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