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, pat_util};
  24 use middle::def_id::{DefId};
  25 use middle::infer;
  26 use middle::mem_categorization as mc;
  27 use middle::ty;
  28 use middle::ty::adjustment;
  29
  30 use rustc_front::hir;
  31
  32 use syntax::ast;
  33 use syntax::ptr::P;
  34 use syntax::codemap::Span;
  35
  36 ///////////////////////////////////////////////////////////////////////////
  37 // The Delegate trait
  38
  39 /// This trait defines the callbacks you can expect to receive when
  40 /// employing the ExprUseVisitor.
  41 pub trait Delegate<'tcx> {
  42     // The value found at `cmt` is either copied or moved, depending
  43     // on mode.
  44     fn consume(&mut self,
  45                consume_id: ast::NodeId,
  46                consume_span: Span,
  47                cmt: mc::cmt<'tcx>,
  48                mode: ConsumeMode);
  49
  50     // The value found at `cmt` has been determined to match the
  51     // pattern binding `matched_pat`, and its subparts are being
  52     // copied or moved depending on `mode`.  Note that `matched_pat`
  53     // is called on all variant/structs in the pattern (i.e., the
  54     // interior nodes of the pattern's tree structure) while
  55     // consume_pat is called on the binding identifiers in the pattern
  56     // (which are leaves of the pattern's tree structure).
  57     //
  58     // Note that variants/structs and identifiers are disjoint; thus
  59     // `matched_pat` and `consume_pat` are never both called on the
  60     // same input pattern structure (though of `consume_pat` can be
  61     // called on a subpart of an input passed to `matched_pat).
  62     fn matched_pat(&mut self,
  63                    matched_pat: &hir::Pat,
  64                    cmt: mc::cmt<'tcx>,
  65                    mode: MatchMode);
  66
  67     // The value found at `cmt` is either copied or moved via the
  68     // pattern binding `consume_pat`, depending on mode.
  69     fn consume_pat(&mut self,
  70                    consume_pat: &hir::Pat,
  71                    cmt: mc::cmt<'tcx>,
  72                    mode: ConsumeMode);
  73
  74     // The value found at `borrow` is being borrowed at the point
  75     // `borrow_id` for the region `loan_region` with kind `bk`.
  76     fn borrow(&mut self,
  77               borrow_id: ast::NodeId,
  78               borrow_span: Span,
  79               cmt: mc::cmt<'tcx>,
  80               loan_region: ty::Region,
  81               bk: ty::BorrowKind,
  82               loan_cause: LoanCause);
  83
  84     // The local variable `id` is declared but not initialized.
  85     fn decl_without_init(&mut self,
  86                          id: ast::NodeId,
  87                          span: Span);
  88
  89     // The path at `cmt` is being assigned to.
  90     fn mutate(&mut self,
  91               assignment_id: ast::NodeId,
  92               assignment_span: Span,
  93               assignee_cmt: mc::cmt<'tcx>,
  94               mode: MutateMode);
  95 }
  96
  97 #[derive(Copy, Clone, PartialEq, Debug)]
  98 pub enum LoanCause {
  99     ClosureCapture(Span),
 100     AddrOf,
 101     AutoRef,
 102     AutoUnsafe,
 103     RefBinding,
 104     OverloadedOperator,
 105     ClosureInvocation,
 106     ForLoop,
 107     MatchDiscriminant
 108 }
 109
 110 #[derive(Copy, Clone, PartialEq, Debug)]
 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 #[derive(Copy, Clone, PartialEq, Debug)]
 117 pub enum MoveReason {
 118     DirectRefMove,
 119     PatBindingMove,
 120     CaptureMove,
 121 }
 122
 123 #[derive(Copy, Clone, PartialEq, Debug)]
 124 pub enum MatchMode {
 125     NonBindingMatch,
 126     BorrowingMatch,
 127     CopyingMatch,
 128     MovingMatch,
 129 }
 130
 131 #[derive(Copy, Clone, PartialEq, Debug)]
 132 enum TrackMatchMode {
 133     Unknown,
 134     Definite(MatchMode),
 135     Conflicting,
 136 }
 137
 138 impl TrackMatchMode {
 139     // Builds up the whole match mode for a pattern from its constituent
 140     // parts.  The lattice looks like this:
 141     //
 142     //          Conflicting
 143     //            /     \
 144     //           /       \
 145     //      Borrowing   Moving
 146     //           \       /
 147     //            \     /
 148     //            Copying
 149     //               |
 150     //          NonBinding
 151     //               |
 152     //            Unknown
 153     //
 154     // examples:
 155     //
 156     // * `(_, some_int)` pattern is Copying, since
 157     //   NonBinding + Copying => Copying
 158     //
 159     // * `(some_int, some_box)` pattern is Moving, since
 160     //   Copying + Moving => Moving
 161     //
 162     // * `(ref x, some_box)` pattern is Conflicting, since
 163     //   Borrowing + Moving => Conflicting
 164     //
 165     // Note that the `Unknown` and `Conflicting` states are
 166     // represented separately from the other more interesting
 167     // `Definite` states, which simplifies logic here somewhat.
 168     fn lub(&mut self, mode: MatchMode) {
 169         *self = match (*self, mode) {
 170             // Note that clause order below is very significant.
 171             (Unknown, new) => Definite(new),
 172             (Definite(old), new) if old == new => Definite(old),
 173
 174             (Definite(old), NonBindingMatch) => Definite(old),
 175             (Definite(NonBindingMatch), new) => Definite(new),
 176
 177             (Definite(old), CopyingMatch) => Definite(old),
 178             (Definite(CopyingMatch), new) => Definite(new),
 179
 180             (Definite(_), _) => Conflicting,
 181             (Conflicting, _) => *self,
 182         };
 183     }
 184
 185     fn match_mode(&self) -> MatchMode {
 186         match *self {
 187             Unknown => NonBindingMatch,
 188             Definite(mode) => mode,
 189             Conflicting => {
 190                 // Conservatively return MovingMatch to let the
 191                 // compiler continue to make progress.
 192                 MovingMatch
 193             }
 194         }
 195     }
 196 }
 197
 198 #[derive(Copy, Clone, PartialEq, Debug)]
 199 pub enum MutateMode {
 200     Init,
 201     JustWrite,    // x = y
 202     WriteAndRead, // x += y
 203 }
 204
 205 #[derive(Copy, Clone)]
 206 enum OverloadedCallType {
 207     FnOverloadedCall,
 208     FnMutOverloadedCall,
 209     FnOnceOverloadedCall,
 210 }
 211
 212 impl OverloadedCallType {
 213     fn from_trait_id(tcx: &ty::ctxt, trait_id: DefId)
 214                      -> OverloadedCallType {
 215         for &(maybe_function_trait, overloaded_call_type) in &[
 216             (tcx.lang_items.fn_once_trait(), FnOnceOverloadedCall),
 217             (tcx.lang_items.fn_mut_trait(), FnMutOverloadedCall),
 218             (tcx.lang_items.fn_trait(), FnOverloadedCall)
 219         ] {
 220             match maybe_function_trait {
 221                 Some(function_trait) if function_trait == trait_id => {
 222                     return overloaded_call_type
 223                 }
 224                 _ => continue,
 225             }
 226         }
 227
 228         tcx.sess.bug("overloaded call didn't map to known function trait")
 229     }
 230
 231     fn from_method_id(tcx: &ty::ctxt, method_id: DefId)
 232                       -> OverloadedCallType {
 233         let method = tcx.impl_or_trait_item(method_id);
 234         OverloadedCallType::from_trait_id(tcx, method.container().id())
 235     }
 236 }
 237
 238 ///////////////////////////////////////////////////////////////////////////
 239 // The ExprUseVisitor type
 240 //
 241 // This is the code that actually walks the tree. Like
 242 // mem_categorization, it requires a TYPER, which is a type that
 243 // supplies types from the tree. After type checking is complete, you
 244 // can just use the tcx as the typer.
 245 //
 246 // FIXME(stage0): the :'t here is probably only important for stage0
 247 pub struct ExprUseVisitor<'d, 't, 'a: 't, 'tcx:'a+'d> {
 248     typer: &'t infer::InferCtxt<'a, 'tcx>,
 249     mc: mc::MemCategorizationContext<'t, 'a, 'tcx>,
 250     delegate: &'d mut Delegate<'tcx>,
 251 }
 252
 253 // If the TYPER results in an error, it's because the type check
 254 // failed (or will fail, when the error is uncovered and reported
 255 // during writeback). In this case, we just ignore this part of the
 256 // code.
 257 //
 258 // Note that this macro appears similar to try!(), but, unlike try!(),
 259 // it does not propagate the error.
 260 macro_rules! return_if_err {
 261     ($inp: expr) => (
 262         match $inp {
 263             Ok(v) => v,
 264             Err(()) => {
 265                 debug!("mc reported err");
 266                 return
 267             }
 268         }
 269     )
 270 }
 271
 272 /// Whether the elements of an overloaded operation are passed by value or by reference
 273 enum PassArgs {
 274     ByValue,
 275     ByRef,
 276 }
 277
 278 impl<'d,'t,'a,'tcx> ExprUseVisitor<'d,'t,'a,'tcx> {
 279     pub fn new(delegate: &'d mut (Delegate<'tcx>),
 280                typer: &'t infer::InferCtxt<'a, 'tcx>)
 281                -> ExprUseVisitor<'d,'t,'a,'tcx> where 'tcx:'a
 282     {
 283         let mc: mc::MemCategorizationContext<'t, 'a, 'tcx> =
 284             mc::MemCategorizationContext::new(typer);
 285         ExprUseVisitor { typer: typer, mc: mc, delegate: delegate }
 286     }
 287
 288     pub fn walk_fn(&mut self,
 289                    decl: &hir::FnDecl,
 290                    body: &hir::Block) {
 291         self.walk_arg_patterns(decl, body);
 292         self.walk_block(body);
 293     }
 294
 295     fn walk_arg_patterns(&mut self,
 296                          decl: &hir::FnDecl,
 297                          body: &hir::Block) {
 298         for arg in &decl.inputs {
 299             let arg_ty = return_if_err!(self.typer.node_ty(arg.pat.id));
 300
 301             let fn_body_scope = self.tcx().region_maps.node_extent(body.id);
 302             let arg_cmt = self.mc.cat_rvalue(
 303                 arg.id,
 304                 arg.pat.span,
 305                 ty::ReScope(fn_body_scope), // Args live only as long as the fn body.
 306                 arg_ty);
 307
 308             self.walk_irrefutable_pat(arg_cmt, &*arg.pat);
 309         }
 310     }
 311
 312     fn tcx(&self) -> &'t ty::ctxt<'tcx> {
 313         self.typer.tcx
 314     }
 315
 316     fn delegate_consume(&mut self,
 317                         consume_id: ast::NodeId,
 318                         consume_span: Span,
 319                         cmt: mc::cmt<'tcx>) {
 320         debug!("delegate_consume(consume_id={}, cmt={:?})",
 321                consume_id, cmt);
 322
 323         let mode = copy_or_move(self.typer, &cmt, DirectRefMove);
 324         self.delegate.consume(consume_id, consume_span, cmt, mode);
 325     }
 326
 327     fn consume_exprs(&mut self, exprs: &Vec<P<hir::Expr>>) {
 328         for expr in exprs {
 329             self.consume_expr(&**expr);
 330         }
 331     }
 332
 333     pub fn consume_expr(&mut self, expr: &hir::Expr) {
 334         debug!("consume_expr(expr={:?})", expr);
 335
 336         let cmt = return_if_err!(self.mc.cat_expr(expr));
 337         self.delegate_consume(expr.id, expr.span, cmt);
 338         self.walk_expr(expr);
 339     }
 340
 341     fn mutate_expr(&mut self,
 342                    assignment_expr: &hir::Expr,
 343                    expr: &hir::Expr,
 344                    mode: MutateMode) {
 345         let cmt = return_if_err!(self.mc.cat_expr(expr));
 346         self.delegate.mutate(assignment_expr.id, assignment_expr.span, cmt, mode);
 347         self.walk_expr(expr);
 348     }
 349
 350     fn borrow_expr(&mut self,
 351                    expr: &hir::Expr,
 352                    r: ty::Region,
 353                    bk: ty::BorrowKind,
 354                    cause: LoanCause) {
 355         debug!("borrow_expr(expr={:?}, r={:?}, bk={:?})",
 356                expr, r, bk);
 357
 358         let cmt = return_if_err!(self.mc.cat_expr(expr));
 359         self.delegate.borrow(expr.id, expr.span, cmt, r, bk, cause);
 360
 361         self.walk_expr(expr)
 362     }
 363
 364     fn select_from_expr(&mut self, expr: &hir::Expr) {
 365         self.walk_expr(expr)
 366     }
 367
 368     pub fn walk_expr(&mut self, expr: &hir::Expr) {
 369         debug!("walk_expr(expr={:?})", expr);
 370
 371         self.walk_adjustment(expr);
 372
 373         match expr.node {
 374             hir::ExprPath(..) => { }
 375
 376             hir::ExprUnary(hir::UnDeref, ref base) => {      // *base
 377                 if !self.walk_overloaded_operator(expr, &**base, Vec::new(), PassArgs::ByRef) {
 378                     self.select_from_expr(&**base);
 379                 }
 380             }
 381
 382             hir::ExprField(ref base, _) => {         // base.f
 383                 self.select_from_expr(&**base);
 384             }
 385
 386             hir::ExprTupField(ref base, _) => {         // base.<n>
 387                 self.select_from_expr(&**base);
 388             }
 389
 390             hir::ExprIndex(ref lhs, ref rhs) => {       // lhs[rhs]
 391                 if !self.walk_overloaded_operator(expr,
 392                                                   &**lhs,
 393                                                   vec![&**rhs],
 394                                                   PassArgs::ByValue) {
 395                     self.select_from_expr(&**lhs);
 396                     self.consume_expr(&**rhs);
 397                 }
 398             }
 399
 400             hir::ExprRange(ref start, ref end) => {
 401                 start.as_ref().map(|e| self.consume_expr(&**e));
 402                 end.as_ref().map(|e| self.consume_expr(&**e));
 403             }
 404
 405             hir::ExprCall(ref callee, ref args) => {    // callee(args)
 406                 self.walk_callee(expr, &**callee);
 407                 self.consume_exprs(args);
 408             }
 409
 410             hir::ExprMethodCall(_, _, ref args) => { // callee.m(args)
 411                 self.consume_exprs(args);
 412             }
 413
 414             hir::ExprStruct(_, ref fields, ref opt_with) => {
 415                 self.walk_struct_expr(expr, fields, opt_with);
 416             }
 417
 418             hir::ExprTup(ref exprs) => {
 419                 self.consume_exprs(exprs);
 420             }
 421
 422             hir::ExprIf(ref cond_expr, ref then_blk, ref opt_else_expr) => {
 423                 self.consume_expr(&**cond_expr);
 424                 self.walk_block(&**then_blk);
 425                 if let Some(ref else_expr) = *opt_else_expr {
 426                     self.consume_expr(&**else_expr);
 427                 }
 428             }
 429
 430             hir::ExprMatch(ref discr, ref arms, _) => {
 431                 let discr_cmt = return_if_err!(self.mc.cat_expr(&**discr));
 432                 self.borrow_expr(&**discr, ty::ReEmpty, ty::ImmBorrow, MatchDiscriminant);
 433
 434                 // treatment of the discriminant is handled while walking the arms.
 435                 for arm in arms {
 436                     let mode = self.arm_move_mode(discr_cmt.clone(), arm);
 437                     let mode = mode.match_mode();
 438                     self.walk_arm(discr_cmt.clone(), arm, mode);
 439                 }
 440             }
 441
 442             hir::ExprVec(ref exprs) => {
 443                 self.consume_exprs(exprs);
 444             }
 445
 446             hir::ExprAddrOf(m, ref base) => {   // &base
 447                 // make sure that the thing we are pointing out stays valid
 448                 // for the lifetime `scope_r` of the resulting ptr:
 449                 let expr_ty = return_if_err!(self.typer.node_ty(expr.id));
 450                 if let ty::TyRef(&r, _) = expr_ty.sty {
 451                     let bk = ty::BorrowKind::from_mutbl(m);
 452                     self.borrow_expr(&**base, r, bk, AddrOf);
 453                 }
 454             }
 455
 456             hir::ExprInlineAsm(ref ia) => {
 457                 for &(_, ref input) in &ia.inputs {
 458                     self.consume_expr(&**input);
 459                 }
 460
 461                 for &(_, ref output, is_rw) in &ia.outputs {
 462                     self.mutate_expr(expr, &**output,
 463                                            if is_rw { WriteAndRead } else { JustWrite });
 464                 }
 465             }
 466
 467             hir::ExprBreak(..) |
 468             hir::ExprAgain(..) |
 469             hir::ExprLit(..) => {}
 470
 471             hir::ExprLoop(ref blk, _) => {
 472                 self.walk_block(&**blk);
 473             }
 474
 475             hir::ExprWhile(ref cond_expr, ref blk, _) => {
 476                 self.consume_expr(&**cond_expr);
 477                 self.walk_block(&**blk);
 478             }
 479
 480             hir::ExprUnary(op, ref lhs) => {
 481                 let pass_args = if ::rustc_front::util::is_by_value_unop(op) {
 482                     PassArgs::ByValue
 483                 } else {
 484                     PassArgs::ByRef
 485                 };
 486
 487                 if !self.walk_overloaded_operator(expr, &**lhs, Vec::new(), pass_args) {
 488                     self.consume_expr(&**lhs);
 489                 }
 490             }
 491
 492             hir::ExprBinary(op, ref lhs, ref rhs) => {
 493                 let pass_args = if ::rustc_front::util::is_by_value_binop(op.node) {
 494                     PassArgs::ByValue
 495                 } else {
 496                     PassArgs::ByRef
 497                 };
 498
 499                 if !self.walk_overloaded_operator(expr, &**lhs, vec![&**rhs], pass_args) {
 500                     self.consume_expr(&**lhs);
 501                     self.consume_expr(&**rhs);
 502                 }
 503             }
 504
 505             hir::ExprBlock(ref blk) => {
 506                 self.walk_block(&**blk);
 507             }
 508
 509             hir::ExprRet(ref opt_expr) => {
 510                 if let Some(ref expr) = *opt_expr {
 511                     self.consume_expr(&**expr);
 512                 }
 513             }
 514
 515             hir::ExprAssign(ref lhs, ref rhs) => {
 516                 self.mutate_expr(expr, &**lhs, JustWrite);
 517                 self.consume_expr(&**rhs);
 518             }
 519
 520             hir::ExprCast(ref base, _) => {
 521                 self.consume_expr(&**base);
 522             }
 523
 524             hir::ExprAssignOp(op, ref lhs, ref rhs) => {
 525                 // NB All our assignment operations take the RHS by value
 526                 assert!(::rustc_front::util::is_by_value_binop(op.node));
 527
 528                 if !self.walk_overloaded_operator(expr, lhs, vec![rhs], PassArgs::ByValue) {
 529                     self.mutate_expr(expr, &**lhs, WriteAndRead);
 530                     self.consume_expr(&**rhs);
 531                 }
 532             }
 533
 534             hir::ExprRepeat(ref base, ref count) => {
 535                 self.consume_expr(&**base);
 536                 self.consume_expr(&**count);
 537             }
 538
 539             hir::ExprClosure(..) => {
 540                 self.walk_captures(expr)
 541             }
 542
 543             hir::ExprBox(ref base) => {
 544                 self.consume_expr(&**base);
 545             }
 546         }
 547     }
 548
 549     fn walk_callee(&mut self, call: &hir::Expr, callee: &hir::Expr) {
 550         let callee_ty = return_if_err!(self.typer.expr_ty_adjusted(callee));
 551         debug!("walk_callee: callee={:?} callee_ty={:?}",
 552                callee, callee_ty);
 553         let call_scope = self.tcx().region_maps.node_extent(call.id);
 554         match callee_ty.sty {
 555             ty::TyBareFn(..) => {
 556                 self.consume_expr(callee);
 557             }
 558             ty::TyError => { }
 559             _ => {
 560                 let overloaded_call_type =
 561                     match self.typer.node_method_id(ty::MethodCall::expr(call.id)) {
 562                         Some(method_id) => {
 563                             OverloadedCallType::from_method_id(self.tcx(), method_id)
 564                         }
 565                         None => {
 566                             self.tcx().sess.span_bug(
 567                                 callee.span,
 568                                 &format!("unexpected callee type {}", callee_ty))
 569                         }
 570                     };
 571                 match overloaded_call_type {
 572                     FnMutOverloadedCall => {
 573                         self.borrow_expr(callee,
 574                                          ty::ReScope(call_scope),
 575                                          ty::MutBorrow,
 576                                          ClosureInvocation);
 577                     }
 578                     FnOverloadedCall => {
 579                         self.borrow_expr(callee,
 580                                          ty::ReScope(call_scope),
 581                                          ty::ImmBorrow,
 582                                          ClosureInvocation);
 583                     }
 584                     FnOnceOverloadedCall => self.consume_expr(callee),
 585                 }
 586             }
 587         }
 588     }
 589
 590     fn walk_stmt(&mut self, stmt: &hir::Stmt) {
 591         match stmt.node {
 592             hir::StmtDecl(ref decl, _) => {
 593                 match decl.node {
 594                     hir::DeclLocal(ref local) => {
 595                         self.walk_local(&**local);
 596                     }
 597
 598                     hir::DeclItem(_) => {
 599                         // we don't visit nested items in this visitor,
 600                         // only the fn body we were given.
 601                     }
 602                 }
 603             }
 604
 605             hir::StmtExpr(ref expr, _) |
 606             hir::StmtSemi(ref expr, _) => {
 607                 self.consume_expr(&**expr);
 608             }
 609         }
 610     }
 611
 612     fn walk_local(&mut self, local: &hir::Local) {
 613         match local.init {
 614             None => {
 615                 let delegate = &mut self.delegate;
 616                 pat_util::pat_bindings(&self.typer.tcx.def_map, &*local.pat,
 617                                        |_, id, span, _| {
 618                     delegate.decl_without_init(id, span);
 619                 })
 620             }
 621
 622             Some(ref expr) => {
 623                 // Variable declarations with
 624                 // initializers are considered
 625                 // "assigns", which is handled by
 626                 // `walk_pat`:
 627                 self.walk_expr(&**expr);
 628                 let init_cmt = return_if_err!(self.mc.cat_expr(&**expr));
 629                 self.walk_irrefutable_pat(init_cmt, &*local.pat);
 630             }
 631         }
 632     }
 633
 634     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 635     /// depending on its type.
 636     fn walk_block(&mut self, blk: &hir::Block) {
 637         debug!("walk_block(blk.id={})", blk.id);
 638
 639         for stmt in &blk.stmts {
 640             self.walk_stmt(&**stmt);
 641         }
 642
 643         if let Some(ref tail_expr) = blk.expr {
 644             self.consume_expr(&**tail_expr);
 645         }
 646     }
 647
 648     fn walk_struct_expr(&mut self,
 649                         _expr: &hir::Expr,
 650                         fields: &Vec<hir::Field>,
 651                         opt_with: &Option<P<hir::Expr>>) {
 652         // Consume the expressions supplying values for each field.
 653         for field in fields {
 654             self.consume_expr(&*field.expr);
 655         }
 656
 657         let with_expr = match *opt_with {
 658             Some(ref w) => &**w,
 659             None => { return; }
 660         };
 661
 662         let with_cmt = return_if_err!(self.mc.cat_expr(&*with_expr));
 663
 664         // Select just those fields of the `with`
 665         // expression that will actually be used
 666         if let ty::TyStruct(def, substs) = with_cmt.ty.sty {
 667             // Consume those fields of the with expression that are needed.
 668             for with_field in &def.struct_variant().fields {
 669                 if !contains_field_named(with_field, fields) {
 670                     let cmt_field = self.mc.cat_field(
 671                         &*with_expr,
 672                         with_cmt.clone(),
 673                         with_field.name,
 674                         with_field.ty(self.tcx(), substs)
 675                     );
 676                     self.delegate_consume(with_expr.id, with_expr.span, cmt_field);
 677                 }
 678             }
 679         } else {
 680             // the base expression should always evaluate to a
 681             // struct; however, when EUV is run during typeck, it
 682             // may not. This will generate an error earlier in typeck,
 683             // so we can just ignore it.
 684             if !self.tcx().sess.has_errors() {
 685                 self.tcx().sess.span_bug(
 686                     with_expr.span,
 687                     "with expression doesn't evaluate to a struct");
 688             }
 689         };
 690
 691         // walk the with expression so that complex expressions
 692         // are properly handled.
 693         self.walk_expr(with_expr);
 694
 695         fn contains_field_named(field: ty::FieldDef,
 696                                 fields: &Vec<hir::Field>)
 697                                 -> bool
 698         {
 699             fields.iter().any(
 700                 |f| f.name.node == field.name)
 701         }
 702     }
 703
 704     // Invoke the appropriate delegate calls for anything that gets
 705     // consumed or borrowed as part of the automatic adjustment
 706     // process.
 707     fn walk_adjustment(&mut self, expr: &hir::Expr) {
 708         let typer = self.typer;
 709         //NOTE(@jroesch): mixed RefCell borrow causes crash
 710         let adj = typer.adjustments().get(&expr.id).map(|x| x.clone());
 711         if let Some(adjustment) = adj {
 712             match adjustment {
 713                 adjustment::AdjustReifyFnPointer |
 714                 adjustment::AdjustUnsafeFnPointer => {
 715                     // Creating a closure/fn-pointer or unsizing consumes
 716                     // the input and stores it into the resulting rvalue.
 717                     debug!("walk_adjustment(AdjustReifyFnPointer|AdjustUnsafeFnPointer)");
 718                     let cmt_unadjusted =
 719                         return_if_err!(self.mc.cat_expr_unadjusted(expr));
 720                     self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
 721                 }
 722                 adjustment::AdjustDerefRef(ref adj) => {
 723                     self.walk_autoderefref(expr, adj);
 724                 }
 725             }
 726         }
 727     }
 728
 729     /// Autoderefs for overloaded Deref calls in fact reference their receiver. That is, if we have
 730     /// `(*x)` where `x` is of type `Rc<T>`, then this in fact is equivalent to `x.deref()`. Since
 731     /// `deref()` is declared with `&self`, this is an autoref of `x`.
 732     fn walk_autoderefs(&mut self,
 733                        expr: &hir::Expr,
 734                        autoderefs: usize) {
 735         debug!("walk_autoderefs expr={:?} autoderefs={}", expr, autoderefs);
 736
 737         for i in 0..autoderefs {
 738             let deref_id = ty::MethodCall::autoderef(expr.id, i as u32);
 739             match self.typer.node_method_ty(deref_id) {
 740                 None => {}
 741                 Some(method_ty) => {
 742                     let cmt = return_if_err!(self.mc.cat_expr_autoderefd(expr, i));
 743
 744                     // the method call infrastructure should have
 745                     // replaced all late-bound regions with variables:
 746                     let self_ty = method_ty.fn_sig().input(0);
 747                     let self_ty = self.tcx().no_late_bound_regions(&self_ty).unwrap();
 748
 749                     let (m, r) = match self_ty.sty {
 750                         ty::TyRef(r, ref m) => (m.mutbl, r),
 751                         _ => self.tcx().sess.span_bug(expr.span,
 752                                 &format!("bad overloaded deref type {:?}",
 753                                     method_ty))
 754                     };
 755                     let bk = ty::BorrowKind::from_mutbl(m);
 756                     self.delegate.borrow(expr.id, expr.span, cmt,
 757                                          *r, bk, AutoRef);
 758                 }
 759             }
 760         }
 761     }
 762
 763     fn walk_autoderefref(&mut self,
 764                          expr: &hir::Expr,
 765                          adj: &adjustment::AutoDerefRef<'tcx>) {
 766         debug!("walk_autoderefref expr={:?} adj={:?}",
 767                expr,
 768                adj);
 769
 770         self.walk_autoderefs(expr, adj.autoderefs);
 771
 772         let cmt_derefd =
 773             return_if_err!(self.mc.cat_expr_autoderefd(expr, adj.autoderefs));
 774
 775         let cmt_refd =
 776             self.walk_autoref(expr, cmt_derefd, adj.autoref);
 777
 778         if adj.unsize.is_some() {
 779             // Unsizing consumes the thin pointer and produces a fat one.
 780             self.delegate_consume(expr.id, expr.span, cmt_refd);
 781         }
 782     }
 783
 784
 785     /// Walks the autoref `opt_autoref` applied to the autoderef'd
 786     /// `expr`. `cmt_derefd` is the mem-categorized form of `expr`
 787     /// after all relevant autoderefs have occurred. Because AutoRefs
 788     /// can be recursive, this function is recursive: it first walks
 789     /// deeply all the way down the autoref chain, and then processes
 790     /// the autorefs on the way out. At each point, it returns the
 791     /// `cmt` for the rvalue that will be produced by introduced an
 792     /// autoref.
 793     fn walk_autoref(&mut self,
 794                     expr: &hir::Expr,
 795                     cmt_base: mc::cmt<'tcx>,
 796                     opt_autoref: Option<adjustment::AutoRef<'tcx>>)
 797                     -> mc::cmt<'tcx>
 798     {
 799         debug!("walk_autoref(expr.id={} cmt_derefd={:?} opt_autoref={:?})",
 800                expr.id,
 801                cmt_base,
 802                opt_autoref);
 803
 804         let cmt_base_ty = cmt_base.ty;
 805
 806         let autoref = match opt_autoref {
 807             Some(ref autoref) => autoref,
 808             None => {
 809                 // No AutoRef.
 810                 return cmt_base;
 811             }
 812         };
 813
 814         match *autoref {
 815             adjustment::AutoPtr(r, m) => {
 816                 self.delegate.borrow(expr.id,
 817                                      expr.span,
 818                                      cmt_base,
 819                                      *r,
 820                                      ty::BorrowKind::from_mutbl(m),
 821                                      AutoRef);
 822             }
 823
 824             adjustment::AutoUnsafe(m) => {
 825                 debug!("walk_autoref: expr.id={} cmt_base={:?}",
 826                        expr.id,
 827                        cmt_base);
 828
 829                 // Converting from a &T to *T (or &mut T to *mut T) is
 830                 // treated as borrowing it for the enclosing temporary
 831                 // scope.
 832                 let r = ty::ReScope(self.tcx().region_maps.node_extent(expr.id));
 833
 834                 self.delegate.borrow(expr.id,
 835                                      expr.span,
 836                                      cmt_base,
 837                                      r,
 838                                      ty::BorrowKind::from_mutbl(m),
 839                                      AutoUnsafe);
 840             }
 841         }
 842
 843         // Construct the categorization for the result of the autoref.
 844         // This is always an rvalue, since we are producing a new
 845         // (temporary) indirection.
 846
 847         let adj_ty = cmt_base_ty.adjust_for_autoref(self.tcx(), opt_autoref);
 848
 849         self.mc.cat_rvalue_node(expr.id, expr.span, adj_ty)
 850     }
 851
 852
 853     // When this returns true, it means that the expression *is* a
 854     // method-call (i.e. via the operator-overload).  This true result
 855     // also implies that walk_overloaded_operator already took care of
 856     // recursively processing the input arguments, and thus the caller
 857     // should not do so.
 858     fn walk_overloaded_operator(&mut self,
 859                                 expr: &hir::Expr,
 860                                 receiver: &hir::Expr,
 861                                 rhs: Vec<&hir::Expr>,
 862                                 pass_args: PassArgs)
 863                                 -> bool
 864     {
 865         if !self.typer.is_method_call(expr.id) {
 866             return false;
 867         }
 868
 869         match pass_args {
 870             PassArgs::ByValue => {
 871                 self.consume_expr(receiver);
 872                 for &arg in &rhs {
 873                     self.consume_expr(arg);
 874                 }
 875
 876                 return true;
 877             },
 878             PassArgs::ByRef => {},
 879         }
 880
 881         self.walk_expr(receiver);
 882
 883         // Arguments (but not receivers) to overloaded operator
 884         // methods are implicitly autoref'd which sadly does not use
 885         // adjustments, so we must hardcode the borrow here.
 886
 887         let r = ty::ReScope(self.tcx().region_maps.node_extent(expr.id));
 888         let bk = ty::ImmBorrow;
 889
 890         for &arg in &rhs {
 891             self.borrow_expr(arg, r, bk, OverloadedOperator);
 892         }
 893         return true;
 894     }
 895
 896     fn arm_move_mode(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm) -> TrackMatchMode {
 897         let mut mode = Unknown;
 898         for pat in &arm.pats {
 899             self.determine_pat_move_mode(discr_cmt.clone(), &**pat, &mut mode);
 900         }
 901         mode
 902     }
 903
 904     fn walk_arm(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm, mode: MatchMode) {
 905         for pat in &arm.pats {
 906             self.walk_pat(discr_cmt.clone(), &**pat, mode);
 907         }
 908
 909         if let Some(ref guard) = arm.guard {
 910             self.consume_expr(&**guard);
 911         }
 912
 913         self.consume_expr(&*arm.body);
 914     }
 915
 916     /// Walks an pat that occurs in isolation (i.e. top-level of fn
 917     /// arg or let binding.  *Not* a match arm or nested pat.)
 918     fn walk_irrefutable_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat) {
 919         let mut mode = Unknown;
 920         self.determine_pat_move_mode(cmt_discr.clone(), pat, &mut mode);
 921         let mode = mode.match_mode();
 922         self.walk_pat(cmt_discr, pat, mode);
 923     }
 924
 925     /// Identifies any bindings within `pat` and accumulates within
 926     /// `mode` whether the overall pattern/match structure is a move,
 927     /// copy, or borrow.
 928     fn determine_pat_move_mode(&mut self,
 929                                cmt_discr: mc::cmt<'tcx>,
 930                                pat: &hir::Pat,
 931                                mode: &mut TrackMatchMode) {
 932         debug!("determine_pat_move_mode cmt_discr={:?} pat={:?}", cmt_discr,
 933                pat);
 934         return_if_err!(self.mc.cat_pattern(cmt_discr, pat, |_mc, cmt_pat, pat| {
 935             let tcx = self.tcx();
 936             let def_map = &self.tcx().def_map;
 937             if pat_util::pat_is_binding(def_map, pat) {
 938                 match pat.node {
 939                     hir::PatIdent(hir::BindByRef(_), _, _) =>
 940                         mode.lub(BorrowingMatch),
 941                     hir::PatIdent(hir::BindByValue(_), _, _) => {
 942                         match copy_or_move(self.typer, &cmt_pat, PatBindingMove) {
 943                             Copy => mode.lub(CopyingMatch),
 944                             Move(_) => mode.lub(MovingMatch),
 945                         }
 946                     }
 947                     _ => {
 948                         tcx.sess.span_bug(
 949                             pat.span,
 950                             "binding pattern not an identifier");
 951                     }
 952                 }
 953             }
 954         }));
 955     }
 956
 957     /// The core driver for walking a pattern; `match_mode` must be
 958     /// established up front, e.g. via `determine_pat_move_mode` (see
 959     /// also `walk_irrefutable_pat` for patterns that stand alone).
 960     fn walk_pat(&mut self,
 961                 cmt_discr: mc::cmt<'tcx>,
 962                 pat: &hir::Pat,
 963                 match_mode: MatchMode) {
 964         debug!("walk_pat cmt_discr={:?} pat={:?}", cmt_discr,
 965                pat);
 966
 967         let mc = &self.mc;
 968         let typer = self.typer;
 969         let def_map = &self.tcx().def_map;
 970         let delegate = &mut self.delegate;
 971         return_if_err!(mc.cat_pattern(cmt_discr.clone(), pat, |mc, cmt_pat, pat| {
 972             if pat_util::pat_is_binding(def_map, pat) {
 973                 let tcx = typer.tcx;
 974
 975                 debug!("binding cmt_pat={:?} pat={:?} match_mode={:?}",
 976                        cmt_pat,
 977                        pat,
 978                        match_mode);
 979
 980                 // pat_ty: the type of the binding being produced.
 981                 let pat_ty = return_if_err!(typer.node_ty(pat.id));
 982
 983                 // Each match binding is effectively an assignment to the
 984                 // binding being produced.
 985                 let def = def_map.borrow().get(&pat.id).unwrap().full_def();
 986                 match mc.cat_def(pat.id, pat.span, pat_ty, def) {
 987                     Ok(binding_cmt) => {
 988                         delegate.mutate(pat.id, pat.span, binding_cmt, Init);
 989                     }
 990                     Err(_) => { }
 991                 }
 992
 993                 // It is also a borrow or copy/move of the value being matched.
 994                 match pat.node {
 995                     hir::PatIdent(hir::BindByRef(m), _, _) => {
 996                         if let ty::TyRef(&r, _) = pat_ty.sty {
 997                             let bk = ty::BorrowKind::from_mutbl(m);
 998                             delegate.borrow(pat.id, pat.span, cmt_pat,
 999                                             r, bk, RefBinding);
1000                         }
1001                     }
1002                     hir::PatIdent(hir::BindByValue(_), _, _) => {
1003                         let mode = copy_or_move(typer, &cmt_pat, PatBindingMove);
1004                         debug!("walk_pat binding consuming pat");
1005                         delegate.consume_pat(pat, cmt_pat, mode);
1006                     }
1007                     _ => {
1008                         tcx.sess.span_bug(
1009                             pat.span,
1010                             "binding pattern not an identifier");
1011                     }
1012                 }
1013             } else {
1014                 match pat.node {
1015                     hir::PatVec(_, Some(ref slice_pat), _) => {
1016                         // The `slice_pat` here creates a slice into
1017                         // the original vector.  This is effectively a
1018                         // borrow of the elements of the vector being
1019                         // matched.
1020
1021                         let (slice_cmt, slice_mutbl, slice_r) =
1022                             return_if_err!(mc.cat_slice_pattern(cmt_pat, &**slice_pat));
1023
1024                         // Note: We declare here that the borrow
1025                         // occurs upon entering the `[...]`
1026                         // pattern. This implies that something like
1027                         // `[a; b]` where `a` is a move is illegal,
1028                         // because the borrow is already in effect.
1029                         // In fact such a move would be safe-ish, but
1030                         // it effectively *requires* that we use the
1031                         // nulling out semantics to indicate when a
1032                         // value has been moved, which we are trying
1033                         // to move away from.  Otherwise, how can we
1034                         // indicate that the first element in the
1035                         // vector has been moved?  Eventually, we
1036                         // could perhaps modify this rule to permit
1037                         // `[..a, b]` where `b` is a move, because in
1038                         // that case we can adjust the length of the
1039                         // original vec accordingly, but we'd have to
1040                         // make trans do the right thing, and it would
1041                         // only work for `Box<[T]>`s. It seems simpler
1042                         // to just require that people call
1043                         // `vec.pop()` or `vec.unshift()`.
1044                         let slice_bk = ty::BorrowKind::from_mutbl(slice_mutbl);
1045                         delegate.borrow(pat.id, pat.span,
1046                                         slice_cmt, slice_r,
1047                                         slice_bk, RefBinding);
1048                     }
1049                     _ => { }
1050                 }
1051             }
1052         }));
1053
1054         // Do a second pass over the pattern, calling `matched_pat` on
1055         // the interior nodes (enum variants and structs), as opposed
1056         // to the above loop's visit of than the bindings that form
1057         // the leaves of the pattern tree structure.
1058         return_if_err!(mc.cat_pattern(cmt_discr, pat, |mc, cmt_pat, pat| {
1059             let def_map = def_map.borrow();
1060             let tcx = typer.tcx;
1061
1062             match pat.node {
1063                 hir::PatEnum(_, _) | hir::PatQPath(..) |
1064                 hir::PatIdent(_, _, None) | hir::PatStruct(..) => {
1065                     match def_map.get(&pat.id).map(|d| d.full_def()) {
1066                         None => {
1067                             // no definition found: pat is not a
1068                             // struct or enum pattern.
1069                         }
1070
1071                         Some(def::DefVariant(enum_did, variant_did, _is_struct)) => {
1072                             let downcast_cmt =
1073                                 if tcx.lookup_adt_def(enum_did).is_univariant() {
1074                                     cmt_pat
1075                                 } else {
1076                                     let cmt_pat_ty = cmt_pat.ty;
1077                                     mc.cat_downcast(pat, cmt_pat, cmt_pat_ty, variant_did)
1078                                 };
1079
1080                             debug!("variant downcast_cmt={:?} pat={:?}",
1081                                    downcast_cmt,
1082                                    pat);
1083
1084                             delegate.matched_pat(pat, downcast_cmt, match_mode);
1085                         }
1086
1087                         Some(def::DefStruct(..)) | Some(def::DefTy(_, false)) => {
1088                             // A struct (in either the value or type
1089                             // namespace; we encounter the former on
1090                             // e.g. patterns for unit structs).
1091
1092                             debug!("struct cmt_pat={:?} pat={:?}",
1093                                    cmt_pat,
1094                                    pat);
1095
1096                             delegate.matched_pat(pat, cmt_pat, match_mode);
1097                         }
1098
1099                         Some(def::DefConst(..)) |
1100                         Some(def::DefAssociatedConst(..)) |
1101                         Some(def::DefLocal(..)) => {
1102                             // This is a leaf (i.e. identifier binding
1103                             // or constant value to match); thus no
1104                             // `matched_pat` call.
1105                         }
1106
1107                         Some(def @ def::DefTy(_, true)) => {
1108                             // An enum's type -- should never be in a
1109                             // pattern.
1110
1111                             if !tcx.sess.has_errors() {
1112                                 let msg = format!("Pattern has unexpected type: {:?} and type {:?}",
1113                                                   def,
1114                                                   cmt_pat.ty);
1115                                 tcx.sess.span_bug(pat.span, &msg)
1116                             }
1117                         }
1118
1119                         Some(def) => {
1120                             // Remaining cases are e.g. DefFn, to
1121                             // which identifiers within patterns
1122                             // should not resolve. However, we do
1123                             // encouter this when using the
1124                             // expr-use-visitor during typeck. So just
1125                             // ignore it, an error should have been
1126                             // reported.
1127
1128                             if !tcx.sess.has_errors() {
1129                                 let msg = format!("Pattern has unexpected def: {:?} and type {:?}",
1130                                                   def,
1131                                                   cmt_pat.ty);
1132                                 tcx.sess.span_bug(pat.span, &msg[..])
1133                             }
1134                         }
1135                     }
1136                 }
1137
1138                 hir::PatIdent(_, _, Some(_)) => {
1139                     // Do nothing; this is a binding (not a enum
1140                     // variant or struct), and the cat_pattern call
1141                     // will visit the substructure recursively.
1142                 }
1143
1144                 hir::PatWild(_) | hir::PatTup(..) | hir::PatBox(..) |
1145                 hir::PatRegion(..) | hir::PatLit(..) | hir::PatRange(..) |
1146                 hir::PatVec(..) => {
1147                     // Similarly, each of these cases does not
1148                     // correspond to a enum variant or struct, so we
1149                     // do not do any `matched_pat` calls for these
1150                     // cases either.
1151                 }
1152             }
1153         }));
1154     }
1155
1156     fn walk_captures(&mut self, closure_expr: &hir::Expr) {
1157         debug!("walk_captures({:?})", closure_expr);
1158
1159         self.tcx().with_freevars(closure_expr.id, |freevars| {
1160             for freevar in freevars {
1161                 let id_var = freevar.def.var_id();
1162                 let upvar_id = ty::UpvarId { var_id: id_var,
1163                                              closure_expr_id: closure_expr.id };
1164                 let upvar_capture = self.typer.upvar_capture(upvar_id).unwrap();
1165                 let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.id,
1166                                                                    closure_expr.span,
1167                                                                    freevar.def));
1168                 match upvar_capture {
1169                     ty::UpvarCapture::ByValue => {
1170                         let mode = copy_or_move(self.typer, &cmt_var, CaptureMove);
1171                         self.delegate.consume(closure_expr.id, freevar.span, cmt_var, mode);
1172                     }
1173                     ty::UpvarCapture::ByRef(upvar_borrow) => {
1174                         self.delegate.borrow(closure_expr.id,
1175                                              closure_expr.span,
1176                                              cmt_var,
1177                                              upvar_borrow.region,
1178                                              upvar_borrow.kind,
1179                                              ClosureCapture(freevar.span));
1180                     }
1181                 }
1182             }
1183         });
1184     }
1185
1186     fn cat_captured_var(&mut self,
1187                         closure_id: ast::NodeId,
1188                         closure_span: Span,
1189                         upvar_def: def::Def)
1190                         -> mc::McResult<mc::cmt<'tcx>> {
1191         // Create the cmt for the variable being borrowed, from the
1192         // caller's perspective
1193         let var_id = upvar_def.var_id();
1194         let var_ty = try!(self.typer.node_ty(var_id));
1195         self.mc.cat_def(closure_id, closure_span, var_ty, upvar_def)
1196     }
1197 }
1198
1199 fn copy_or_move<'a, 'tcx>(typer: &infer::InferCtxt<'a, 'tcx>,
1200                       cmt: &mc::cmt<'tcx>,
1201                       move_reason: MoveReason)
1202                       -> ConsumeMode
1203 {
1204     if typer.type_moves_by_default(cmt.ty, cmt.span) {
1205         Move(move_reason)
1206     } else {
1207         Copy
1208     }
1209 }