src/librustc/middle/expr_use_visitor.rs

   1 //! A different sort of visitor for walking fn bodies. Unlike the
   2 //! normal visitor, which just walks the entire body in one shot, the
   3 //! `ExprUseVisitor` determines how expressions are being used.
   4
   5 pub use self::LoanCause::*;
   6 pub use self::ConsumeMode::*;
   7 pub use self::MoveReason::*;
   8 pub use self::MatchMode::*;
   9 use self::TrackMatchMode::*;
  10 use self::OverloadedCallType::*;
  11
  12 use crate::hir::def::{CtorOf, Res, DefKind};
  13 use crate::hir::def_id::DefId;
  14 use crate::hir::ptr::P;
  15 use crate::infer::InferCtxt;
  16 use crate::middle::mem_categorization as mc;
  17 use crate::middle::region;
  18 use crate::ty::{self, DefIdTree, TyCtxt, adjustment};
  19
  20 use crate::hir::{self, PatKind};
  21 use std::rc::Rc;
  22 use syntax_pos::Span;
  23
  24 ///////////////////////////////////////////////////////////////////////////
  25 // The Delegate trait
  26
  27 /// This trait defines the callbacks you can expect to receive when
  28 /// employing the ExprUseVisitor.
  29 pub trait Delegate<'tcx> {
  30     // The value found at `cmt` is either copied or moved, depending
  31     // on mode.
  32     fn consume(&mut self,
  33                consume_id: hir::HirId,
  34                consume_span: Span,
  35                cmt: &mc::cmt_<'tcx>,
  36                mode: ConsumeMode);
  37
  38     // The value found at `cmt` has been determined to match the
  39     // pattern binding `matched_pat`, and its subparts are being
  40     // copied or moved depending on `mode`.  Note that `matched_pat`
  41     // is called on all variant/structs in the pattern (i.e., the
  42     // interior nodes of the pattern's tree structure) while
  43     // consume_pat is called on the binding identifiers in the pattern
  44     // (which are leaves of the pattern's tree structure).
  45     //
  46     // Note that variants/structs and identifiers are disjoint; thus
  47     // `matched_pat` and `consume_pat` are never both called on the
  48     // same input pattern structure (though of `consume_pat` can be
  49     // called on a subpart of an input passed to `matched_pat).
  50     fn matched_pat(&mut self,
  51                    matched_pat: &hir::Pat,
  52                    cmt: &mc::cmt_<'tcx>,
  53                    mode: MatchMode);
  54
  55     // The value found at `cmt` is either copied or moved via the
  56     // pattern binding `consume_pat`, depending on mode.
  57     fn consume_pat(&mut self,
  58                    consume_pat: &hir::Pat,
  59                    cmt: &mc::cmt_<'tcx>,
  60                    mode: ConsumeMode);
  61
  62     // The value found at `borrow` is being borrowed at the point
  63     // `borrow_id` for the region `loan_region` with kind `bk`.
  64     fn borrow(&mut self,
  65               borrow_id: hir::HirId,
  66               borrow_span: Span,
  67               cmt: &mc::cmt_<'tcx>,
  68               loan_region: ty::Region<'tcx>,
  69               bk: ty::BorrowKind,
  70               loan_cause: LoanCause);
  71
  72     // The local variable `id` is declared but not initialized.
  73     fn decl_without_init(&mut self,
  74                          id: hir::HirId,
  75                          span: Span);
  76
  77     // The path at `cmt` is being assigned to.
  78     fn mutate(&mut self,
  79               assignment_id: hir::HirId,
  80               assignment_span: Span,
  81               assignee_cmt: &mc::cmt_<'tcx>,
  82               mode: MutateMode);
  83
  84     // A nested closure or generator - only one layer deep.
  85     fn nested_body(&mut self, _body_id: hir::BodyId) {}
  86 }
  87
  88 #[derive(Copy, Clone, PartialEq, Debug)]
  89 pub enum LoanCause {
  90     ClosureCapture(Span),
  91     AddrOf,
  92     AutoRef,
  93     AutoUnsafe,
  94     RefBinding,
  95     OverloadedOperator,
  96     ClosureInvocation,
  97     ForLoop,
  98     MatchDiscriminant
  99 }
 100
 101 #[derive(Copy, Clone, PartialEq, Debug)]
 102 pub enum ConsumeMode {
 103     Copy,                // reference to x where x has a type that copies
 104     Move(MoveReason),    // reference to x where x has a type that moves
 105 }
 106
 107 #[derive(Copy, Clone, PartialEq, Debug)]
 108 pub enum MoveReason {
 109     DirectRefMove,
 110     PatBindingMove,
 111     CaptureMove,
 112 }
 113
 114 #[derive(Copy, Clone, PartialEq, Debug)]
 115 pub enum MatchMode {
 116     NonBindingMatch,
 117     BorrowingMatch,
 118     CopyingMatch,
 119     MovingMatch,
 120 }
 121
 122 #[derive(Copy, Clone, PartialEq, Debug)]
 123 enum TrackMatchMode {
 124     Unknown,
 125     Definite(MatchMode),
 126     Conflicting,
 127 }
 128
 129 impl TrackMatchMode {
 130     // Builds up the whole match mode for a pattern from its constituent
 131     // parts.  The lattice looks like this:
 132     //
 133     //          Conflicting
 134     //            /     \
 135     //           /       \
 136     //      Borrowing   Moving
 137     //           \       /
 138     //            \     /
 139     //            Copying
 140     //               |
 141     //          NonBinding
 142     //               |
 143     //            Unknown
 144     //
 145     // examples:
 146     //
 147     // * `(_, some_int)` pattern is Copying, since
 148     //   NonBinding + Copying => Copying
 149     //
 150     // * `(some_int, some_box)` pattern is Moving, since
 151     //   Copying + Moving => Moving
 152     //
 153     // * `(ref x, some_box)` pattern is Conflicting, since
 154     //   Borrowing + Moving => Conflicting
 155     //
 156     // Note that the `Unknown` and `Conflicting` states are
 157     // represented separately from the other more interesting
 158     // `Definite` states, which simplifies logic here somewhat.
 159     fn lub(&mut self, mode: MatchMode) {
 160         *self = match (*self, mode) {
 161             // Note that clause order below is very significant.
 162             (Unknown, new) => Definite(new),
 163             (Definite(old), new) if old == new => Definite(old),
 164
 165             (Definite(old), NonBindingMatch) => Definite(old),
 166             (Definite(NonBindingMatch), new) => Definite(new),
 167
 168             (Definite(old), CopyingMatch) => Definite(old),
 169             (Definite(CopyingMatch), new) => Definite(new),
 170
 171             (Definite(_), _) => Conflicting,
 172             (Conflicting, _) => *self,
 173         };
 174     }
 175
 176     fn match_mode(&self) -> MatchMode {
 177         match *self {
 178             Unknown => NonBindingMatch,
 179             Definite(mode) => mode,
 180             Conflicting => {
 181                 // Conservatively return MovingMatch to let the
 182                 // compiler continue to make progress.
 183                 MovingMatch
 184             }
 185         }
 186     }
 187 }
 188
 189 #[derive(Copy, Clone, PartialEq, Debug)]
 190 pub enum MutateMode {
 191     Init,
 192     JustWrite,    // x = y
 193     WriteAndRead, // x += y
 194 }
 195
 196 #[derive(Copy, Clone)]
 197 enum OverloadedCallType {
 198     FnOverloadedCall,
 199     FnMutOverloadedCall,
 200     FnOnceOverloadedCall,
 201 }
 202
 203 impl OverloadedCallType {
 204     fn from_trait_id(tcx: TyCtxt<'_>, trait_id: DefId) -> OverloadedCallType {
 205         for &(maybe_function_trait, overloaded_call_type) in &[
 206             (tcx.lang_items().fn_once_trait(), FnOnceOverloadedCall),
 207             (tcx.lang_items().fn_mut_trait(), FnMutOverloadedCall),
 208             (tcx.lang_items().fn_trait(), FnOverloadedCall)
 209         ] {
 210             match maybe_function_trait {
 211                 Some(function_trait) if function_trait == trait_id => {
 212                     return overloaded_call_type
 213                 }
 214                 _ => continue,
 215             }
 216         }
 217
 218         bug!("overloaded call didn't map to known function trait")
 219     }
 220
 221     fn from_method_id(tcx: TyCtxt<'_>, method_id: DefId) -> OverloadedCallType {
 222         let method = tcx.associated_item(method_id);
 223         OverloadedCallType::from_trait_id(tcx, method.container.id())
 224     }
 225 }
 226
 227 ///////////////////////////////////////////////////////////////////////////
 228 // The ExprUseVisitor type
 229 //
 230 // This is the code that actually walks the tree.
 231 pub struct ExprUseVisitor<'a, 'tcx> {
 232     mc: mc::MemCategorizationContext<'a, 'tcx>,
 233     delegate: &'a mut dyn Delegate<'tcx>,
 234     param_env: ty::ParamEnv<'tcx>,
 235 }
 236
 237 // If the MC results in an error, it's because the type check
 238 // failed (or will fail, when the error is uncovered and reported
 239 // during writeback). In this case, we just ignore this part of the
 240 // code.
 241 //
 242 // Note that this macro appears similar to try!(), but, unlike try!(),
 243 // it does not propagate the error.
 244 macro_rules! return_if_err {
 245     ($inp: expr) => (
 246         match $inp {
 247             Ok(v) => v,
 248             Err(()) => {
 249                 debug!("mc reported err");
 250                 return
 251             }
 252         }
 253     )
 254 }
 255
 256 impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
 257     /// Creates the ExprUseVisitor, configuring it with the various options provided:
 258     ///
 259     /// - `delegate` -- who receives the callbacks
 260     /// - `param_env` --- parameter environment for trait lookups (esp. pertaining to `Copy`)
 261     /// - `region_scope_tree` --- region scope tree for the code being analyzed
 262     /// - `tables` --- typeck results for the code being analyzed
 263     ///
 264     /// See also `with_infer`, which is used *during* typeck.
 265     pub fn new(
 266         delegate: &'a mut (dyn Delegate<'tcx> + 'a),
 267         tcx: TyCtxt<'tcx>,
 268         body_owner: DefId,
 269         param_env: ty::ParamEnv<'tcx>,
 270         region_scope_tree: &'a region::ScopeTree,
 271         tables: &'a ty::TypeckTables<'tcx>,
 272     ) -> Self {
 273         ExprUseVisitor {
 274             mc: mc::MemCategorizationContext::new(tcx,
 275                                                   param_env,
 276                                                   body_owner,
 277                                                   region_scope_tree,
 278                                                   tables),
 279             delegate,
 280             param_env,
 281         }
 282     }
 283 }
 284
 285 impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
 286     pub fn with_infer(
 287         delegate: &'a mut (dyn Delegate<'tcx> + 'a),
 288         infcx: &'a InferCtxt<'a, 'tcx>,
 289         body_owner: DefId,
 290         param_env: ty::ParamEnv<'tcx>,
 291         region_scope_tree: &'a region::ScopeTree,
 292         tables: &'a ty::TypeckTables<'tcx>,
 293     ) -> Self {
 294         ExprUseVisitor {
 295             mc: mc::MemCategorizationContext::with_infer(
 296                 infcx,
 297                 param_env,
 298                 body_owner,
 299                 region_scope_tree,
 300                 tables,
 301             ),
 302             delegate,
 303             param_env,
 304         }
 305     }
 306
 307     pub fn consume_body(&mut self, body: &hir::Body) {
 308         debug!("consume_body(body={:?})", body);
 309
 310         for param in &body.params {
 311             let param_ty = return_if_err!(self.mc.pat_ty_adjusted(&param.pat));
 312             debug!("consume_body: param_ty = {:?}", param_ty);
 313
 314             let param_cmt = Rc::new(self.mc.cat_rvalue(
 315                 param.hir_id,
 316                 param.pat.span,
 317                 param_ty));
 318
 319             self.walk_irrefutable_pat(param_cmt, &param.pat);
 320         }
 321
 322         self.consume_expr(&body.value);
 323     }
 324
 325     fn tcx(&self) -> TyCtxt<'tcx> {
 326         self.mc.tcx
 327     }
 328
 329     fn delegate_consume(&mut self,
 330                         consume_id: hir::HirId,
 331                         consume_span: Span,
 332                         cmt: &mc::cmt_<'tcx>) {
 333         debug!("delegate_consume(consume_id={}, cmt={:?})",
 334                consume_id, cmt);
 335
 336         let mode = copy_or_move(&self.mc, self.param_env, cmt, DirectRefMove);
 337         self.delegate.consume(consume_id, consume_span, cmt, mode);
 338     }
 339
 340     fn consume_exprs(&mut self, exprs: &[hir::Expr]) {
 341         for expr in exprs {
 342             self.consume_expr(&expr);
 343         }
 344     }
 345
 346     pub fn consume_expr(&mut self, expr: &hir::Expr) {
 347         debug!("consume_expr(expr={:?})", expr);
 348
 349         let cmt = return_if_err!(self.mc.cat_expr(expr));
 350         self.delegate_consume(expr.hir_id, expr.span, &cmt);
 351         self.walk_expr(expr);
 352     }
 353
 354     fn mutate_expr(&mut self,
 355                    span: Span,
 356                    assignment_expr: &hir::Expr,
 357                    expr: &hir::Expr,
 358                    mode: MutateMode) {
 359         let cmt = return_if_err!(self.mc.cat_expr(expr));
 360         self.delegate.mutate(assignment_expr.hir_id, span, &cmt, mode);
 361         self.walk_expr(expr);
 362     }
 363
 364     fn borrow_expr(&mut self,
 365                    expr: &hir::Expr,
 366                    r: ty::Region<'tcx>,
 367                    bk: ty::BorrowKind,
 368                    cause: LoanCause) {
 369         debug!("borrow_expr(expr={:?}, r={:?}, bk={:?})",
 370                expr, r, bk);
 371
 372         let cmt = return_if_err!(self.mc.cat_expr(expr));
 373         self.delegate.borrow(expr.hir_id, expr.span, &cmt, r, bk, cause);
 374
 375         self.walk_expr(expr)
 376     }
 377
 378     fn select_from_expr(&mut self, expr: &hir::Expr) {
 379         self.walk_expr(expr)
 380     }
 381
 382     pub fn walk_expr(&mut self, expr: &hir::Expr) {
 383         debug!("walk_expr(expr={:?})", expr);
 384
 385         self.walk_adjustment(expr);
 386
 387         match expr.kind {
 388             hir::ExprKind::Path(_) => { }
 389
 390             hir::ExprKind::Type(ref subexpr, _) => {
 391                 self.walk_expr(&subexpr)
 392             }
 393
 394             hir::ExprKind::Unary(hir::UnDeref, ref base) => { // *base
 395                 self.select_from_expr(&base);
 396             }
 397
 398             hir::ExprKind::Field(ref base, _) => { // base.f
 399                 self.select_from_expr(&base);
 400             }
 401
 402             hir::ExprKind::Index(ref lhs, ref rhs) => { // lhs[rhs]
 403                 self.select_from_expr(&lhs);
 404                 self.consume_expr(&rhs);
 405             }
 406
 407             hir::ExprKind::Call(ref callee, ref args) => { // callee(args)
 408                 self.walk_callee(expr, &callee);
 409                 self.consume_exprs(args);
 410             }
 411
 412             hir::ExprKind::MethodCall(.., ref args) => { // callee.m(args)
 413                 self.consume_exprs(args);
 414             }
 415
 416             hir::ExprKind::Struct(_, ref fields, ref opt_with) => {
 417                 self.walk_struct_expr(fields, opt_with);
 418             }
 419
 420             hir::ExprKind::Tup(ref exprs) => {
 421                 self.consume_exprs(exprs);
 422             }
 423
 424             hir::ExprKind::Match(ref discr, ref arms, _) => {
 425                 let discr_cmt = Rc::new(return_if_err!(self.mc.cat_expr(&discr)));
 426                 let r = self.tcx().lifetimes.re_empty;
 427                 self.borrow_expr(&discr, r, ty::ImmBorrow, MatchDiscriminant);
 428
 429                 // treatment of the discriminant is handled while walking the arms.
 430                 for arm in arms {
 431                     let mode = self.arm_move_mode(discr_cmt.clone(), arm);
 432                     let mode = mode.match_mode();
 433                     self.walk_arm(discr_cmt.clone(), arm, mode);
 434                 }
 435             }
 436
 437             hir::ExprKind::Array(ref exprs) => {
 438                 self.consume_exprs(exprs);
 439             }
 440
 441             hir::ExprKind::AddrOf(m, ref base) => {   // &base
 442                 // make sure that the thing we are pointing out stays valid
 443                 // for the lifetime `scope_r` of the resulting ptr:
 444                 let expr_ty = return_if_err!(self.mc.expr_ty(expr));
 445                 if let ty::Ref(r, _, _) = expr_ty.kind {
 446                     let bk = ty::BorrowKind::from_mutbl(m);
 447                     self.borrow_expr(&base, r, bk, AddrOf);
 448                 }
 449             }
 450
 451             hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
 452                 for (o, output) in ia.outputs.iter().zip(outputs) {
 453                     if o.is_indirect {
 454                         self.consume_expr(output);
 455                     } else {
 456                         self.mutate_expr(
 457                             output.span,
 458                             expr,
 459                             output,
 460                             if o.is_rw {
 461                                 MutateMode::WriteAndRead
 462                             } else {
 463                                 MutateMode::JustWrite
 464                             },
 465                         );
 466                     }
 467                 }
 468                 self.consume_exprs(inputs);
 469             }
 470
 471             hir::ExprKind::Continue(..) |
 472             hir::ExprKind::Lit(..) |
 473             hir::ExprKind::Err => {}
 474
 475             hir::ExprKind::Loop(ref blk, _, _) => {
 476                 self.walk_block(&blk);
 477             }
 478
 479             hir::ExprKind::Unary(_, ref lhs) => {
 480                 self.consume_expr(&lhs);
 481             }
 482
 483             hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
 484                 self.consume_expr(&lhs);
 485                 self.consume_expr(&rhs);
 486             }
 487
 488             hir::ExprKind::Block(ref blk, _) => {
 489                 self.walk_block(&blk);
 490             }
 491
 492             hir::ExprKind::Break(_, ref opt_expr) | hir::ExprKind::Ret(ref opt_expr) => {
 493                 if let Some(ref expr) = *opt_expr {
 494                     self.consume_expr(&expr);
 495                 }
 496             }
 497
 498             hir::ExprKind::Assign(ref lhs, ref rhs) => {
 499                 self.mutate_expr(expr.span, expr, &lhs, MutateMode::JustWrite);
 500                 self.consume_expr(&rhs);
 501             }
 502
 503             hir::ExprKind::Cast(ref base, _) => {
 504                 self.consume_expr(&base);
 505             }
 506
 507             hir::ExprKind::DropTemps(ref expr) => {
 508                 self.consume_expr(&expr);
 509             }
 510
 511             hir::ExprKind::AssignOp(_, ref lhs, ref rhs) => {
 512                 if self.mc.tables.is_method_call(expr) {
 513                     self.consume_expr(lhs);
 514                 } else {
 515                     self.mutate_expr(expr.span, expr, &lhs, MutateMode::WriteAndRead);
 516                 }
 517                 self.consume_expr(&rhs);
 518             }
 519
 520             hir::ExprKind::Repeat(ref base, _) => {
 521                 self.consume_expr(&base);
 522             }
 523
 524             hir::ExprKind::Closure(_, _, body_id, fn_decl_span, _) => {
 525                 self.delegate.nested_body(body_id);
 526                 self.walk_captures(expr, fn_decl_span);
 527             }
 528
 529             hir::ExprKind::Box(ref base) => {
 530                 self.consume_expr(&base);
 531             }
 532
 533             hir::ExprKind::Yield(ref value, _) => {
 534                 self.consume_expr(&value);
 535             }
 536         }
 537     }
 538
 539     fn walk_callee(&mut self, call: &hir::Expr, callee: &hir::Expr) {
 540         let callee_ty = return_if_err!(self.mc.expr_ty_adjusted(callee));
 541         debug!("walk_callee: callee={:?} callee_ty={:?}",
 542                callee, callee_ty);
 543         match callee_ty.kind {
 544             ty::FnDef(..) | ty::FnPtr(_) => {
 545                 self.consume_expr(callee);
 546             }
 547             ty::Error => { }
 548             _ => {
 549                 if let Some(def_id) = self.mc.tables.type_dependent_def_id(call.hir_id) {
 550                     let call_scope = region::Scope {
 551                         id: call.hir_id.local_id,
 552                         data: region::ScopeData::Node
 553                     };
 554                     match OverloadedCallType::from_method_id(self.tcx(), def_id) {
 555                         FnMutOverloadedCall => {
 556                             let call_scope_r = self.tcx().mk_region(ty::ReScope(call_scope));
 557                             self.borrow_expr(callee,
 558                                             call_scope_r,
 559                                             ty::MutBorrow,
 560                                             ClosureInvocation);
 561                         }
 562                         FnOverloadedCall => {
 563                             let call_scope_r = self.tcx().mk_region(ty::ReScope(call_scope));
 564                             self.borrow_expr(callee,
 565                                             call_scope_r,
 566                                             ty::ImmBorrow,
 567                                             ClosureInvocation);
 568                         }
 569                         FnOnceOverloadedCall => self.consume_expr(callee),
 570                     }
 571                 } else {
 572                     self.tcx().sess.delay_span_bug(call.span,
 573                                                    "no type-dependent def for overloaded call");
 574                 }
 575             }
 576         }
 577     }
 578
 579     fn walk_stmt(&mut self, stmt: &hir::Stmt) {
 580         match stmt.kind {
 581             hir::StmtKind::Local(ref local) => {
 582                 self.walk_local(&local);
 583             }
 584
 585             hir::StmtKind::Item(_) => {
 586                 // We don't visit nested items in this visitor,
 587                 // only the fn body we were given.
 588             }
 589
 590             hir::StmtKind::Expr(ref expr) |
 591             hir::StmtKind::Semi(ref expr) => {
 592                 self.consume_expr(&expr);
 593             }
 594         }
 595     }
 596
 597     fn walk_local(&mut self, local: &hir::Local) {
 598         match local.init {
 599             None => {
 600                 local.pat.each_binding(|_, hir_id, span, _| {
 601                     self.delegate.decl_without_init(hir_id, span);
 602                 })
 603             }
 604
 605             Some(ref expr) => {
 606                 // Variable declarations with
 607                 // initializers are considered
 608                 // "assigns", which is handled by
 609                 // `walk_pat`:
 610                 self.walk_expr(&expr);
 611                 let init_cmt = Rc::new(return_if_err!(self.mc.cat_expr(&expr)));
 612                 self.walk_irrefutable_pat(init_cmt, &local.pat);
 613             }
 614         }
 615     }
 616
 617     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 618     /// depending on its type.
 619     fn walk_block(&mut self, blk: &hir::Block) {
 620         debug!("walk_block(blk.hir_id={})", blk.hir_id);
 621
 622         for stmt in &blk.stmts {
 623             self.walk_stmt(stmt);
 624         }
 625
 626         if let Some(ref tail_expr) = blk.expr {
 627             self.consume_expr(&tail_expr);
 628         }
 629     }
 630
 631     fn walk_struct_expr(&mut self,
 632                         fields: &[hir::Field],
 633                         opt_with: &Option<P<hir::Expr>>) {
 634         // Consume the expressions supplying values for each field.
 635         for field in fields {
 636             self.consume_expr(&field.expr);
 637         }
 638
 639         let with_expr = match *opt_with {
 640             Some(ref w) => &**w,
 641             None => { return; }
 642         };
 643
 644         let with_cmt = Rc::new(return_if_err!(self.mc.cat_expr(&with_expr)));
 645
 646         // Select just those fields of the `with`
 647         // expression that will actually be used
 648         match with_cmt.ty.kind {
 649             ty::Adt(adt, substs) if adt.is_struct() => {
 650                 // Consume those fields of the with expression that are needed.
 651                 for (f_index, with_field) in adt.non_enum_variant().fields.iter().enumerate() {
 652                     let is_mentioned = fields.iter().any(|f| {
 653                         self.tcx().field_index(f.hir_id, self.mc.tables) == f_index
 654                     });
 655                     if !is_mentioned {
 656                         let cmt_field = self.mc.cat_field(
 657                             &*with_expr,
 658                             with_cmt.clone(),
 659                             f_index,
 660                             with_field.ident,
 661                             with_field.ty(self.tcx(), substs)
 662                         );
 663                         self.delegate_consume(with_expr.hir_id, with_expr.span, &cmt_field);
 664                     }
 665                 }
 666             }
 667             _ => {
 668                 // the base expression should always evaluate to a
 669                 // struct; however, when EUV is run during typeck, it
 670                 // may not. This will generate an error earlier in typeck,
 671                 // so we can just ignore it.
 672                 if !self.tcx().sess.has_errors() {
 673                     span_bug!(
 674                         with_expr.span,
 675                         "with expression doesn't evaluate to a struct");
 676                 }
 677             }
 678         }
 679
 680         // walk the with expression so that complex expressions
 681         // are properly handled.
 682         self.walk_expr(with_expr);
 683     }
 684
 685     // Invoke the appropriate delegate calls for anything that gets
 686     // consumed or borrowed as part of the automatic adjustment
 687     // process.
 688     fn walk_adjustment(&mut self, expr: &hir::Expr) {
 689         let adjustments = self.mc.tables.expr_adjustments(expr);
 690         let mut cmt = return_if_err!(self.mc.cat_expr_unadjusted(expr));
 691         for adjustment in adjustments {
 692             debug!("walk_adjustment expr={:?} adj={:?}", expr, adjustment);
 693             match adjustment.kind {
 694                 adjustment::Adjust::NeverToAny |
 695                 adjustment::Adjust::Pointer(_)  => {
 696                     // Creating a closure/fn-pointer or unsizing consumes
 697                     // the input and stores it into the resulting rvalue.
 698                     self.delegate_consume(expr.hir_id, expr.span, &cmt);
 699                 }
 700
 701                 adjustment::Adjust::Deref(None) => {}
 702
 703                 // Autoderefs for overloaded Deref calls in fact reference
 704                 // their receiver. That is, if we have `(*x)` where `x`
 705                 // is of type `Rc<T>`, then this in fact is equivalent to
 706                 // `x.deref()`. Since `deref()` is declared with `&self`,
 707                 // this is an autoref of `x`.
 708                 adjustment::Adjust::Deref(Some(ref deref)) => {
 709                     let bk = ty::BorrowKind::from_mutbl(deref.mutbl);
 710                     self.delegate.borrow(expr.hir_id, expr.span, &cmt, deref.region, bk, AutoRef);
 711                 }
 712
 713                 adjustment::Adjust::Borrow(ref autoref) => {
 714                     self.walk_autoref(expr, &cmt, autoref);
 715                 }
 716             }
 717             cmt = return_if_err!(self.mc.cat_expr_adjusted(expr, cmt, &adjustment));
 718         }
 719     }
 720
 721     /// Walks the autoref `autoref` applied to the autoderef'd
 722     /// `expr`. `cmt_base` is the mem-categorized form of `expr`
 723     /// after all relevant autoderefs have occurred.
 724     fn walk_autoref(&mut self,
 725                     expr: &hir::Expr,
 726                     cmt_base: &mc::cmt_<'tcx>,
 727                     autoref: &adjustment::AutoBorrow<'tcx>) {
 728         debug!("walk_autoref(expr.hir_id={} cmt_base={:?} autoref={:?})",
 729                expr.hir_id,
 730                cmt_base,
 731                autoref);
 732
 733         match *autoref {
 734             adjustment::AutoBorrow::Ref(r, m) => {
 735                 self.delegate.borrow(expr.hir_id,
 736                                      expr.span,
 737                                      cmt_base,
 738                                      r,
 739                                      ty::BorrowKind::from_mutbl(m.into()),
 740                                      AutoRef);
 741             }
 742
 743             adjustment::AutoBorrow::RawPtr(m) => {
 744                 debug!("walk_autoref: expr.hir_id={} cmt_base={:?}",
 745                        expr.hir_id,
 746                        cmt_base);
 747
 748                 // Converting from a &T to *T (or &mut T to *mut T) is
 749                 // treated as borrowing it for the enclosing temporary
 750                 // scope.
 751                 let r = self.tcx().mk_region(ty::ReScope(
 752                     region::Scope {
 753                         id: expr.hir_id.local_id,
 754                         data: region::ScopeData::Node
 755                     }));
 756
 757                 self.delegate.borrow(expr.hir_id,
 758                                      expr.span,
 759                                      cmt_base,
 760                                      r,
 761                                      ty::BorrowKind::from_mutbl(m),
 762                                      AutoUnsafe);
 763             }
 764         }
 765     }
 766
 767     fn arm_move_mode(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm) -> TrackMatchMode {
 768         let mut mode = Unknown;
 769         self.determine_pat_move_mode(discr_cmt.clone(), &arm.pat, &mut mode);
 770         mode
 771     }
 772
 773     fn walk_arm(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm, mode: MatchMode) {
 774         self.walk_pat(discr_cmt.clone(), &arm.pat, mode);
 775
 776         if let Some(hir::Guard::If(ref e)) = arm.guard {
 777             self.consume_expr(e)
 778         }
 779
 780         self.consume_expr(&arm.body);
 781     }
 782
 783     /// Walks a pat that occurs in isolation (i.e., top-level of fn argument or
 784     /// let binding, and *not* a match arm or nested pat.)
 785     fn walk_irrefutable_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat) {
 786         let mut mode = Unknown;
 787         self.determine_pat_move_mode(cmt_discr.clone(), pat, &mut mode);
 788         let mode = mode.match_mode();
 789         self.walk_pat(cmt_discr, pat, mode);
 790     }
 791
 792     /// Identifies any bindings within `pat` and accumulates within
 793     /// `mode` whether the overall pattern/match structure is a move,
 794     /// copy, or borrow.
 795     fn determine_pat_move_mode(&mut self,
 796                                cmt_discr: mc::cmt<'tcx>,
 797                                pat: &hir::Pat,
 798                                mode: &mut TrackMatchMode) {
 799         debug!("determine_pat_move_mode cmt_discr={:?} pat={:?}", cmt_discr, pat);
 800
 801         return_if_err!(self.mc.cat_pattern(cmt_discr, pat, |cmt_pat, pat| {
 802             if let PatKind::Binding(..) = pat.kind {
 803                 let bm = *self.mc.tables.pat_binding_modes()
 804                                         .get(pat.hir_id)
 805                                         .expect("missing binding mode");
 806                 match bm {
 807                     ty::BindByReference(..) =>
 808                         mode.lub(BorrowingMatch),
 809                     ty::BindByValue(..) => {
 810                         match copy_or_move(&self.mc, self.param_env, &cmt_pat, PatBindingMove) {
 811                             Copy => mode.lub(CopyingMatch),
 812                             Move(..) => mode.lub(MovingMatch),
 813                         }
 814                     }
 815                 }
 816             }
 817         }));
 818     }
 819
 820     /// The core driver for walking a pattern; `match_mode` must be
 821     /// established up front, e.g., via `determine_pat_move_mode` (see
 822     /// also `walk_irrefutable_pat` for patterns that stand alone).
 823     fn walk_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat, match_mode: MatchMode) {
 824         debug!("walk_pat(cmt_discr={:?}, pat={:?})", cmt_discr, pat);
 825
 826         let tcx = self.tcx();
 827         let ExprUseVisitor { ref mc, ref mut delegate, param_env } = *self;
 828         return_if_err!(mc.cat_pattern(cmt_discr.clone(), pat, |cmt_pat, pat| {
 829             if let PatKind::Binding(_, canonical_id, ..) = pat.kind {
 830                 debug!(
 831                     "walk_pat: binding cmt_pat={:?} pat={:?} match_mode={:?}",
 832                     cmt_pat,
 833                     pat,
 834                     match_mode,
 835                 );
 836                 if let Some(&bm) = mc.tables.pat_binding_modes().get(pat.hir_id) {
 837                     debug!("walk_pat: pat.hir_id={:?} bm={:?}", pat.hir_id, bm);
 838
 839                     // pat_ty: the type of the binding being produced.
 840                     let pat_ty = return_if_err!(mc.node_ty(pat.hir_id));
 841                     debug!("walk_pat: pat_ty={:?}", pat_ty);
 842
 843                     // Each match binding is effectively an assignment to the
 844                     // binding being produced.
 845                     let def = Res::Local(canonical_id);
 846                     if let Ok(ref binding_cmt) = mc.cat_res(pat.hir_id, pat.span, pat_ty, def) {
 847                         delegate.mutate(pat.hir_id, pat.span, binding_cmt, MutateMode::Init);
 848                     }
 849
 850                     // It is also a borrow or copy/move of the value being matched.
 851                     match bm {
 852                         ty::BindByReference(m) => {
 853                             if let ty::Ref(r, _, _) = pat_ty.kind {
 854                                 let bk = ty::BorrowKind::from_mutbl(m);
 855                                 delegate.borrow(pat.hir_id, pat.span, &cmt_pat, r, bk, RefBinding);
 856                             }
 857                         }
 858                         ty::BindByValue(..) => {
 859                             let mode = copy_or_move(mc, param_env, &cmt_pat, PatBindingMove);
 860                             debug!("walk_pat binding consuming pat");
 861                             delegate.consume_pat(pat, &cmt_pat, mode);
 862                         }
 863                     }
 864                 } else {
 865                     tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 866                 }
 867             }
 868         }));
 869
 870         // Do a second pass over the pattern, calling `matched_pat` on
 871         // the interior nodes (enum variants and structs), as opposed
 872         // to the above loop's visit of than the bindings that form
 873         // the leaves of the pattern tree structure.
 874         return_if_err!(mc.cat_pattern(cmt_discr, pat, |cmt_pat, pat| {
 875             let qpath = match pat.kind {
 876                 PatKind::Path(ref qpath) |
 877                 PatKind::TupleStruct(ref qpath, ..) |
 878                 PatKind::Struct(ref qpath, ..) => qpath,
 879                 _ => return
 880             };
 881             let res = mc.tables.qpath_res(qpath, pat.hir_id);
 882             match res {
 883                 Res::Def(DefKind::Ctor(CtorOf::Variant, ..), variant_ctor_did) => {
 884                     let variant_did = mc.tcx.parent(variant_ctor_did).unwrap();
 885                     let downcast_cmt = mc.cat_downcast_if_needed(pat, cmt_pat, variant_did);
 886
 887                     debug!("variantctor downcast_cmt={:?} pat={:?}", downcast_cmt, pat);
 888                     delegate.matched_pat(pat, &downcast_cmt, match_mode);
 889                 }
 890                 Res::Def(DefKind::Variant, variant_did) => {
 891                     let downcast_cmt = mc.cat_downcast_if_needed(pat, cmt_pat, variant_did);
 892
 893                     debug!("variant downcast_cmt={:?} pat={:?}", downcast_cmt, pat);
 894                     delegate.matched_pat(pat, &downcast_cmt, match_mode);
 895                 }
 896                 Res::Def(DefKind::Struct, _)
 897                 | Res::Def(DefKind::Ctor(..), _)
 898                 | Res::Def(DefKind::Union, _)
 899                 | Res::Def(DefKind::TyAlias, _)
 900                 | Res::Def(DefKind::AssocTy, _)
 901                 | Res::SelfTy(..) => {
 902                     debug!("struct cmt_pat={:?} pat={:?}", cmt_pat, pat);
 903                     delegate.matched_pat(pat, &cmt_pat, match_mode);
 904                 }
 905                 _ => {}
 906             }
 907         }));
 908     }
 909
 910     fn walk_captures(&mut self, closure_expr: &hir::Expr, fn_decl_span: Span) {
 911         debug!("walk_captures({:?})", closure_expr);
 912
 913         let closure_def_id = self.tcx().hir().local_def_id(closure_expr.hir_id);
 914         if let Some(upvars) = self.tcx().upvars(closure_def_id) {
 915             for (&var_id, upvar) in upvars.iter() {
 916                 let upvar_id = ty::UpvarId {
 917                     var_path: ty::UpvarPath { hir_id: var_id },
 918                     closure_expr_id: closure_def_id.to_local(),
 919                 };
 920                 let upvar_capture = self.mc.tables.upvar_capture(upvar_id);
 921                 let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.hir_id,
 922                                                                    fn_decl_span,
 923                                                                    var_id));
 924                 match upvar_capture {
 925                     ty::UpvarCapture::ByValue => {
 926                         let mode = copy_or_move(&self.mc,
 927                                                 self.param_env,
 928                                                 &cmt_var,
 929                                                 CaptureMove);
 930                         self.delegate.consume(closure_expr.hir_id, upvar.span, &cmt_var, mode);
 931                     }
 932                     ty::UpvarCapture::ByRef(upvar_borrow) => {
 933                         self.delegate.borrow(closure_expr.hir_id,
 934                                              fn_decl_span,
 935                                              &cmt_var,
 936                                              upvar_borrow.region,
 937                                              upvar_borrow.kind,
 938                                              ClosureCapture(upvar.span));
 939                     }
 940                 }
 941             }
 942         }
 943     }
 944
 945     fn cat_captured_var(&mut self,
 946                         closure_hir_id: hir::HirId,
 947                         closure_span: Span,
 948                         var_id: hir::HirId)
 949                         -> mc::McResult<mc::cmt_<'tcx>> {
 950         // Create the cmt for the variable being borrowed, from the
 951         // perspective of the creator (parent) of the closure.
 952         let var_ty = self.mc.node_ty(var_id)?;
 953         self.mc.cat_res(closure_hir_id, closure_span, var_ty, Res::Local(var_id))
 954     }
 955 }
 956
 957 fn copy_or_move<'a, 'tcx>(
 958     mc: &mc::MemCategorizationContext<'a, 'tcx>,
 959     param_env: ty::ParamEnv<'tcx>,
 960     cmt: &mc::cmt_<'tcx>,
 961     move_reason: MoveReason,
 962 ) -> ConsumeMode {
 963     if !mc.type_is_copy_modulo_regions(param_env, cmt.ty, cmt.span) {
 964         Move(move_reason)
 965     } else {
 966         Copy
 967     }
 968 }