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::Def;
  13 use crate::hir::def_id::DefId;
  14 use crate::infer::InferCtxt;
  15 use crate::middle::mem_categorization as mc;
  16 use crate::middle::region;
  17 use crate::ty::{self, TyCtxt, adjustment};
  18
  19 use crate::hir::{self, PatKind};
  20 use rustc_data_structures::sync::Lrc;
  21 use std::rc::Rc;
  22 use syntax::ptr::P;
  23 use syntax_pos::Span;
  24 use crate::util::nodemap::ItemLocalSet;
  25
  26 ///////////////////////////////////////////////////////////////////////////
  27 // The Delegate trait
  28
  29 /// This trait defines the callbacks you can expect to receive when
  30 /// employing the ExprUseVisitor.
  31 pub trait Delegate<'tcx> {
  32     // The value found at `cmt` is either copied or moved, depending
  33     // on mode.
  34     fn consume(&mut self,
  35                consume_id: hir::HirId,
  36                consume_span: Span,
  37                cmt: &mc::cmt_<'tcx>,
  38                mode: ConsumeMode);
  39
  40     // The value found at `cmt` has been determined to match the
  41     // pattern binding `matched_pat`, and its subparts are being
  42     // copied or moved depending on `mode`.  Note that `matched_pat`
  43     // is called on all variant/structs in the pattern (i.e., the
  44     // interior nodes of the pattern's tree structure) while
  45     // consume_pat is called on the binding identifiers in the pattern
  46     // (which are leaves of the pattern's tree structure).
  47     //
  48     // Note that variants/structs and identifiers are disjoint; thus
  49     // `matched_pat` and `consume_pat` are never both called on the
  50     // same input pattern structure (though of `consume_pat` can be
  51     // called on a subpart of an input passed to `matched_pat).
  52     fn matched_pat(&mut self,
  53                    matched_pat: &hir::Pat,
  54                    cmt: &mc::cmt_<'tcx>,
  55                    mode: MatchMode);
  56
  57     // The value found at `cmt` is either copied or moved via the
  58     // pattern binding `consume_pat`, depending on mode.
  59     fn consume_pat(&mut self,
  60                    consume_pat: &hir::Pat,
  61                    cmt: &mc::cmt_<'tcx>,
  62                    mode: ConsumeMode);
  63
  64     // The value found at `borrow` is being borrowed at the point
  65     // `borrow_id` for the region `loan_region` with kind `bk`.
  66     fn borrow(&mut self,
  67               borrow_id: hir::HirId,
  68               borrow_span: Span,
  69               cmt: &mc::cmt_<'tcx>,
  70               loan_region: ty::Region<'tcx>,
  71               bk: ty::BorrowKind,
  72               loan_cause: LoanCause);
  73
  74     // The local variable `id` is declared but not initialized.
  75     fn decl_without_init(&mut self,
  76                          id: hir::HirId,
  77                          span: Span);
  78
  79     // The path at `cmt` is being assigned to.
  80     fn mutate(&mut self,
  81               assignment_id: hir::HirId,
  82               assignment_span: Span,
  83               assignee_cmt: &mc::cmt_<'tcx>,
  84               mode: MutateMode);
  85
  86     // A nested closure or generator - only one layer deep.
  87     fn nested_body(&mut self, _body_id: hir::BodyId) {}
  88 }
  89
  90 #[derive(Copy, Clone, PartialEq, Debug)]
  91 pub enum LoanCause {
  92     ClosureCapture(Span),
  93     AddrOf,
  94     AutoRef,
  95     AutoUnsafe,
  96     RefBinding,
  97     OverloadedOperator,
  98     ClosureInvocation,
  99     ForLoop,
 100     MatchDiscriminant
 101 }
 102
 103 #[derive(Copy, Clone, PartialEq, Debug)]
 104 pub enum ConsumeMode {
 105     Copy,                // reference to x where x has a type that copies
 106     Move(MoveReason),    // reference to x where x has a type that moves
 107 }
 108
 109 #[derive(Copy, Clone, PartialEq, Debug)]
 110 pub enum MoveReason {
 111     DirectRefMove,
 112     PatBindingMove,
 113     CaptureMove,
 114 }
 115
 116 #[derive(Copy, Clone, PartialEq, Debug)]
 117 pub enum MatchMode {
 118     NonBindingMatch,
 119     BorrowingMatch,
 120     CopyingMatch,
 121     MovingMatch,
 122 }
 123
 124 #[derive(Copy, Clone, PartialEq, Debug)]
 125 enum TrackMatchMode {
 126     Unknown,
 127     Definite(MatchMode),
 128     Conflicting,
 129 }
 130
 131 impl TrackMatchMode {
 132     // Builds up the whole match mode for a pattern from its constituent
 133     // parts.  The lattice looks like this:
 134     //
 135     //          Conflicting
 136     //            /     \
 137     //           /       \
 138     //      Borrowing   Moving
 139     //           \       /
 140     //            \     /
 141     //            Copying
 142     //               |
 143     //          NonBinding
 144     //               |
 145     //            Unknown
 146     //
 147     // examples:
 148     //
 149     // * `(_, some_int)` pattern is Copying, since
 150     //   NonBinding + Copying => Copying
 151     //
 152     // * `(some_int, some_box)` pattern is Moving, since
 153     //   Copying + Moving => Moving
 154     //
 155     // * `(ref x, some_box)` pattern is Conflicting, since
 156     //   Borrowing + Moving => Conflicting
 157     //
 158     // Note that the `Unknown` and `Conflicting` states are
 159     // represented separately from the other more interesting
 160     // `Definite` states, which simplifies logic here somewhat.
 161     fn lub(&mut self, mode: MatchMode) {
 162         *self = match (*self, mode) {
 163             // Note that clause order below is very significant.
 164             (Unknown, new) => Definite(new),
 165             (Definite(old), new) if old == new => Definite(old),
 166
 167             (Definite(old), NonBindingMatch) => Definite(old),
 168             (Definite(NonBindingMatch), new) => Definite(new),
 169
 170             (Definite(old), CopyingMatch) => Definite(old),
 171             (Definite(CopyingMatch), new) => Definite(new),
 172
 173             (Definite(_), _) => Conflicting,
 174             (Conflicting, _) => *self,
 175         };
 176     }
 177
 178     fn match_mode(&self) -> MatchMode {
 179         match *self {
 180             Unknown => NonBindingMatch,
 181             Definite(mode) => mode,
 182             Conflicting => {
 183                 // Conservatively return MovingMatch to let the
 184                 // compiler continue to make progress.
 185                 MovingMatch
 186             }
 187         }
 188     }
 189 }
 190
 191 #[derive(Copy, Clone, PartialEq, Debug)]
 192 pub enum MutateMode {
 193     Init,
 194     JustWrite,    // x = y
 195     WriteAndRead, // x += y
 196 }
 197
 198 #[derive(Copy, Clone)]
 199 enum OverloadedCallType {
 200     FnOverloadedCall,
 201     FnMutOverloadedCall,
 202     FnOnceOverloadedCall,
 203 }
 204
 205 impl OverloadedCallType {
 206     fn from_trait_id(tcx: TyCtxt<'_, '_, '_>, trait_id: DefId) -> OverloadedCallType {
 207         for &(maybe_function_trait, overloaded_call_type) in &[
 208             (tcx.lang_items().fn_once_trait(), FnOnceOverloadedCall),
 209             (tcx.lang_items().fn_mut_trait(), FnMutOverloadedCall),
 210             (tcx.lang_items().fn_trait(), FnOverloadedCall)
 211         ] {
 212             match maybe_function_trait {
 213                 Some(function_trait) if function_trait == trait_id => {
 214                     return overloaded_call_type
 215                 }
 216                 _ => continue,
 217             }
 218         }
 219
 220         bug!("overloaded call didn't map to known function trait")
 221     }
 222
 223     fn from_method_id(tcx: TyCtxt<'_, '_, '_>, method_id: DefId) -> OverloadedCallType {
 224         let method = tcx.associated_item(method_id);
 225         OverloadedCallType::from_trait_id(tcx, method.container.id())
 226     }
 227 }
 228
 229 ///////////////////////////////////////////////////////////////////////////
 230 // The ExprUseVisitor type
 231 //
 232 // This is the code that actually walks the tree.
 233 pub struct ExprUseVisitor<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 234     mc: mc::MemCategorizationContext<'a, 'gcx, 'tcx>,
 235     delegate: &'a mut dyn Delegate<'tcx>,
 236     param_env: ty::ParamEnv<'tcx>,
 237 }
 238
 239 // If the MC results in an error, it's because the type check
 240 // failed (or will fail, when the error is uncovered and reported
 241 // during writeback). In this case, we just ignore this part of the
 242 // code.
 243 //
 244 // Note that this macro appears similar to try!(), but, unlike try!(),
 245 // it does not propagate the error.
 246 macro_rules! return_if_err {
 247     ($inp: expr) => (
 248         match $inp {
 249             Ok(v) => v,
 250             Err(()) => {
 251                 debug!("mc reported err");
 252                 return
 253             }
 254         }
 255     )
 256 }
 257
 258 impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx, 'tcx> {
 259     /// Creates the ExprUseVisitor, configuring it with the various options provided:
 260     ///
 261     /// - `delegate` -- who receives the callbacks
 262     /// - `param_env` --- parameter environment for trait lookups (esp. pertaining to `Copy`)
 263     /// - `region_scope_tree` --- region scope tree for the code being analyzed
 264     /// - `tables` --- typeck results for the code being analyzed
 265     /// - `rvalue_promotable_map` --- if you care about rvalue promotion, then provide
 266     ///   the map here (it can be computed with `tcx.rvalue_promotable_map(def_id)`).
 267     ///   `None` means that rvalues will be given more conservative lifetimes.
 268     ///
 269     /// See also `with_infer`, which is used *during* typeck.
 270     pub fn new(delegate: &'a mut (dyn Delegate<'tcx>+'a),
 271                tcx: TyCtxt<'a, 'tcx, 'tcx>,
 272                param_env: ty::ParamEnv<'tcx>,
 273                region_scope_tree: &'a region::ScopeTree,
 274                tables: &'a ty::TypeckTables<'tcx>,
 275                rvalue_promotable_map: Option<Lrc<ItemLocalSet>>)
 276                -> Self
 277     {
 278         ExprUseVisitor {
 279             mc: mc::MemCategorizationContext::new(tcx,
 280                                                   region_scope_tree,
 281                                                   tables,
 282                                                   rvalue_promotable_map),
 283             delegate,
 284             param_env,
 285         }
 286     }
 287 }
 288
 289 impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
 290     pub fn with_infer(delegate: &'a mut (dyn Delegate<'tcx>+'a),
 291                       infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 292                       param_env: ty::ParamEnv<'tcx>,
 293                       region_scope_tree: &'a region::ScopeTree,
 294                       tables: &'a ty::TypeckTables<'tcx>)
 295                       -> Self
 296     {
 297         ExprUseVisitor {
 298             mc: mc::MemCategorizationContext::with_infer(infcx, region_scope_tree, tables),
 299             delegate,
 300             param_env,
 301         }
 302     }
 303
 304     pub fn consume_body(&mut self, body: &hir::Body) {
 305         debug!("consume_body(body={:?})", body);
 306
 307         for arg in &body.arguments {
 308             let arg_ty = return_if_err!(self.mc.pat_ty_adjusted(&arg.pat));
 309             debug!("consume_body: arg_ty = {:?}", arg_ty);
 310
 311             let fn_body_scope_r =
 312                 self.tcx().mk_region(ty::ReScope(
 313                     region::Scope {
 314                         id: body.value.hir_id.local_id,
 315                         data: region::ScopeData::Node
 316                 }));
 317             let arg_cmt = Rc::new(self.mc.cat_rvalue(
 318                 arg.hir_id,
 319                 arg.pat.span,
 320                 fn_body_scope_r, // Args live only as long as the fn body.
 321                 arg_ty));
 322
 323             self.walk_irrefutable_pat(arg_cmt, &arg.pat);
 324         }
 325
 326         self.consume_expr(&body.value);
 327     }
 328
 329     fn tcx(&self) -> TyCtxt<'a, 'gcx, 'tcx> {
 330         self.mc.tcx
 331     }
 332
 333     fn delegate_consume(&mut self,
 334                         consume_id: hir::HirId,
 335                         consume_span: Span,
 336                         cmt: &mc::cmt_<'tcx>) {
 337         debug!("delegate_consume(consume_id={}, cmt={:?})",
 338                consume_id, cmt);
 339
 340         let mode = copy_or_move(&self.mc, self.param_env, cmt, DirectRefMove);
 341         self.delegate.consume(consume_id, consume_span, cmt, mode);
 342     }
 343
 344     fn consume_exprs(&mut self, exprs: &[hir::Expr]) {
 345         for expr in exprs {
 346             self.consume_expr(&expr);
 347         }
 348     }
 349
 350     pub fn consume_expr(&mut self, expr: &hir::Expr) {
 351         debug!("consume_expr(expr={:?})", expr);
 352
 353         let cmt = return_if_err!(self.mc.cat_expr(expr));
 354         self.delegate_consume(expr.hir_id, expr.span, &cmt);
 355         self.walk_expr(expr);
 356     }
 357
 358     fn mutate_expr(&mut self,
 359                    span: Span,
 360                    assignment_expr: &hir::Expr,
 361                    expr: &hir::Expr,
 362                    mode: MutateMode) {
 363         let cmt = return_if_err!(self.mc.cat_expr(expr));
 364         self.delegate.mutate(assignment_expr.hir_id, span, &cmt, mode);
 365         self.walk_expr(expr);
 366     }
 367
 368     fn borrow_expr(&mut self,
 369                    expr: &hir::Expr,
 370                    r: ty::Region<'tcx>,
 371                    bk: ty::BorrowKind,
 372                    cause: LoanCause) {
 373         debug!("borrow_expr(expr={:?}, r={:?}, bk={:?})",
 374                expr, r, bk);
 375
 376         let cmt = return_if_err!(self.mc.cat_expr(expr));
 377         self.delegate.borrow(expr.hir_id, expr.span, &cmt, r, bk, cause);
 378
 379         self.walk_expr(expr)
 380     }
 381
 382     fn select_from_expr(&mut self, expr: &hir::Expr) {
 383         self.walk_expr(expr)
 384     }
 385
 386     pub fn walk_expr(&mut self, expr: &hir::Expr) {
 387         debug!("walk_expr(expr={:?})", expr);
 388
 389         self.walk_adjustment(expr);
 390
 391         match expr.node {
 392             hir::ExprKind::Path(_) => { }
 393
 394             hir::ExprKind::Type(ref subexpr, _) => {
 395                 self.walk_expr(&subexpr)
 396             }
 397
 398             hir::ExprKind::Unary(hir::UnDeref, ref base) => { // *base
 399                 self.select_from_expr(&base);
 400             }
 401
 402             hir::ExprKind::Field(ref base, _) => { // base.f
 403                 self.select_from_expr(&base);
 404             }
 405
 406             hir::ExprKind::Index(ref lhs, ref rhs) => { // lhs[rhs]
 407                 self.select_from_expr(&lhs);
 408                 self.consume_expr(&rhs);
 409             }
 410
 411             hir::ExprKind::Call(ref callee, ref args) => { // callee(args)
 412                 self.walk_callee(expr, &callee);
 413                 self.consume_exprs(args);
 414             }
 415
 416             hir::ExprKind::MethodCall(.., ref args) => { // callee.m(args)
 417                 self.consume_exprs(args);
 418             }
 419
 420             hir::ExprKind::Struct(_, ref fields, ref opt_with) => {
 421                 self.walk_struct_expr(fields, opt_with);
 422             }
 423
 424             hir::ExprKind::Tup(ref exprs) => {
 425                 self.consume_exprs(exprs);
 426             }
 427
 428             hir::ExprKind::If(ref cond_expr, ref then_expr, ref opt_else_expr) => {
 429                 self.consume_expr(&cond_expr);
 430                 self.walk_expr(&then_expr);
 431                 if let Some(ref else_expr) = *opt_else_expr {
 432                     self.consume_expr(&else_expr);
 433                 }
 434             }
 435
 436             hir::ExprKind::Match(ref discr, ref arms, _) => {
 437                 let discr_cmt = Rc::new(return_if_err!(self.mc.cat_expr(&discr)));
 438                 let r = self.tcx().types.re_empty;
 439                 self.borrow_expr(&discr, r, ty::ImmBorrow, MatchDiscriminant);
 440
 441                 // treatment of the discriminant is handled while walking the arms.
 442                 for arm in arms {
 443                     let mode = self.arm_move_mode(discr_cmt.clone(), arm);
 444                     let mode = mode.match_mode();
 445                     self.walk_arm(discr_cmt.clone(), arm, mode);
 446                 }
 447             }
 448
 449             hir::ExprKind::Array(ref exprs) => {
 450                 self.consume_exprs(exprs);
 451             }
 452
 453             hir::ExprKind::AddrOf(m, ref base) => {   // &base
 454                 // make sure that the thing we are pointing out stays valid
 455                 // for the lifetime `scope_r` of the resulting ptr:
 456                 let expr_ty = return_if_err!(self.mc.expr_ty(expr));
 457                 if let ty::Ref(r, _, _) = expr_ty.sty {
 458                     let bk = ty::BorrowKind::from_mutbl(m);
 459                     self.borrow_expr(&base, r, bk, AddrOf);
 460                 }
 461             }
 462
 463             hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
 464                 for (o, output) in ia.outputs.iter().zip(outputs) {
 465                     if o.is_indirect {
 466                         self.consume_expr(output);
 467                     } else {
 468                         self.mutate_expr(
 469                             output.span,
 470                             expr,
 471                             output,
 472                             if o.is_rw {
 473                                 MutateMode::WriteAndRead
 474                             } else {
 475                                 MutateMode::JustWrite
 476                             },
 477                         );
 478                     }
 479                 }
 480                 self.consume_exprs(inputs);
 481             }
 482
 483             hir::ExprKind::Continue(..) |
 484             hir::ExprKind::Lit(..) |
 485             hir::ExprKind::Err => {}
 486
 487             hir::ExprKind::Loop(ref blk, _, _) => {
 488                 self.walk_block(&blk);
 489             }
 490
 491             hir::ExprKind::While(ref cond_expr, ref blk, _) => {
 492                 self.consume_expr(&cond_expr);
 493                 self.walk_block(&blk);
 494             }
 495
 496             hir::ExprKind::Unary(_, ref lhs) => {
 497                 self.consume_expr(&lhs);
 498             }
 499
 500             hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
 501                 self.consume_expr(&lhs);
 502                 self.consume_expr(&rhs);
 503             }
 504
 505             hir::ExprKind::Block(ref blk, _) => {
 506                 self.walk_block(&blk);
 507             }
 508
 509             hir::ExprKind::Break(_, ref opt_expr) | hir::ExprKind::Ret(ref opt_expr) => {
 510                 if let Some(ref expr) = *opt_expr {
 511                     self.consume_expr(&expr);
 512                 }
 513             }
 514
 515             hir::ExprKind::Assign(ref lhs, ref rhs) => {
 516                 self.mutate_expr(expr.span, expr, &lhs, MutateMode::JustWrite);
 517                 self.consume_expr(&rhs);
 518             }
 519
 520             hir::ExprKind::Cast(ref base, _) => {
 521                 self.consume_expr(&base);
 522             }
 523
 524             hir::ExprKind::AssignOp(_, ref lhs, ref rhs) => {
 525                 if self.mc.tables.is_method_call(expr) {
 526                     self.consume_expr(lhs);
 527                 } else {
 528                     self.mutate_expr(expr.span, expr, &lhs, MutateMode::WriteAndRead);
 529                 }
 530                 self.consume_expr(&rhs);
 531             }
 532
 533             hir::ExprKind::Repeat(ref base, _) => {
 534                 self.consume_expr(&base);
 535             }
 536
 537             hir::ExprKind::Closure(_, _, body_id, fn_decl_span, _) => {
 538                 self.delegate.nested_body(body_id);
 539                 self.walk_captures(expr, fn_decl_span);
 540             }
 541
 542             hir::ExprKind::Box(ref base) => {
 543                 self.consume_expr(&base);
 544             }
 545
 546             hir::ExprKind::Yield(ref value) => {
 547                 self.consume_expr(&value);
 548             }
 549         }
 550     }
 551
 552     fn walk_callee(&mut self, call: &hir::Expr, callee: &hir::Expr) {
 553         let callee_ty = return_if_err!(self.mc.expr_ty_adjusted(callee));
 554         debug!("walk_callee: callee={:?} callee_ty={:?}",
 555                callee, callee_ty);
 556         match callee_ty.sty {
 557             ty::FnDef(..) | ty::FnPtr(_) => {
 558                 self.consume_expr(callee);
 559             }
 560             ty::Error => { }
 561             _ => {
 562                 if let Some(def) = self.mc.tables.type_dependent_defs().get(call.hir_id) {
 563                     let def_id = def.def_id();
 564                     let call_scope = region::Scope {
 565                         id: call.hir_id.local_id,
 566                         data: region::ScopeData::Node
 567                     };
 568                     match OverloadedCallType::from_method_id(self.tcx(), def_id) {
 569                         FnMutOverloadedCall => {
 570                             let call_scope_r = self.tcx().mk_region(ty::ReScope(call_scope));
 571                             self.borrow_expr(callee,
 572                                             call_scope_r,
 573                                             ty::MutBorrow,
 574                                             ClosureInvocation);
 575                         }
 576                         FnOverloadedCall => {
 577                             let call_scope_r = self.tcx().mk_region(ty::ReScope(call_scope));
 578                             self.borrow_expr(callee,
 579                                             call_scope_r,
 580                                             ty::ImmBorrow,
 581                                             ClosureInvocation);
 582                         }
 583                         FnOnceOverloadedCall => self.consume_expr(callee),
 584                     }
 585                 } else {
 586                     self.tcx().sess.delay_span_bug(call.span,
 587                                                    "no type-dependent def for overloaded call");
 588                 }
 589             }
 590         }
 591     }
 592
 593     fn walk_stmt(&mut self, stmt: &hir::Stmt) {
 594         match stmt.node {
 595             hir::StmtKind::Local(ref local) => {
 596                 self.walk_local(&local);
 597             }
 598
 599             hir::StmtKind::Item(_) => {
 600                 // we don't visit nested items in this visitor,
 601                 // only the fn body we were given.
 602             }
 603
 604             hir::StmtKind::Expr(ref expr) |
 605             hir::StmtKind::Semi(ref expr) => {
 606                 self.consume_expr(&expr);
 607             }
 608         }
 609     }
 610
 611     fn walk_local(&mut self, local: &hir::Local) {
 612         match local.init {
 613             None => {
 614                 local.pat.each_binding(|_, hir_id, span, _| {
 615                     self.delegate.decl_without_init(hir_id, span);
 616                 })
 617             }
 618
 619             Some(ref expr) => {
 620                 // Variable declarations with
 621                 // initializers are considered
 622                 // "assigns", which is handled by
 623                 // `walk_pat`:
 624                 self.walk_expr(&expr);
 625                 let init_cmt = Rc::new(return_if_err!(self.mc.cat_expr(&expr)));
 626                 self.walk_irrefutable_pat(init_cmt, &local.pat);
 627             }
 628         }
 629     }
 630
 631     /// Indicates that the value of `blk` will be consumed, meaning either copied or moved
 632     /// depending on its type.
 633     fn walk_block(&mut self, blk: &hir::Block) {
 634         debug!("walk_block(blk.hir_id={})", blk.hir_id);
 635
 636         for stmt in &blk.stmts {
 637             self.walk_stmt(stmt);
 638         }
 639
 640         if let Some(ref tail_expr) = blk.expr {
 641             self.consume_expr(&tail_expr);
 642         }
 643     }
 644
 645     fn walk_struct_expr(&mut self,
 646                         fields: &[hir::Field],
 647                         opt_with: &Option<P<hir::Expr>>) {
 648         // Consume the expressions supplying values for each field.
 649         for field in fields {
 650             self.consume_expr(&field.expr);
 651         }
 652
 653         let with_expr = match *opt_with {
 654             Some(ref w) => &**w,
 655             None => { return; }
 656         };
 657
 658         let with_cmt = Rc::new(return_if_err!(self.mc.cat_expr(&with_expr)));
 659
 660         // Select just those fields of the `with`
 661         // expression that will actually be used
 662         match with_cmt.ty.sty {
 663             ty::Adt(adt, substs) if adt.is_struct() => {
 664                 // Consume those fields of the with expression that are needed.
 665                 for (f_index, with_field) in adt.non_enum_variant().fields.iter().enumerate() {
 666                     let is_mentioned = fields.iter().any(|f| {
 667                         self.tcx().field_index(f.hir_id, self.mc.tables) == f_index
 668                     });
 669                     if !is_mentioned {
 670                         let cmt_field = self.mc.cat_field(
 671                             &*with_expr,
 672                             with_cmt.clone(),
 673                             f_index,
 674                             with_field.ident,
 675                             with_field.ty(self.tcx(), substs)
 676                         );
 677                         self.delegate_consume(with_expr.hir_id, with_expr.span, &cmt_field);
 678                     }
 679                 }
 680             }
 681             _ => {
 682                 // the base expression should always evaluate to a
 683                 // struct; however, when EUV is run during typeck, it
 684                 // may not. This will generate an error earlier in typeck,
 685                 // so we can just ignore it.
 686                 if !self.tcx().sess.has_errors() {
 687                     span_bug!(
 688                         with_expr.span,
 689                         "with expression doesn't evaluate to a struct");
 690                 }
 691             }
 692         }
 693
 694         // walk the with expression so that complex expressions
 695         // are properly handled.
 696         self.walk_expr(with_expr);
 697     }
 698
 699     // Invoke the appropriate delegate calls for anything that gets
 700     // consumed or borrowed as part of the automatic adjustment
 701     // process.
 702     fn walk_adjustment(&mut self, expr: &hir::Expr) {
 703         let adjustments = self.mc.tables.expr_adjustments(expr);
 704         let mut cmt = return_if_err!(self.mc.cat_expr_unadjusted(expr));
 705         for adjustment in adjustments {
 706             debug!("walk_adjustment expr={:?} adj={:?}", expr, adjustment);
 707             match adjustment.kind {
 708                 adjustment::Adjust::NeverToAny |
 709                 adjustment::Adjust::ReifyFnPointer |
 710                 adjustment::Adjust::UnsafeFnPointer |
 711                 adjustment::Adjust::ClosureFnPointer |
 712                 adjustment::Adjust::MutToConstPointer |
 713                 adjustment::Adjust::Unsize => {
 714                     // Creating a closure/fn-pointer or unsizing consumes
 715                     // the input and stores it into the resulting rvalue.
 716                     self.delegate_consume(expr.hir_id, expr.span, &cmt);
 717                 }
 718
 719                 adjustment::Adjust::Deref(None) => {}
 720
 721                 // Autoderefs for overloaded Deref calls in fact reference
 722                 // their receiver. That is, if we have `(*x)` where `x`
 723                 // is of type `Rc<T>`, then this in fact is equivalent to
 724                 // `x.deref()`. Since `deref()` is declared with `&self`,
 725                 // this is an autoref of `x`.
 726                 adjustment::Adjust::Deref(Some(ref deref)) => {
 727                     let bk = ty::BorrowKind::from_mutbl(deref.mutbl);
 728                     self.delegate.borrow(expr.hir_id, expr.span, &cmt, deref.region, bk, AutoRef);
 729                 }
 730
 731                 adjustment::Adjust::Borrow(ref autoref) => {
 732                     self.walk_autoref(expr, &cmt, autoref);
 733                 }
 734             }
 735             cmt = return_if_err!(self.mc.cat_expr_adjusted(expr, cmt, &adjustment));
 736         }
 737     }
 738
 739     /// Walks the autoref `autoref` applied to the autoderef'd
 740     /// `expr`. `cmt_base` is the mem-categorized form of `expr`
 741     /// after all relevant autoderefs have occurred.
 742     fn walk_autoref(&mut self,
 743                     expr: &hir::Expr,
 744                     cmt_base: &mc::cmt_<'tcx>,
 745                     autoref: &adjustment::AutoBorrow<'tcx>) {
 746         debug!("walk_autoref(expr.hir_id={} cmt_base={:?} autoref={:?})",
 747                expr.hir_id,
 748                cmt_base,
 749                autoref);
 750
 751         match *autoref {
 752             adjustment::AutoBorrow::Ref(r, m) => {
 753                 self.delegate.borrow(expr.hir_id,
 754                                      expr.span,
 755                                      cmt_base,
 756                                      r,
 757                                      ty::BorrowKind::from_mutbl(m.into()),
 758                                      AutoRef);
 759             }
 760
 761             adjustment::AutoBorrow::RawPtr(m) => {
 762                 debug!("walk_autoref: expr.hir_id={} cmt_base={:?}",
 763                        expr.hir_id,
 764                        cmt_base);
 765
 766                 // Converting from a &T to *T (or &mut T to *mut T) is
 767                 // treated as borrowing it for the enclosing temporary
 768                 // scope.
 769                 let r = self.tcx().mk_region(ty::ReScope(
 770                     region::Scope {
 771                         id: expr.hir_id.local_id,
 772                         data: region::ScopeData::Node
 773                     }));
 774
 775                 self.delegate.borrow(expr.hir_id,
 776                                      expr.span,
 777                                      cmt_base,
 778                                      r,
 779                                      ty::BorrowKind::from_mutbl(m),
 780                                      AutoUnsafe);
 781             }
 782         }
 783     }
 784
 785     fn arm_move_mode(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm) -> TrackMatchMode {
 786         let mut mode = Unknown;
 787         for pat in &arm.pats {
 788             self.determine_pat_move_mode(discr_cmt.clone(), &pat, &mut mode);
 789         }
 790         mode
 791     }
 792
 793     fn walk_arm(&mut self, discr_cmt: mc::cmt<'tcx>, arm: &hir::Arm, mode: MatchMode) {
 794         for pat in &arm.pats {
 795             self.walk_pat(discr_cmt.clone(), &pat, mode);
 796         }
 797
 798         if let Some(hir::Guard::If(ref e)) = arm.guard {
 799             self.consume_expr(e)
 800         }
 801
 802         self.consume_expr(&arm.body);
 803     }
 804
 805     /// Walks a pat that occurs in isolation (i.e., top-level of fn argument or
 806     /// let binding, and *not* a match arm or nested pat.)
 807     fn walk_irrefutable_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat) {
 808         let mut mode = Unknown;
 809         self.determine_pat_move_mode(cmt_discr.clone(), pat, &mut mode);
 810         let mode = mode.match_mode();
 811         self.walk_pat(cmt_discr, pat, mode);
 812     }
 813
 814     /// Identifies any bindings within `pat` and accumulates within
 815     /// `mode` whether the overall pattern/match structure is a move,
 816     /// copy, or borrow.
 817     fn determine_pat_move_mode(&mut self,
 818                                cmt_discr: mc::cmt<'tcx>,
 819                                pat: &hir::Pat,
 820                                mode: &mut TrackMatchMode) {
 821         debug!("determine_pat_move_mode cmt_discr={:?} pat={:?}", cmt_discr, pat);
 822
 823         return_if_err!(self.mc.cat_pattern(cmt_discr, pat, |cmt_pat, pat| {
 824             if let PatKind::Binding(..) = pat.node {
 825                 let bm = *self.mc.tables.pat_binding_modes()
 826                                         .get(pat.hir_id)
 827                                         .expect("missing binding mode");
 828                 match bm {
 829                     ty::BindByReference(..) =>
 830                         mode.lub(BorrowingMatch),
 831                     ty::BindByValue(..) => {
 832                         match copy_or_move(&self.mc, self.param_env, &cmt_pat, PatBindingMove) {
 833                             Copy => mode.lub(CopyingMatch),
 834                             Move(..) => mode.lub(MovingMatch),
 835                         }
 836                     }
 837                 }
 838             }
 839         }));
 840     }
 841
 842     /// The core driver for walking a pattern; `match_mode` must be
 843     /// established up front, e.g., via `determine_pat_move_mode` (see
 844     /// also `walk_irrefutable_pat` for patterns that stand alone).
 845     fn walk_pat(&mut self, cmt_discr: mc::cmt<'tcx>, pat: &hir::Pat, match_mode: MatchMode) {
 846         debug!("walk_pat(cmt_discr={:?}, pat={:?})", cmt_discr, pat);
 847
 848         let tcx = self.tcx();
 849         let ExprUseVisitor { ref mc, ref mut delegate, param_env } = *self;
 850         return_if_err!(mc.cat_pattern(cmt_discr.clone(), pat, |cmt_pat, pat| {
 851             if let PatKind::Binding(_, canonical_id, ..) = pat.node {
 852                 debug!(
 853                     "walk_pat: binding cmt_pat={:?} pat={:?} match_mode={:?}",
 854                     cmt_pat,
 855                     pat,
 856                     match_mode,
 857                 );
 858                 if let Some(&bm) = mc.tables.pat_binding_modes().get(pat.hir_id) {
 859                     debug!("walk_pat: pat.hir_id={:?} bm={:?}", pat.hir_id, bm);
 860
 861                     // pat_ty: the type of the binding being produced.
 862                     let pat_ty = return_if_err!(mc.node_ty(pat.hir_id));
 863                     debug!("walk_pat: pat_ty={:?}", pat_ty);
 864
 865                     // Each match binding is effectively an assignment to the
 866                     // binding being produced.
 867                     let def = Def::Local(mc.tcx.hir().hir_to_node_id(canonical_id));
 868                     if let Ok(ref binding_cmt) = mc.cat_def(pat.hir_id, pat.span, pat_ty, def) {
 869                         delegate.mutate(pat.hir_id, pat.span, binding_cmt, MutateMode::Init);
 870                     }
 871
 872                     // It is also a borrow or copy/move of the value being matched.
 873                     match bm {
 874                         ty::BindByReference(m) => {
 875                             if let ty::Ref(r, _, _) = pat_ty.sty {
 876                                 let bk = ty::BorrowKind::from_mutbl(m);
 877                                 delegate.borrow(pat.hir_id, pat.span, &cmt_pat, r, bk, RefBinding);
 878                             }
 879                         }
 880                         ty::BindByValue(..) => {
 881                             let mode = copy_or_move(mc, param_env, &cmt_pat, PatBindingMove);
 882                             debug!("walk_pat binding consuming pat");
 883                             delegate.consume_pat(pat, &cmt_pat, mode);
 884                         }
 885                     }
 886                 } else {
 887                     tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 888                 }
 889             }
 890         }));
 891
 892         // Do a second pass over the pattern, calling `matched_pat` on
 893         // the interior nodes (enum variants and structs), as opposed
 894         // to the above loop's visit of than the bindings that form
 895         // the leaves of the pattern tree structure.
 896         return_if_err!(mc.cat_pattern(cmt_discr, pat, |cmt_pat, pat| {
 897             let qpath = match pat.node {
 898                 PatKind::Path(ref qpath) |
 899                 PatKind::TupleStruct(ref qpath, ..) |
 900                 PatKind::Struct(ref qpath, ..) => qpath,
 901                 _ => return
 902             };
 903             let def = mc.tables.qpath_def(qpath, pat.hir_id);
 904             match def {
 905                 Def::Variant(variant_did) |
 906                 Def::VariantCtor(variant_did, ..) => {
 907                     let downcast_cmt = mc.cat_downcast_if_needed(pat, cmt_pat, variant_did);
 908
 909                     debug!("variant downcast_cmt={:?} pat={:?}", downcast_cmt, pat);
 910                     delegate.matched_pat(pat, &downcast_cmt, match_mode);
 911                 }
 912                 Def::Struct(..) | Def::StructCtor(..) | Def::Union(..) |
 913                 Def::TyAlias(..) | Def::AssociatedTy(..) | Def::SelfTy(..) => {
 914                     debug!("struct cmt_pat={:?} pat={:?}", cmt_pat, pat);
 915                     delegate.matched_pat(pat, &cmt_pat, match_mode);
 916                 }
 917                 _ => {}
 918             }
 919         }));
 920     }
 921
 922     fn walk_captures(&mut self, closure_expr: &hir::Expr, fn_decl_span: Span) {
 923         debug!("walk_captures({:?})", closure_expr);
 924
 925         let closure_def_id = self.tcx().hir().local_def_id_from_hir_id(closure_expr.hir_id);
 926         self.tcx().with_freevars(closure_expr.hir_id, |freevars| {
 927             for freevar in freevars {
 928                 let var_hir_id = self.tcx().hir().node_to_hir_id(freevar.var_id());
 929                 let upvar_id = ty::UpvarId {
 930                     var_path: ty::UpvarPath { hir_id: var_hir_id },
 931                     closure_expr_id: closure_def_id.to_local(),
 932                 };
 933                 let upvar_capture = self.mc.tables.upvar_capture(upvar_id);
 934                 let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.hir_id,
 935                                                                    fn_decl_span,
 936                                                                    freevar));
 937                 match upvar_capture {
 938                     ty::UpvarCapture::ByValue => {
 939                         let mode = copy_or_move(&self.mc,
 940                                                 self.param_env,
 941                                                 &cmt_var,
 942                                                 CaptureMove);
 943                         self.delegate.consume(closure_expr.hir_id, freevar.span, &cmt_var, mode);
 944                     }
 945                     ty::UpvarCapture::ByRef(upvar_borrow) => {
 946                         self.delegate.borrow(closure_expr.hir_id,
 947                                              fn_decl_span,
 948                                              &cmt_var,
 949                                              upvar_borrow.region,
 950                                              upvar_borrow.kind,
 951                                              ClosureCapture(freevar.span));
 952                     }
 953                 }
 954             }
 955         });
 956     }
 957
 958     fn cat_captured_var(&mut self,
 959                         closure_hir_id: hir::HirId,
 960                         closure_span: Span,
 961                         upvar: &hir::Freevar)
 962                         -> mc::McResult<mc::cmt_<'tcx>> {
 963         // Create the cmt for the variable being borrowed, from the
 964         // caller's perspective
 965         let var_hir_id = self.tcx().hir().node_to_hir_id(upvar.var_id());
 966         let var_ty = self.mc.node_ty(var_hir_id)?;
 967         self.mc.cat_def(closure_hir_id, closure_span, var_ty, upvar.def)
 968     }
 969 }
 970
 971 fn copy_or_move<'a, 'gcx, 'tcx>(mc: &mc::MemCategorizationContext<'a, 'gcx, 'tcx>,
 972                                 param_env: ty::ParamEnv<'tcx>,
 973                                 cmt: &mc::cmt_<'tcx>,
 974                                 move_reason: MoveReason)
 975                                 -> ConsumeMode
 976 {
 977     if !mc.type_is_copy_modulo_regions(param_env, cmt.ty, cmt.span) {
 978         Move(move_reason)
 979     } else {
 980         Copy
 981     }
 982 }