  61     // same input pattern structure (though of `consume_pat` can be
  62     // called on a subpart of an input passed to `matched_pat).
  63     fn matched_pat(&mut self,
  64                    matched_pat: &ast::Pat,
  65                    cmt: mc::cmt<'tcx>,
  66                    mode: MatchMode);
  67
  68     // The value found at `cmt` is either copied or moved via the
  69     // pattern binding `consume_pat`, depending on mode.
  70     fn consume_pat(&mut self,
  71                    consume_pat: &ast::Pat,
  72                    cmt: mc::cmt<'tcx>,
  73                    mode: ConsumeMode);
  74
  75     // The value found at `borrow` is being borrowed at the point
  76     // `borrow_id` for the region `loan_region` with kind `bk`.
  77     fn borrow(&mut self,
  78               borrow_id: ast::NodeId,
  79               borrow_span: Span,
  80               cmt: mc::cmt<'tcx>,
  81               loan_region: ty::Region,
  82               bk: ty::BorrowKind,
  83               loan_cause: LoanCause);
  84
  85     // The local variable `id` is declared but not initialized.
  86     fn decl_without_init(&mut self,
  87                          id: ast::NodeId,
  88                          span: Span);
  89
  90     // The path at `cmt` is being assigned to.
  91     fn mutate(&mut self,
  92               assignment_id: ast::NodeId,
  93               assignment_span: Span,
  94               assignee_cmt: mc::cmt<'tcx>,
  95               mode: MutateMode);
  96 }
  97
  98 #[deriving(Copy, PartialEq, Show)]
  99 pub enum LoanCause {
 100     ClosureCapture(Span),
 101     AddrOf,
 102     AutoRef,
 103     RefBinding,
 104     OverloadedOperator,
 105     ClosureInvocation,
 106     ForLoop,
 107     MatchDiscriminant
 108 }
 109
 110 #[deriving(Copy, PartialEq, Show)]
 111 pub enum ConsumeMode {
 112     Copy,                // reference to x where x has a type that copies
 113     Move(MoveReason),    // reference to x where x has a type that moves
 114 }
 115
 116 #[deriving(Copy, PartialEq, Show)]
 117 pub enum MoveReason {
 118     DirectRefMove,
 119     PatBindingMove,
 120     CaptureMove,
 121 }
 122
 123 #[deriving(Copy, PartialEq, Show)]
 124 pub enum MatchMode {
 125     NonBindingMatch,
 126     BorrowingMatch,
 127     CopyingMatch,
 128     MovingMatch,
 129 }
 130
 131 #[deriving(PartialEq,Show)]
 132 enum TrackMatchMode<T> {
 133     Unknown,
 134     Definite(MatchMode),
 135     Conflicting,
 136 }
 137
 138 impl<T> kinds::Copy for TrackMatchMode<T> {}
 139
 140 impl<T> TrackMatchMode<T> {
 141     // Builds up the whole match mode for a pattern from its constituent
 142     // parts.  The lattice looks like this:
 143     //
 144     //          Conflicting
 145     //            /     \
 146     //           /       \
 147     //      Borrowing   Moving
 148     //           \       /
 149     //            \     /
 150     //            Copying
 151     //               |
 152     //          NonBinding
 153     //               |
 154     //            Unknown
 155     //
 156     // examples:
 157     //
 158     // * `(_, some_int)` pattern is Copying, since
 159     //   NonBinding + Copying => Copying
 160     //
 161     // * `(some_int, some_box)` pattern is Moving, since
 162     //   Copying + Moving => Moving
 163     //
 164     // * `(ref x, some_box)` pattern is Conflicting, since
 165     //   Borrowing + Moving => Conflicting
 166     //
 167     // Note that the `Unknown` and `Conflicting` states are
 168     // represented separately from the other more interesting
 169     // `Definite` states, which simplifies logic here somewhat.
 170     fn lub(&mut self, mode: MatchMode) {
 171         *self = match (*self, mode) {
 172             // Note that clause order below is very significant.
 173             (Unknown, new) => Definite(new),
 174             (Definite(old), new) if old == new => Definite(old),
 175
 176             (Definite(old), NonBindingMatch) => Definite(old),
 177             (Definite(NonBindingMatch), new) => Definite(new),
 178
 179             (Definite(old), CopyingMatch) => Definite(old),
 180             (Definite(CopyingMatch), new) => Definite(new),
 181
 182             (Definite(_), _) => Conflicting,
 183             (Conflicting, _) => *self,
 184         };
 185     }
 186
 187     fn match_mode(&self) -> MatchMode {
 188         match *self {
 189             Unknown => NonBindingMatch,
 190             Definite(mode) => mode,
 191             Conflicting => {
 192                 // Conservatively return MovingMatch to let the
 193                 // compiler continue to make progress.
 194                 MovingMatch
 195             }
 196         }
 197     }
 198 }
 199
 200 #[deriving(Copy, PartialEq, Show)]
 201 pub enum MutateMode {
 202     Init,
 203     JustWrite,    // x = y
 204     WriteAndRead, // x += y
 205 }
 206
 207 #[deriving(Copy)]
 208 enum OverloadedCallType {
 209     FnOverloadedCall,
 210     FnMutOverloadedCall,
 211     FnOnceOverloadedCall,
 212 }
 213
 214 impl OverloadedCallType {
 215     fn from_trait_id(tcx: &ty::ctxt, trait_id: ast::DefId)
 216                      -> OverloadedCallType {
 217         for &(maybe_function_trait, overloaded_call_type) in [
 218             (tcx.lang_items.fn_once_trait(), FnOnceOverloadedCall),
 219             (tcx.lang_items.fn_mut_trait(), FnMutOverloadedCall),
 220             (tcx.lang_items.fn_trait(), FnOverloadedCall)
 221         ].iter() {
 222             match maybe_function_trait {
 223                 Some(function_trait) if function_trait == trait_id => {
 224                     return overloaded_call_type
 225                 }
 226                 _ => continue,
 227             }
 228         }
 229
 230         tcx.sess.bug("overloaded call didn't map to known function trait")
 231     }
 232
 233     fn from_method_id(tcx: &ty::ctxt, method_id: ast::DefId)
 234                       -> OverloadedCallType {
 235         let method_descriptor = match ty::impl_or_trait_item(tcx, method_id) {
 236             ty::MethodTraitItem(ref method_descriptor) => {
 237                 (*method_descriptor).clone()
 238             }
 239             ty::TypeTraitItem(_) => {
 240                 tcx.sess.bug("overloaded call method wasn't in method map")
 241             }
 242         };
 243         let impl_id = match method_descriptor.container {
 244             ty::TraitContainer(_) => {
 245                 tcx.sess.bug("statically resolved overloaded call method \
 246                               belonged to a trait?!")
 247             }
 248             ty::ImplContainer(impl_id) => impl_id,
 249         };
 250         let trait_ref = match ty::impl_trait_ref(tcx, impl_id) {
 251             None => {
 252                 tcx.sess.bug("statically resolved overloaded call impl \
 253                               didn't implement a trait?!")
 254             }
 255             Some(ref trait_ref) => (*trait_ref).clone(),
 256         };
 257         OverloadedCallType::from_trait_id(tcx, trait_ref.def_id)
 258     }
 259
 260     fn from_unboxed_closure(tcx: &ty::ctxt, closure_did: ast::DefId)
 261                             -> OverloadedCallType {
 262         let trait_did =
 263             tcx.unboxed_closures
 264                .borrow()
 265                .get(&closure_did)
 266                .expect("OverloadedCallType::from_unboxed_closure: didn't \
 267                         find closure id")
 268                .kind
 269                .trait_did(tcx);
 270         OverloadedCallType::from_trait_id(tcx, trait_did)
 271     }
 272
 273     fn from_method_origin(tcx: &ty::ctxt, origin: &MethodOrigin)
 274                           -> OverloadedCallType {
 275         match *origin {
 276             MethodStatic(def_id) => {
 277                 OverloadedCallType::from_method_id(tcx, def_id)
 278             }
 279             MethodStaticUnboxedClosure(def_id) => {
 280                 OverloadedCallType::from_unboxed_closure(tcx, def_id)
 281             }
 282             MethodTypeParam(MethodParam { ref trait_ref, .. }) |
 283             MethodTraitObject(MethodObject { ref trait_ref, .. }) => {
 284                 OverloadedCallType::from_trait_id(tcx, trait_ref.def_id)
 285             }
 286         }
 287     }
 288 }
 289
 290 ///////////////////////////////////////////////////////////////////////////
 291 // The ExprUseVisitor type
 292 //
 293 // This is the code that actually walks the tree. Like
 294 // mem_categorization, it requires a TYPER, which is a type that
 295 // supplies types from the tree. After type checking is complete, you
 296 // can just use the tcx as the typer.
 297
 298 pub struct ExprUseVisitor<'d,'t,'tcx:'t,TYPER:'t> {
 299     typer: &'t TYPER,
 300     mc: mc::MemCategorizationContext<'t,TYPER>,
 301     delegate: &'d mut (Delegate<'tcx>+'d),
 302 }
 303
 304 // If the TYPER results in an error, it's because the type check
 305 // failed (or will fail, when the error is uncovered and reported
 306 // during writeback). In this case, we just ignore this part of the
 307 // code.
 308 //
 309 // Note that this macro appears similar to try!(), but, unlike try!(),
 310 // it does not propagate the error.
 311 macro_rules! return_if_err {
 312     ($inp: expr) => (
 313         match $inp {
 314             Ok(v) => v,
 315             Err(()) => return
 316         }
 317     )
 318 }
 319
 320 /// Whether the elements of an overloaded operation are passed by value or by reference
 321 enum PassArgs {
 322     ByValue,
 323     ByRef,
 324 }
 325
 326 impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,'tcx,TYPER> {
 327     pub fn new(delegate: &'d mut Delegate<'tcx>,
 328                typer: &'t TYPER)
 329                -> ExprUseVisitor<'d,'t,'tcx,TYPER> {
 330         ExprUseVisitor {
 331             typer: typer,
 332             mc: mc::MemCategorizationContext::new(typer),
 333             delegate: delegate,
 334         }
 335     }
 336
 337     pub fn walk_fn(&mut self,
 338                    decl: &ast::FnDecl,
 339                    body: &ast::Block) {
 340         self.walk_arg_patterns(decl, body);
 341         self.walk_block(body);
 342     }
 343
 344     fn walk_arg_patterns(&mut self,
 345                          decl: &ast::FnDecl,
 346                          body: &ast::Block) {
 347         for arg in decl.inputs.iter() {
 348             let arg_ty = return_if_err!(self.typer.node_ty(arg.pat.id));
 349
 350             let fn_body_scope = region::CodeExtent::from_node_id(body.id);
 351             let arg_cmt = self.mc.cat_rvalue(
 352                 arg.id,
 353                 arg.pat.span,
 354                 ty::ReScope(fn_body_scope), // Args live only as long as the fn body.
 355                 arg_ty);
 356
 357             self.walk_irrefutable_pat(arg_cmt, &*arg.pat);
 358         }
 359     }
 360
 361     fn tcx(&self) -> &'t ty::ctxt<'tcx> {
 362         self.typer.tcx()
 363     }
 364
 365     fn delegate_consume(&mut self,
 366                         consume_id: ast::NodeId,
 367                         consume_span: Span,
 368                         cmt: mc::cmt<'tcx>) {
 369         let mode = copy_or_move(self.typer, &cmt, DirectRefMove);
 370         self.delegate.consume(consume_id, consume_span, cmt, mode);
 371     }
 372
 373     fn consume_exprs(&mut self, exprs: &Vec<P<ast::Expr>>) {
 374         for expr in exprs.iter() {
 375             self.consume_expr(&**expr);
 376         }
 377     }
 378
 379     pub fn consume_expr(&mut self, expr: &ast::Expr) {
 380         debug!("consume_expr(expr={})", expr.repr(self.tcx()));
 381
 382         let cmt = return_if_err!(self.mc.cat_expr(expr));
 383         self.delegate_consume(expr.id, expr.span, cmt);
 384         self.walk_expr(expr);
 385     }
 386
 387     fn mutate_expr(&mut self,
 388                    assignment_expr: &ast::Expr,
 389                    expr: &ast::Expr,
 390                    mode: MutateMode) {
 391         let cmt = return_if_err!(self.mc.cat_expr(expr));
 392         self.delegate.mutate(assignment_expr.id, assignment_expr.span, cmt, mode);
 393         self.walk_expr(expr);
 394     }
 395
 396     fn borrow_expr(&mut self,
 397                    expr: &ast::Expr,
 398                    r: ty::Region,
 399                    bk: ty::BorrowKind,
 400                    cause: LoanCause) {
 401         debug!("borrow_expr(expr={}, r={}, bk={})",
 402                expr.repr(self.tcx()), r.repr(self.tcx()), bk.repr(self.tcx()));
 403
 404         let cmt = return_if_err!(self.mc.cat_expr(expr));
 405         self.delegate.borrow(expr.id, expr.span, cmt, r, bk, cause);
 406
 407         // Note: Unlike consume, we can ignore ExprParen. cat_expr
 408         // already skips over them, and walk will uncover any
 409         // attachments or whatever.
 410         self.walk_expr(expr)
 411     }
 412
 413     fn select_from_expr(&mut self, expr: &ast::Expr) {
 414         self.walk_expr(expr)
 415     }
 416
 417     pub fn walk_expr(&mut self, expr: &ast::Expr) {
 418         debug!("walk_expr(expr={})", expr.repr(self.tcx()));
 419
 420         self.walk_adjustment(expr);
 421
 422         match expr.node {
 423             ast::ExprParen(ref subexpr) => {
 424                 self.walk_expr(&**subexpr)
 425             }
 426
 427             ast::ExprPath(..) => { }
 428
 429             ast::ExprUnary(ast::UnDeref, ref base) => {      // *base
 430                 if !self.walk_overloaded_operator(expr, &**base, Vec::new(), PassArgs::ByRef) {
 431                     self.select_from_expr(&**base);
 432                 }
 433             }
 434
 435             ast::ExprField(ref base, _) => {         // base.f
 436                 self.select_from_expr(&**base);
 437             }
 438
 439             ast::ExprTupField(ref base, _) => {         // base.<n>
 440                 self.select_from_expr(&**base);
 441             }
 442
 443             ast::ExprIndex(ref lhs, ref rhs) => {       // lhs[rhs]
 444                 match rhs.node {
 445                     ast::ExprRange(ref start, ref end) => {
 446                         // Hacked slicing syntax (KILLME).
 447                         let args = match (start, end) {
 448                             (&Some(ref e1), &Some(ref e2)) => vec![&**e1, &**e2],
 449                             (&Some(ref e), &None) => vec![&**e],
 450                             (&None, &Some(ref e)) => vec![&**e],
 451                             (&None, &None) => Vec::new()
 452                         };
 453                         let overloaded =
 454                             self.walk_overloaded_operator(expr, &**lhs, args, PassArgs::ByRef);
 455                         assert!(overloaded);
 456                     }
 457                     _ => {
 458                         if !self.walk_overloaded_operator(expr,
 459                                                           &**lhs,
 460                                                           vec![&**rhs],
 461                                                           PassArgs::ByRef) {
 462                             self.select_from_expr(&**lhs);
 463                             self.consume_expr(&**rhs);
 464                         }
 465                     }
 466                 }
 467             }
 468
 469             ast::ExprRange(ref start, ref end) => {
 470                 start.as_ref().map(|e| self.consume_expr(&**e));
 471                 end.as_ref().map(|e| self.consume_expr(&**e));
 472             }
 473
 474             ast::ExprCall(ref callee, ref args) => {    // callee(args)
 475                 self.walk_callee(expr, &**callee);
 476                 self.consume_exprs(args);
 477             }
 478
 479             ast::ExprMethodCall(_, _, ref args) => { // callee.m(args)
 480                 self.consume_exprs(args);
 481             }
 482
 483             ast::ExprStruct(_, ref fields, ref opt_with) => {
 484                 self.walk_struct_expr(expr, fields, opt_with);
 485             }
 486
 487             ast::ExprTup(ref exprs) => {
 488                 self.consume_exprs(exprs);
 489             }
 490
 491             ast::ExprIf(ref cond_expr, ref then_blk, ref opt_else_expr) => {
 492                 self.consume_expr(&**cond_expr);
 493                 self.walk_block(&**then_blk);
 494                 for else_expr in opt_else_expr.iter() {
 495                     self.consume_expr(&**else_expr);
 496                 }
 497             }
 498
 499             ast::ExprIfLet(..) => {
 500                 self.tcx().sess.span_bug(expr.span, "non-desugared ExprIfLet");
 501             }
 502
 503             ast::ExprMatch(ref discr, ref arms, _) => {
 504                 let discr_cmt = return_if_err!(self.mc.cat_expr(&**discr));
 505                 self.borrow_expr(&**discr, ty::ReEmpty, ty::ImmBorrow, MatchDiscriminant);
 506
 507                 // treatment of the discriminant is handled while walking the arms.
 508                 for arm in arms.iter() {
 509                     let mode = self.arm_move_mode(discr_cmt.clone(), arm);
 510                     let mode = mode.match_mode();
 511                     self.walk_arm(discr_cmt.clone(), arm, mode);
 512                 }
 513             }
 514
 515             ast::ExprVec(ref exprs) => {
 516                 self.consume_exprs(exprs);
 517             }
 518
 519             ast::ExprAddrOf(m, ref base) => {   // &base
 520                 // make sure that the thing we are pointing out stays valid
 521                 // for the lifetime `scope_r` of the resulting ptr:
 522                 let expr_ty = return_if_err!(self.typer.node_ty(expr.id));
 523                 let r = ty::ty_region(self.tcx(), expr.span, expr_ty);
 524                 let bk = ty::BorrowKind::from_mutbl(m);
 525                 self.borrow_expr(&**base, r, bk, AddrOf);
 526             }
 527
 528             ast::ExprInlineAsm(ref ia) => {
 529                 for &(_, ref input) in ia.inputs.iter() {
 530                     self.consume_expr(&**input);
 531                 }
 532
 533                 for &(_, ref output, is_rw) in ia.outputs.iter() {
 534                     self.mutate_expr(expr, &**output,
 535                                            if is_rw { WriteAndRead } else { JustWrite });
 536                 }
 537             }
 538
 539             ast::ExprBreak(..) |
 540             ast::ExprAgain(..) |
 541             ast::ExprLit(..) => {}
 542
 543             ast::ExprLoop(ref blk, _) => {
 544                 self.walk_block(&**blk);
 545             }
 546
 547             ast::ExprWhile(ref cond_expr, ref blk, _) => {
 548                 self.consume_expr(&**cond_expr);
 549                 self.walk_block(&**blk);
 550             }
 551
 552             ast::ExprWhileLet(..) => {
 553                 self.tcx().sess.span_bug(expr.span, "non-desugared ExprWhileLet");
 554             }
 555
 556             ast::ExprForLoop(ref pat, ref head, ref blk, _) => {
 557                 // The pattern lives as long as the block.
 558                 debug!("walk_expr for loop case: blk id={}", blk.id);
 559                 self.consume_expr(&**head);
 560
 561                 // Fetch the type of the value that the iteration yields to
 562                 // produce the pattern's categorized mutable type.
 563                 let pattern_type = return_if_err!(self.typer.node_ty(pat.id));
 564                 let blk_scope = region::CodeExtent::from_node_id(blk.id);
 565                 let pat_cmt = self.mc.cat_rvalue(pat.id,
 566                                                  pat.span,
 567                                                  ty::ReScope(blk_scope),
 568                                                  pattern_type);
 569                 self.walk_irrefutable_pat(pat_cmt, &**pat);
 570
 571                 self.walk_block(&**blk);
 572             }
 573
 574             ast::ExprUnary(op, ref lhs) => {
 575                 let pass_args = if ast_util::is_by_value_unop(op) {
 576                     PassArgs::ByValue
 577                 } else {
 578                     PassArgs::ByRef
 579                 };
 580
 581                 if !self.walk_overloaded_operator(expr, &**lhs, Vec::new(), pass_args) {
 582                     self.consume_expr(&**lhs);
 583                 }
 584             }
 585
 586             ast::ExprBinary(op, ref lhs, ref rhs) => {
 587                 let pass_args = if ast_util::is_by_value_binop(op) {
 588                     PassArgs::ByValue
 589                 } else {
 590                     PassArgs::ByRef
 591                 };
 592
 593                 if !self.walk_overloaded_operator(expr, &**lhs, vec![&**rhs], pass_args) {
 594                     self.consume_expr(&**lhs);
 595                     self.consume_expr(&**rhs);
 596                 }
 597             }
 598
 599             ast::ExprBlock(ref blk) => {
 600                 self.walk_block(&**blk);
 601             }
 602
 603             ast::ExprRet(ref opt_expr) => {
 604                 for expr in opt_expr.iter() {
 605                     self.consume_expr(&**expr);
 606                 }
 607             }
 608
 609             ast::ExprAssign(ref lhs, ref rhs) => {
 610                 self.mutate_expr(expr, &**lhs, JustWrite);
 611                 self.consume_expr(&**rhs);
 612             }
 613
 614             ast::ExprCast(ref base, _) => {
 615                 self.consume_expr(&**base);
 616             }
 617
 618             ast::ExprAssignOp(_, ref lhs, ref rhs) => {
 619                 // This will have to change if/when we support
 620                 // overloaded operators for `+=` and so forth.
 621                 self.mutate_expr(expr, &**lhs, WriteAndRead);
 622                 self.consume_expr(&**rhs);
 623             }
 624
 625             ast::ExprRepeat(ref base, ref count) => {
 626                 self.consume_expr(&**base);
 627                 self.consume_expr(&**count);
 628             }
 629
 630             ast::ExprClosure(..) => {
 631                 self.walk_captures(expr)
 632             }
 633
 634             ast::ExprBox(ref place, ref base) => {
 635                 match *place {
 636                     Some(ref place) => self.consume_expr(&**place),
 637                     None => {}
 638                 }
 639                 self.consume_expr(&**base);
 640             }
 641
 642             ast::ExprMac(..) => {
 643                 self.tcx().sess.span_bug(
 644                     expr.span,
 645                     "macro expression remains after expansion");
 646             }
 647         }
 648     }
 649
 650     fn walk_callee(&mut self, call: &ast::Expr, callee: &ast::Expr) {
 651         let callee_ty = return_if_err!(self.typer.expr_ty_adjusted(callee));
 652         debug!("walk_callee: callee={} callee_ty={}",
 653                callee.repr(self.tcx()), callee_ty.repr(self.tcx()));
 654         let call_scope = region::CodeExtent::from_node_id(call.id);
 655         match callee_ty.sty {
 656             ty::ty_bare_fn(..) => {
 657                 self.consume_expr(callee);
 658             }
 659             ty::ty_closure(ref f) => {
 660                 match f.onceness {
 661                     ast::Many => {
 662                         self.borrow_expr(callee,
 663                                          ty::ReScope(call_scope),
 664                                          ty::UniqueImmBorrow,
 665                                          ClosureInvocation);
 666                     }
 667                     ast::Once => {
 668                         self.consume_expr(callee);
 669                     }
 670                 }
 671             }
 672             ty::ty_err => { }
 673             _ => {
 674                 let overloaded_call_type =
 675                     match self.typer.node_method_origin(MethodCall::expr(call.id)) {
 676                         Some(method_origin) => {
 677                             OverloadedCallType::from_method_origin(
 678                                 self.tcx(),
 679                                 &method_origin)
 680                         }
 681                         None => {
 682                             self.tcx().sess.span_bug(
 683                                 callee.span,
 684                                 format!("unexpected callee type {}",
 685                                         callee_ty.repr(self.tcx())).as_slice())
 686                         }
 687                     };
 688                 match overloaded_call_type {
 689                     FnMutOverloadedCall => {
 690                         self.borrow_expr(callee,
 691                                          ty::ReScope(call_scope),
 692                                          ty::MutBorrow,
 693                                          ClosureInvocation);
 694                     }
 695                     FnOverloadedCall => {
 696                         self.borrow_expr(callee,
 697                                          ty::ReScope(call_scope),
 698                                          ty::ImmBorrow,
 699                                          ClosureInvocation);
 700                     }
 701                     FnOnceOverloadedCall => self.consume_expr(callee),
 702                 }
 703             }
 704         }
 705     }
 706
 707     fn walk_stmt(&mut self, stmt: &ast::Stmt) {
 708         match stmt.node {
 709             ast::StmtDecl(ref decl, _) => {
 710                 match decl.node {
 711                     ast::DeclLocal(ref local) => {
 712                         self.walk_local(&**local);
 713                     }
 714
 715                     ast::DeclItem(_) => {
 716                         // we don't visit nested items in this visitor,
 717                         // only the fn body we were given.
 718                     }
 719                 }
 720             }
 721
 722             ast::StmtExpr(ref expr, _) |
 723             ast::StmtSemi(ref expr, _) => {
 724                 self.consume_expr(&**expr);
 725             }
 726
 727             ast::StmtMac(..) => {
 728                 self.tcx().sess.span_bug(stmt.span, "unexpanded stmt macro");
 729             }
 730         }
 731     }
 732
 733     fn walk_local(&mut self, local: &ast::Local) {
 734         match local.init {
 735             None => {
 736                 let delegate = &mut self.delegate;
 737                 pat_util::pat_bindings(&self.typer.tcx().def_map, &*local.pat,
 738                                        |_, id, span, _| {
 739                     delegate.decl_without_init(id, span);
 740                 })
 741             }
 742
 743             Some(ref expr) => {
 744                 // Variable declarations with
 745                 // initializers are considered
 746                 // "assigns", which is handled by
 747                 // `walk_pat`:
 748                 self.walk_expr(&**expr);
 749                 let init_cmt = return_if_err!(self.mc.cat_expr(&**expr));
 750                 self.walk_irrefutable_pat(init_cmt, &*local.pat);
 751             }
 752         }
 753     }
 754
 755     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 756     /// depending on its type.
 757     fn walk_block(&mut self, blk: &ast::Block) {
 758         debug!("walk_block(blk.id={})", blk.id);
 759
 760         for stmt in blk.stmts.iter() {
 761             self.walk_stmt(&**stmt);
 762         }
 763
 764         for tail_expr in blk.expr.iter() {
 765             self.consume_expr(&**tail_expr);
 766         }
 767     }
 768
 769     fn walk_struct_expr(&mut self,
 770                         _expr: &ast::Expr,
 771                         fields: &Vec<ast::Field>,
 772                         opt_with: &Option<P<ast::Expr>>) {
 773         // Consume the expressions supplying values for each field.
 774         for field in fields.iter() {
 775             self.consume_expr(&*field.expr);
 776         }
 777
 778         let with_expr = match *opt_with {
 779             Some(ref w) => &**w,
 780             None => { return; }
 781         };
 782
 783         let with_cmt = return_if_err!(self.mc.cat_expr(&*with_expr));
 784
 785         // Select just those fields of the `with`
 786         // expression that will actually be used
 787         let with_fields = match with_cmt.ty.sty {
 788             ty::ty_struct(did, substs) => {
 789                 ty::struct_fields(self.tcx(), did, substs)
 790             }
 791             _ => {
 792                 // the base expression should always evaluate to a
 793                 // struct; however, when EUV is run during typeck, it
 794                 // may not. This will generate an error earlier in typeck,
 795                 // so we can just ignore it.
 796                 if !self.tcx().sess.has_errors() {
 797                     self.tcx().sess.span_bug(
 798                         with_expr.span,
 799                         "with expression doesn't evaluate to a struct");
 800                 }
 801                 assert!(self.tcx().sess.has_errors());
 802                 vec!()
 803             }
 804         };
 805
 806         // Consume those fields of the with expression that are needed.
 807         for with_field in with_fields.iter() {
 808             if !contains_field_named(with_field, fields) {
 809                 let cmt_field = self.mc.cat_field(&*with_expr,
 810                                                   with_cmt.clone(),
 811                                                   with_field.name,
 812                                                   with_field.mt.ty);
 813                 self.delegate_consume(with_expr.id, with_expr.span, cmt_field);
 814             }
 815         }
 816
 817         // walk the with expression so that complex expressions
 818         // are properly handled.
 819         self.walk_expr(with_expr);
 820
 821         fn contains_field_named(field: &ty::field,
 822                                 fields: &Vec<ast::Field>)
 823                                 -> bool
 824         {
 825             fields.iter().any(
 826                 |f| f.ident.node.name == field.name)
 827         }
 828     }
 829
 830     // Invoke the appropriate delegate calls for anything that gets
 831     // consumed or borrowed as part of the automatic adjustment
 832     // process.
 833     fn walk_adjustment(&mut self, expr: &ast::Expr) {
 834         let typer = self.typer;
 835         match typer.adjustments().borrow().get(&expr.id) {
 836             None => { }
 837             Some(adjustment) => {
 838                 match *adjustment {
 839                     ty::AdjustAddEnv(..) |
 840                     ty::AdjustReifyFnPointer(..) => {
 841                         // Creating a closure/fn-pointer consumes the
 842                         // input and stores it into the resulting
 843                         // rvalue.
 844                         debug!("walk_adjustment(AutoAddEnv|AdjustReifyFnPointer)");
 845                         let cmt_unadjusted =
 846                             return_if_err!(self.mc.cat_expr_unadjusted(expr));
 847                         self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
 848                     }
 849                     ty::AdjustDerefRef(ty::AutoDerefRef {
 850                         autoref: ref opt_autoref,
 851                         autoderefs: n
 852                     }) => {
 853                         self.walk_autoderefs(expr, n);
 854
 855                         match *opt_autoref {
 856                             None => { }
 857                             Some(ref r) => {
 858                                 self.walk_autoref(expr, r, n);
 859                             }
 860                         }
 861                     }
 862                 }
 863             }
 864         }
 865     }
 866
 867     /// Autoderefs for overloaded Deref calls in fact reference their receiver. That is, if we have
 868     /// `(*x)` where `x` is of type `Rc<T>`, then this in fact is equivalent to `x.deref()`. Since
 869     /// `deref()` is declared with `&self`, this is an autoref of `x`.
 870     fn walk_autoderefs(&mut self,
 871                        expr: &ast::Expr,
 872                        autoderefs: uint) {
 873         debug!("walk_autoderefs expr={} autoderefs={}", expr.repr(self.tcx()), autoderefs);
 874
 875         for i in range(0, autoderefs) {
 876             let deref_id = ty::MethodCall::autoderef(expr.id, i);
 877             match self.typer.node_method_ty(deref_id) {
 878                 None => {}
 879                 Some(method_ty) => {
 880                     let cmt = return_if_err!(self.mc.cat_expr_autoderefd(expr, i));
 881                     let self_ty = ty::ty_fn_args(method_ty)[0];
 882                     let (m, r) = match self_ty.sty {
 883                         ty::ty_rptr(r, ref m) => (m.mutbl, r),
 884                         _ => self.tcx().sess.span_bug(expr.span,
 885                                 format!("bad overloaded deref type {}",
 886                                     method_ty.repr(self.tcx()))[])
 887                     };
 888                     let bk = ty::BorrowKind::from_mutbl(m);
 889                     self.delegate.borrow(expr.id, expr.span, cmt,
 890                                          *r, bk, AutoRef);
 891                 }
 892             }
 893         }
 894     }
 895
 896     fn walk_autoref(&mut self,
 897                     expr: &ast::Expr,
 898                     autoref: &ty::AutoRef,
 899                     n: uint) {
 900         debug!("walk_autoref expr={}", expr.repr(self.tcx()));
 901
 902         // Match for unique trait coercions first, since we don't need the
 903         // call to cat_expr_autoderefd.
 904         match *autoref {
 905             ty::AutoUnsizeUniq(ty::UnsizeVtable(..)) |
 906             ty::AutoUnsize(ty::UnsizeVtable(..)) => {
 907                 assert!(n == 1, format!("Expected exactly 1 deref with Uniq \
 908                                          AutoRefs, found: {}", n));
 909                 let cmt_unadjusted =
 910                     return_if_err!(self.mc.cat_expr_unadjusted(expr));
 911                 self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
 912                 return;
 913             }
 914             _ => {}
 915         }
 916
 917         let cmt_derefd = return_if_err!(
 918             self.mc.cat_expr_autoderefd(expr, n));
 919         debug!("walk_adjustment: cmt_derefd={}",
 920                cmt_derefd.repr(self.tcx()));
 921
 922         match *autoref {
 923             ty::AutoPtr(r, m, _) => {
 924                 self.delegate.borrow(expr.id,
 925                                      expr.span,
 926                                      cmt_derefd,
 927                                      r,
 928                                      ty::BorrowKind::from_mutbl(m),
 929                                      AutoRef);
 930             }
 931             ty::AutoUnsizeUniq(_) | ty::AutoUnsize(_) | ty::AutoUnsafe(..) => {}
 932         }
 933     }
 934
 935     fn walk_overloaded_operator(&mut self,
 936                                 expr: &ast::Expr,
 937                                 receiver: &ast::Expr,
 938                                 rhs: Vec<&ast::Expr>,
 939                                 pass_args: PassArgs)
 940                                 -> bool
 941     {
 942         if !self.typer.is_method_call(expr.id) {
 943             return false;
 944         }
 945
 946         match pass_args {
 947             PassArgs::ByValue => {
 948                 self.consume_expr(receiver);
 949                 for &arg in rhs.iter() {
 950                     self.consume_expr(arg);
 951                 }
 952
 953                 return true;
 954             },
 955             PassArgs::ByRef => {},
 956         }
 957
 958         self.walk_expr(receiver);
 959
 960         // Arguments (but not receivers) to overloaded operator
 961         // methods are implicitly autoref'd which sadly does not use
 962         // adjustments, so we must hardcode the borrow here.
 963
 964         let r = ty::ReScope(region::CodeExtent::from_node_id(expr.id));
 965         let bk = ty::ImmBorrow;
 966
 967         for &arg in rhs.iter() {
 968             self.borrow_expr(arg, r, bk, OverloadedOperator);
 969         }
 970         return true;
 971     }
 972
 973     fn arm_move_mode(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &ast::Arm) -> TrackMatchMode<Span> {
 974         let mut mode = Unknown;
 975         for pat in arm.pats.iter() {
 976             self.determine_pat_move_mode(discr_cmt.clone(), &**pat, &mut mode);
 977         }
 978         mode
 979     }
 980
 981     fn walk_arm(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &ast::Arm, mode: MatchMode) {
 982         for pat in arm.pats.iter() {
 983             self.walk_pat(discr_cmt.clone(), &**pat, mode);
 984         }
 985
 986         for guard in arm.guard.iter() {
 987             self.consume_expr(&**guard);
 988         }
 989
 990         self.consume_expr(&*arm.body);
 991     }
 992
 993     /// Walks an pat that occurs in isolation (i.e. top-level of fn
 994     /// arg or let binding.  *Not* a match arm or nested pat.)
 995     fn walk_irrefutable_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &ast::Pat) {
 996         let mut mode = Unknown;
 997         self.determine_pat_move_mode(cmt_discr.clone(), pat, &mut mode);
 998         let mode = mode.match_mode();
 999         self.walk_pat(cmt_discr, pat, mode);
1000     }
1001
1002     /// Identifies any bindings within `pat` and accumulates within
1003     /// `mode` whether the overall pattern/match structure is a move,
1004     /// copy, or borrow.
1005     fn determine_pat_move_mode(&mut self,
1006                                cmt_discr: mc::cmt<'tcx>,
1007                                pat: &ast::Pat,
1008                                mode: &mut TrackMatchMode<Span>) {
1009         debug!("determine_pat_move_mode cmt_discr={} pat={}", cmt_discr.repr(self.tcx()),
1010                pat.repr(self.tcx()));
1011         return_if_err!(self.mc.cat_pattern(cmt_discr, pat, |_mc, cmt_pat, pat| {
1012             let tcx = self.tcx();
1013             let def_map = &self.tcx().def_map;
1014             if pat_util::pat_is_binding(def_map, pat) {
1015                 match pat.node {
1016                     ast::PatIdent(ast::BindByRef(_), _, _) =>
1017                         mode.lub(BorrowingMatch),
1018                     ast::PatIdent(ast::BindByValue(_), _, _) => {
1019                         match copy_or_move(self.typer, &cmt_pat, PatBindingMove) {
1020                             Copy => mode.lub(CopyingMatch),
1021                             Move(_) => mode.lub(MovingMatch),
1022                         }
1023                     }
1024                     _ => {
1025                         tcx.sess.span_bug(
1026                             pat.span,
1027                             "binding pattern not an identifier");
1028                     }
1029                 }
1030             }
1031         }));
1032     }
1033
1034     /// The core driver for walking a pattern; `match_mode` must be
1035     /// established up front, e.g. via `determine_pat_move_mode` (see
1036     /// also `walk_irrefutable_pat` for patterns that stand alone).
1037     fn walk_pat(&mut self,
1038                 cmt_discr: mc::cmt<'tcx>,
1039                 pat: &ast::Pat,
1040                 match_mode: MatchMode) {
1041         debug!("walk_pat cmt_discr={} pat={}", cmt_discr.repr(self.tcx()),
1042                pat.repr(self.tcx()));
1043
1044         let mc = &self.mc;
1045         let typer = self.typer;
1046         let def_map = &self.tcx().def_map;
1047         let delegate = &mut self.delegate;
1048         return_if_err!(mc.cat_pattern(cmt_discr.clone(), pat, |mc, cmt_pat, pat| {
1049             if pat_util::pat_is_binding(def_map, pat) {
1050                 let tcx = typer.tcx();
1051
1052                 debug!("binding cmt_pat={} pat={} match_mode={}",
1053                        cmt_pat.repr(tcx),
1054                        pat.repr(tcx),
1055                        match_mode);
1056
1057                 // pat_ty: the type of the binding being produced.
1058                 let pat_ty = return_if_err!(typer.node_ty(pat.id));
1059
1060                 // Each match binding is effectively an assignment to the
1061                 // binding being produced.
1062                 let def = def_map.borrow()[pat.id].clone();
1063                 match mc.cat_def(pat.id, pat.span, pat_ty, def) {
1064                     Ok(binding_cmt) => {
1065                         delegate.mutate(pat.id, pat.span, binding_cmt, Init);
1066                     }
1067                     Err(_) => { }
1068                 }
1069
1070                 // It is also a borrow or copy/move of the value being matched.
1071                 match pat.node {
1072                     ast::PatIdent(ast::BindByRef(m), _, _) => {
1073                         let (r, bk) = {
1074                             (ty::ty_region(tcx, pat.span, pat_ty),
1075                              ty::BorrowKind::from_mutbl(m))
1076                         };
1077                         delegate.borrow(pat.id, pat.span, cmt_pat,
1078                                              r, bk, RefBinding);
1079                     }
1080                     ast::PatIdent(ast::BindByValue(_), _, _) => {
1081                         let mode = copy_or_move(typer, &cmt_pat, PatBindingMove);
1082                         debug!("walk_pat binding consuming pat");
1083                         delegate.consume_pat(pat, cmt_pat, mode);
1084                     }
1085                     _ => {
1086                         tcx.sess.span_bug(
1087                             pat.span,
1088                             "binding pattern not an identifier");
1089                     }
1090                 }
1091             } else {
1092                 match pat.node {
1093                     ast::PatVec(_, Some(ref slice_pat), _) => {
1094                         // The `slice_pat` here creates a slice into
1095                         // the original vector.  This is effectively a
1096                         // borrow of the elements of the vector being
1097                         // matched.
1098
1099                         let (slice_cmt, slice_mutbl, slice_r) =
1100                             return_if_err!(mc.cat_slice_pattern(cmt_pat, &**slice_pat));
1101
1102                         // Note: We declare here that the borrow
1103                         // occurs upon entering the `[...]`
1104                         // pattern. This implies that something like
1105                         // `[a; b]` where `a` is a move is illegal,
1106                         // because the borrow is already in effect.
1107                         // In fact such a move would be safe-ish, but
1108                         // it effectively *requires* that we use the
1109                         // nulling out semantics to indicate when a
1110                         // value has been moved, which we are trying
1111                         // to move away from.  Otherwise, how can we
1112                         // indicate that the first element in the
1113                         // vector has been moved?  Eventually, we
1114                         // could perhaps modify this rule to permit
1115                         // `[..a, b]` where `b` is a move, because in
1116                         // that case we can adjust the length of the
1117                         // original vec accordingly, but we'd have to
1118                         // make trans do the right thing, and it would
1119                         // only work for `~` vectors. It seems simpler
1120                         // to just require that people call
1121                         // `vec.pop()` or `vec.unshift()`.
1122                         let slice_bk = ty::BorrowKind::from_mutbl(slice_mutbl);
1123                         delegate.borrow(pat.id, pat.span,
1124                                         slice_cmt, slice_r,
1125                                         slice_bk, RefBinding);
1126                     }
1127                     _ => { }
1128                 }
1129             }
1130         }));
1131
1132         // Do a second pass over the pattern, calling `matched_pat` on
1133         // the interior nodes (enum variants and structs), as opposed
1134         // to the above loop's visit of than the bindings that form
1135         // the leaves of the pattern tree structure.
1136         return_if_err!(mc.cat_pattern(cmt_discr, pat, |mc, cmt_pat, pat| {
1137             let def_map = def_map.borrow();
1138             let tcx = typer.tcx();
1139
1140             match pat.node {
1141                 ast::PatEnum(_, _) | ast::PatIdent(_, _, None) | ast::PatStruct(..) => {
1142                     match def_map.get(&pat.id) {
1143                         None => {
1144                             // no definition found: pat is not a
1145                             // struct or enum pattern.
1146                         }
1147
1148                         Some(&def::DefVariant(enum_did, variant_did, _is_struct)) => {
1149                             let downcast_cmt =
1150                                 if ty::enum_is_univariant(tcx, enum_did) {
1151                                     cmt_pat
1152                                 } else {
1153                                     let cmt_pat_ty = cmt_pat.ty;
1154                                     mc.cat_downcast(pat, cmt_pat, cmt_pat_ty, variant_did)
1155                                 };
1156
1157                             debug!("variant downcast_cmt={} pat={}",
1158                                    downcast_cmt.repr(tcx),
1159                                    pat.repr(tcx));
1160
1161                             delegate.matched_pat(pat, downcast_cmt, match_mode);
1162                         }
1163
1164                         Some(&def::DefStruct(..)) | Some(&def::DefTy(_, false)) => {
1165                             // A struct (in either the value or type
1166                             // namespace; we encounter the former on
1167                             // e.g. patterns for unit structs).
1168
1169                             debug!("struct cmt_pat={} pat={}",
1170                                    cmt_pat.repr(tcx),
1171                                    pat.repr(tcx));
1172
1173                             delegate.matched_pat(pat, cmt_pat, match_mode);
1174                         }
1175
1176                         Some(&def::DefConst(..)) |
1177                         Some(&def::DefLocal(..)) => {
1178                             // This is a leaf (i.e. identifier binding
1179                             // or constant value to match); thus no
1180                             // `matched_pat` call.
1181                         }
1182
1183                         Some(def @ &def::DefTy(_, true)) => {
1184                             // An enum's type -- should never be in a
1185                             // pattern.
1186
1187                             if !tcx.sess.has_errors() {
1188                                 let msg = format!("Pattern has unexpected type: {} and type {}",
1189                                                   def,
1190                                                   cmt_pat.ty.repr(tcx));
1191                                 tcx.sess.span_bug(pat.span, msg[])
1192                             }
1193                         }
1194
1195                         Some(def) => {
1196                             // Remaining cases are e.g. DefFn, to
1197                             // which identifiers within patterns
1198                             // should not resolve. However, we do
1199                             // encouter this when using the
1200                             // expr-use-visitor during typeck. So just
1201                             // ignore it, an error should have been
1202                             // reported.
1203
1204                             if !tcx.sess.has_errors() {
1205                                 let msg = format!("Pattern has unexpected def: {} and type {}",
1206                                                   def,
1207                                                   cmt_pat.ty.repr(tcx));
1208                                 tcx.sess.span_bug(pat.span, msg[])
1209                             }
1210                         }
1211                     }
1212                 }
1213
1214                 ast::PatIdent(_, _, Some(_)) => {
1215                     // Do nothing; this is a binding (not a enum
1216                     // variant or struct), and the cat_pattern call
1217                     // will visit the substructure recursively.
1218                 }
1219
1220                 ast::PatWild(_) | ast::PatTup(..) | ast::PatBox(..) |
1221                 ast::PatRegion(..) | ast::PatLit(..) | ast::PatRange(..) |
1222                 ast::PatVec(..) | ast::PatMac(..) => {
1223                     // Similarly, each of these cases does not
1224                     // correspond to a enum variant or struct, so we
1225                     // do not do any `matched_pat` calls for these
1226                     // cases either.
1227                 }
1228             }
1229         }));
1230     }
1231
1232     fn walk_captures(&mut self, closure_expr: &ast::Expr) {
1233         debug!("walk_captures({})", closure_expr.repr(self.tcx()));
1234
1235         ty::with_freevars(self.tcx(), closure_expr.id, |freevars| {
1236             match self.tcx().capture_mode(closure_expr.id) {
1237                 ast::CaptureByRef => {
1238                     self.walk_by_ref_captures(closure_expr, freevars);
1239                 }
1240                 ast::CaptureByValue => {
1241                     self.walk_by_value_captures(closure_expr, freevars);
1242                 }
1243             }
1244         });
1245     }
1246
1247     fn walk_by_ref_captures(&mut self,
1248                             closure_expr: &ast::Expr,
1249                             freevars: &[ty::Freevar]) {
1250         for freevar in freevars.iter() {
1251             let id_var = freevar.def.def_id().node;
1252             let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.id,
1253                                                                closure_expr.span,
1254                                                                freevar.def));
1255
1256             // Lookup the kind of borrow the callee requires, as
1257             // inferred by regionbk
1258             let upvar_id = ty::UpvarId { var_id: id_var,
1259                                          closure_expr_id: closure_expr.id };
1260             let upvar_borrow = self.typer.upvar_borrow(upvar_id).unwrap();
1261
1262             self.delegate.borrow(closure_expr.id,
1263                                  closure_expr.span,
1264                                  cmt_var,
1265                                  upvar_borrow.region,
1266                                  upvar_borrow.kind,
1267                                  ClosureCapture(freevar.span));
1268         }
1269     }
1270
1271     fn walk_by_value_captures(&mut self,
1272                               closure_expr: &ast::Expr,
1273                               freevars: &[ty::Freevar]) {
1274         for freevar in freevars.iter() {
1275             let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.id,
1276                                                                closure_expr.span,
1277                                                                freevar.def));
1278             let mode = copy_or_move(self.typer, &cmt_var, CaptureMove);
1279             self.delegate.consume(closure_expr.id, freevar.span, cmt_var, mode);
1280         }
1281     }
1282
1283     fn cat_captured_var(&mut self,
1284                         closure_id: ast::NodeId,
1285                         closure_span: Span,
1286                         upvar_def: def::Def)
1287                         -> mc::McResult<mc::cmt<'tcx>> {
1288         // Create the cmt for the variable being borrowed, from the
1289         // caller's perspective
1290         let var_id = upvar_def.def_id().node;
1291         let var_ty = try!(self.typer.node_ty(var_id));
1292         self.mc.cat_def(closure_id, closure_span, var_ty, upvar_def)
1293     }
1294 }
1295
1296 fn copy_or_move<'tcx>(typer: &mc::Typer<'tcx>,
1297                       cmt: &mc::cmt<'tcx>,
1298                       move_reason: MoveReason)
1299                       -> ConsumeMode
1300 {
1301     if typer.type_moves_by_default(cmt.span, cmt.ty) {
1302         Move(move_reason)
1303     } else {
1304         Copy
1305     }
1306 }