compiler/rustc_middle/src/middle/region.rs

   1 //! This file declares the `ScopeTree` type, which describes
   2 //! the parent links in the region hierarchy.
   3 //!
   4 //! For more information about how MIR-based region-checking works,
   5 //! see the [rustc dev guide].
   6 //!
   7 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
   8
   9 use crate::ich::{NodeIdHashingMode, StableHashingContext};
  10 use crate::ty::TyCtxt;
  11 use rustc_hir as hir;
  12 use rustc_hir::Node;
  13
  14 use rustc_data_structures::fx::FxHashMap;
  15 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
  16 use rustc_macros::HashStable;
  17 use rustc_span::{Span, DUMMY_SP};
  18
  19 use std::fmt;
  20
  21 /// Represents a statically-describable scope that can be used to
  22 /// bound the lifetime/region for values.
  23 ///
  24 /// `Node(node_id)`: Any AST node that has any scope at all has the
  25 /// `Node(node_id)` scope. Other variants represent special cases not
  26 /// immediately derivable from the abstract syntax tree structure.
  27 ///
  28 /// `DestructionScope(node_id)` represents the scope of destructors
  29 /// implicitly-attached to `node_id` that run immediately after the
  30 /// expression for `node_id` itself. Not every AST node carries a
  31 /// `DestructionScope`, but those that are `terminating_scopes` do;
  32 /// see discussion with `ScopeTree`.
  33 ///
  34 /// `Remainder { block, statement_index }` represents
  35 /// the scope of user code running immediately after the initializer
  36 /// expression for the indexed statement, until the end of the block.
  37 ///
  38 /// So: the following code can be broken down into the scopes beneath:
  39 ///
  40 /// ```text
  41 /// let a = f().g( 'b: { let x = d(); let y = d(); x.h(y)  }   ) ;
  42 ///
  43 ///                                                              +-+ (D12.)
  44 ///                                                        +-+       (D11.)
  45 ///                                              +---------+         (R10.)
  46 ///                                              +-+                  (D9.)
  47 ///                                   +----------+                    (M8.)
  48 ///                                 +----------------------+          (R7.)
  49 ///                                 +-+                               (D6.)
  50 ///                      +----------+                                 (M5.)
  51 ///                    +-----------------------------------+          (M4.)
  52 ///         +--------------------------------------------------+      (M3.)
  53 ///         +--+                                                      (M2.)
  54 /// +-----------------------------------------------------------+     (M1.)
  55 ///
  56 ///  (M1.): Node scope of the whole `let a = ...;` statement.
  57 ///  (M2.): Node scope of the `f()` expression.
  58 ///  (M3.): Node scope of the `f().g(..)` expression.
  59 ///  (M4.): Node scope of the block labeled `'b:`.
  60 ///  (M5.): Node scope of the `let x = d();` statement
  61 ///  (D6.): DestructionScope for temporaries created during M5.
  62 ///  (R7.): Remainder scope for block `'b:`, stmt 0 (let x = ...).
  63 ///  (M8.): Node scope of the `let y = d();` statement.
  64 ///  (D9.): DestructionScope for temporaries created during M8.
  65 /// (R10.): Remainder scope for block `'b:`, stmt 1 (let y = ...).
  66 /// (D11.): DestructionScope for temporaries and bindings from block `'b:`.
  67 /// (D12.): DestructionScope for temporaries created during M1 (e.g., f()).
  68 /// ```
  69 ///
  70 /// Note that while the above picture shows the destruction scopes
  71 /// as following their corresponding node scopes, in the internal
  72 /// data structures of the compiler the destruction scopes are
  73 /// represented as enclosing parents. This is sound because we use the
  74 /// enclosing parent relationship just to ensure that referenced
  75 /// values live long enough; phrased another way, the starting point
  76 /// of each range is not really the important thing in the above
  77 /// picture, but rather the ending point.
  78 //
  79 // FIXME(pnkfelix): this currently derives `PartialOrd` and `Ord` to
  80 // placate the same deriving in `ty::FreeRegion`, but we may want to
  81 // actually attach a more meaningful ordering to scopes than the one
  82 // generated via deriving here.
  83 #[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Copy, TyEncodable, TyDecodable)]
  84 #[derive(HashStable)]
  85 pub struct Scope {
  86     pub id: hir::ItemLocalId,
  87     pub data: ScopeData,
  88 }
  89
  90 impl fmt::Debug for Scope {
  91     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
  92         match self.data {
  93             ScopeData::Node => write!(fmt, "Node({:?})", self.id),
  94             ScopeData::CallSite => write!(fmt, "CallSite({:?})", self.id),
  95             ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.id),
  96             ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.id),
  97             ScopeData::Remainder(fsi) => write!(
  98                 fmt,
  99                 "Remainder {{ block: {:?}, first_statement_index: {}}}",
 100                 self.id,
 101                 fsi.as_u32(),
 102             ),
 103         }
 104     }
 105 }
 106
 107 #[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Debug, Copy, TyEncodable, TyDecodable)]
 108 #[derive(HashStable)]
 109 pub enum ScopeData {
 110     Node,
 111
 112     /// Scope of the call-site for a function or closure
 113     /// (outlives the arguments as well as the body).
 114     CallSite,
 115
 116     /// Scope of arguments passed to a function or closure
 117     /// (they outlive its body).
 118     Arguments,
 119
 120     /// Scope of destructors for temporaries of node-id.
 121     Destruction,
 122
 123     /// Scope following a `let id = expr;` binding in a block.
 124     Remainder(FirstStatementIndex),
 125 }
 126
 127 rustc_index::newtype_index! {
 128     /// Represents a subscope of `block` for a binding that is introduced
 129     /// by `block.stmts[first_statement_index]`. Such subscopes represent
 130     /// a suffix of the block. Note that each subscope does not include
 131     /// the initializer expression, if any, for the statement indexed by
 132     /// `first_statement_index`.
 133     ///
 134     /// For example, given `{ let (a, b) = EXPR_1; let c = EXPR_2; ... }`:
 135     ///
 136     /// * The subscope with `first_statement_index == 0` is scope of both
 137     ///   `a` and `b`; it does not include EXPR_1, but does include
 138     ///   everything after that first `let`. (If you want a scope that
 139     ///   includes EXPR_1 as well, then do not use `Scope::Remainder`,
 140     ///   but instead another `Scope` that encompasses the whole block,
 141     ///   e.g., `Scope::Node`.
 142     ///
 143     /// * The subscope with `first_statement_index == 1` is scope of `c`,
 144     ///   and thus does not include EXPR_2, but covers the `...`.
 145     pub struct FirstStatementIndex {
 146         derive [HashStable]
 147     }
 148 }
 149
 150 // compilation error if size of `ScopeData` is not the same as a `u32`
 151 static_assert_size!(ScopeData, 4);
 152
 153 impl Scope {
 154     /// Returns a item-local ID associated with this scope.
 155     ///
 156     /// N.B., likely to be replaced as API is refined; e.g., pnkfelix
 157     /// anticipates `fn entry_node_id` and `fn each_exit_node_id`.
 158     pub fn item_local_id(&self) -> hir::ItemLocalId {
 159         self.id
 160     }
 161
 162     pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<hir::HirId> {
 163         scope_tree
 164             .root_body
 165             .map(|hir_id| hir::HirId { owner: hir_id.owner, local_id: self.item_local_id() })
 166     }
 167
 168     /// Returns the span of this `Scope`. Note that in general the
 169     /// returned span may not correspond to the span of any `NodeId` in
 170     /// the AST.
 171     pub fn span(&self, tcx: TyCtxt<'_>, scope_tree: &ScopeTree) -> Span {
 172         let hir_id = match self.hir_id(scope_tree) {
 173             Some(hir_id) => hir_id,
 174             None => return DUMMY_SP,
 175         };
 176         let span = tcx.hir().span(hir_id);
 177         if let ScopeData::Remainder(first_statement_index) = self.data {
 178             if let Node::Block(ref blk) = tcx.hir().get(hir_id) {
 179                 // Want span for scope starting after the
 180                 // indexed statement and ending at end of
 181                 // `blk`; reuse span of `blk` and shift `lo`
 182                 // forward to end of indexed statement.
 183                 //
 184                 // (This is the special case alluded to in the
 185                 // doc-comment for this method)
 186
 187                 let stmt_span = blk.stmts[first_statement_index.index()].span;
 188
 189                 // To avoid issues with macro-generated spans, the span
 190                 // of the statement must be nested in that of the block.
 191                 if span.lo() <= stmt_span.lo() && stmt_span.lo() <= span.hi() {
 192                     return Span::new(stmt_span.lo(), span.hi(), span.ctxt());
 193                 }
 194             }
 195         }
 196         span
 197     }
 198 }
 199
 200 pub type ScopeDepth = u32;
 201
 202 /// The region scope tree encodes information about region relationships.
 203 #[derive(Default, Debug)]
 204 pub struct ScopeTree {
 205     /// If not empty, this body is the root of this region hierarchy.
 206     pub root_body: Option<hir::HirId>,
 207
 208     /// The parent of the root body owner, if the latter is an
 209     /// an associated const or method, as impls/traits can also
 210     /// have lifetime parameters free in this body.
 211     pub root_parent: Option<hir::HirId>,
 212
 213     /// Maps from a scope ID to the enclosing scope id;
 214     /// this is usually corresponding to the lexical nesting, though
 215     /// in the case of closures the parent scope is the innermost
 216     /// conditional expression or repeating block. (Note that the
 217     /// enclosing scope ID for the block associated with a closure is
 218     /// the closure itself.)
 219     pub parent_map: FxHashMap<Scope, (Scope, ScopeDepth)>,
 220
 221     /// Maps from a variable or binding ID to the block in which that
 222     /// variable is declared.
 223     var_map: FxHashMap<hir::ItemLocalId, Scope>,
 224
 225     /// Maps from a `NodeId` to the associated destruction scope (if any).
 226     destruction_scopes: FxHashMap<hir::ItemLocalId, Scope>,
 227
 228     /// `rvalue_scopes` includes entries for those expressions whose
 229     /// cleanup scope is larger than the default. The map goes from the
 230     /// expression ID to the cleanup scope id. For rvalues not present in
 231     /// this table, the appropriate cleanup scope is the innermost
 232     /// enclosing statement, conditional expression, or repeating
 233     /// block (see `terminating_scopes`).
 234     /// In constants, None is used to indicate that certain expressions
 235     /// escape into 'static and should have no local cleanup scope.
 236     rvalue_scopes: FxHashMap<hir::ItemLocalId, Option<Scope>>,
 237
 238     /// Encodes the hierarchy of fn bodies. Every fn body (including
 239     /// closures) forms its own distinct region hierarchy, rooted in
 240     /// the block that is the fn body. This map points from the ID of
 241     /// that root block to the ID of the root block for the enclosing
 242     /// fn, if any. Thus the map structures the fn bodies into a
 243     /// hierarchy based on their lexical mapping. This is used to
 244     /// handle the relationships between regions in a fn and in a
 245     /// closure defined by that fn. See the "Modeling closures"
 246     /// section of the README in infer::region_constraints for
 247     /// more details.
 248     closure_tree: FxHashMap<hir::ItemLocalId, hir::ItemLocalId>,
 249
 250     /// If there are any `yield` nested within a scope, this map
 251     /// stores the `Span` of the last one and its index in the
 252     /// postorder of the Visitor traversal on the HIR.
 253     ///
 254     /// HIR Visitor postorder indexes might seem like a peculiar
 255     /// thing to care about. but it turns out that HIR bindings
 256     /// and the temporary results of HIR expressions are never
 257     /// storage-live at the end of HIR nodes with postorder indexes
 258     /// lower than theirs, and therefore don't need to be suspended
 259     /// at yield-points at these indexes.
 260     ///
 261     /// For an example, suppose we have some code such as:
 262     /// ```rust,ignore (example)
 263     ///     foo(f(), yield y, bar(g()))
 264     /// ```
 265     ///
 266     /// With the HIR tree (calls numbered for expository purposes)
 267     /// ```
 268     ///     Call#0(foo, [Call#1(f), Yield(y), Call#2(bar, Call#3(g))])
 269     /// ```
 270     ///
 271     /// Obviously, the result of `f()` was created before the yield
 272     /// (and therefore needs to be kept valid over the yield) while
 273     /// the result of `g()` occurs after the yield (and therefore
 274     /// doesn't). If we want to infer that, we can look at the
 275     /// postorder traversal:
 276     /// ```plain,ignore
 277     ///     `foo` `f` Call#1 `y` Yield `bar` `g` Call#3 Call#2 Call#0
 278     /// ```
 279     ///
 280     /// In which we can easily see that `Call#1` occurs before the yield,
 281     /// and `Call#3` after it.
 282     ///
 283     /// To see that this method works, consider:
 284     ///
 285     /// Let `D` be our binding/temporary and `U` be our other HIR node, with
 286     /// `HIR-postorder(U) < HIR-postorder(D)`. Suppose, as in our example,
 287     /// U is the yield and D is one of the calls.
 288     /// Let's show that `D` is storage-dead at `U`.
 289     ///
 290     /// Remember that storage-live/storage-dead refers to the state of
 291     /// the *storage*, and does not consider moves/drop flags.
 292     ///
 293     /// Then:
 294     ///
 295     ///   1. From the ordering guarantee of HIR visitors (see
 296     ///   `rustc_hir::intravisit`), `D` does not dominate `U`.
 297     ///
 298     ///   2. Therefore, `D` is *potentially* storage-dead at `U` (because
 299     ///   we might visit `U` without ever getting to `D`).
 300     ///
 301     ///   3. However, we guarantee that at each HIR point, each
 302     ///   binding/temporary is always either always storage-live
 303     ///   or always storage-dead. This is what is being guaranteed
 304     ///   by `terminating_scopes` including all blocks where the
 305     ///   count of executions is not guaranteed.
 306     ///
 307     ///   4. By `2.` and `3.`, `D` is *statically* storage-dead at `U`,
 308     ///   QED.
 309     ///
 310     /// This property ought to not on (3) in an essential way -- it
 311     /// is probably still correct even if we have "unrestricted" terminating
 312     /// scopes. However, why use the complicated proof when a simple one
 313     /// works?
 314     ///
 315     /// A subtle thing: `box` expressions, such as `box (&x, yield 2, &y)`. It
 316     /// might seem that a `box` expression creates a `Box<T>` temporary
 317     /// when it *starts* executing, at `HIR-preorder(BOX-EXPR)`. That might
 318     /// be true in the MIR desugaring, but it is not important in the semantics.
 319     ///
 320     /// The reason is that semantically, until the `box` expression returns,
 321     /// the values are still owned by their containing expressions. So
 322     /// we'll see that `&x`.
 323     pub yield_in_scope: FxHashMap<Scope, YieldData>,
 324
 325     /// The number of visit_expr and visit_pat calls done in the body.
 326     /// Used to sanity check visit_expr/visit_pat call count when
 327     /// calculating generator interiors.
 328     pub body_expr_count: FxHashMap<hir::BodyId, usize>,
 329 }
 330
 331 #[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
 332 pub struct YieldData {
 333     /// The `Span` of the yield.
 334     pub span: Span,
 335     /// The number of expressions and patterns appearing before the `yield` in the body, plus one.
 336     pub expr_and_pat_count: usize,
 337     pub source: hir::YieldSource,
 338 }
 339
 340 impl ScopeTree {
 341     pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) {
 342         debug!("{:?}.parent = {:?}", child, parent);
 343
 344         if let Some(p) = parent {
 345             let prev = self.parent_map.insert(child, p);
 346             assert!(prev.is_none());
 347         }
 348
 349         // Record the destruction scopes for later so we can query them.
 350         if let ScopeData::Destruction = child.data {
 351             self.destruction_scopes.insert(child.item_local_id(), child);
 352         }
 353     }
 354
 355     pub fn opt_destruction_scope(&self, n: hir::ItemLocalId) -> Option<Scope> {
 356         self.destruction_scopes.get(&n).cloned()
 357     }
 358
 359     /// Records that `sub_closure` is defined within `sup_closure`. These IDs
 360     /// should be the ID of the block that is the fn body, which is
 361     /// also the root of the region hierarchy for that fn.
 362     pub fn record_closure_parent(
 363         &mut self,
 364         sub_closure: hir::ItemLocalId,
 365         sup_closure: hir::ItemLocalId,
 366     ) {
 367         debug!(
 368             "record_closure_parent(sub_closure={:?}, sup_closure={:?})",
 369             sub_closure, sup_closure
 370         );
 371         assert!(sub_closure != sup_closure);
 372         let previous = self.closure_tree.insert(sub_closure, sup_closure);
 373         assert!(previous.is_none());
 374     }
 375
 376     pub fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) {
 377         debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime);
 378         assert!(var != lifetime.item_local_id());
 379         self.var_map.insert(var, lifetime);
 380     }
 381
 382     pub fn record_rvalue_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) {
 383         debug!("record_rvalue_scope(sub={:?}, sup={:?})", var, lifetime);
 384         if let Some(lifetime) = lifetime {
 385             assert!(var != lifetime.item_local_id());
 386         }
 387         self.rvalue_scopes.insert(var, lifetime);
 388     }
 389
 390     /// Returns the narrowest scope that encloses `id`, if any.
 391     pub fn opt_encl_scope(&self, id: Scope) -> Option<Scope> {
 392         self.parent_map.get(&id).cloned().map(|(p, _)| p)
 393     }
 394
 395     /// Returns the lifetime of the local variable `var_id`
 396     pub fn var_scope(&self, var_id: hir::ItemLocalId) -> Scope {
 397         self.var_map
 398             .get(&var_id)
 399             .cloned()
 400             .unwrap_or_else(|| bug!("no enclosing scope for id {:?}", var_id))
 401     }
 402
 403     /// Returns the scope when the temp created by `expr_id` will be cleaned up.
 404     pub fn temporary_scope(&self, expr_id: hir::ItemLocalId) -> Option<Scope> {
 405         // Check for a designated rvalue scope.
 406         if let Some(&s) = self.rvalue_scopes.get(&expr_id) {
 407             debug!("temporary_scope({:?}) = {:?} [custom]", expr_id, s);
 408             return s;
 409         }
 410
 411         // Otherwise, locate the innermost terminating scope
 412         // if there's one. Static items, for instance, won't
 413         // have an enclosing scope, hence no scope will be
 414         // returned.
 415         let mut id = Scope { id: expr_id, data: ScopeData::Node };
 416
 417         while let Some(&(p, _)) = self.parent_map.get(&id) {
 418             match p.data {
 419                 ScopeData::Destruction => {
 420                     debug!("temporary_scope({:?}) = {:?} [enclosing]", expr_id, id);
 421                     return Some(id);
 422                 }
 423                 _ => id = p,
 424             }
 425         }
 426
 427         debug!("temporary_scope({:?}) = None", expr_id);
 428         None
 429     }
 430
 431     /// Returns `true` if `subscope` is equal to or is lexically nested inside `superscope`, and
 432     /// `false` otherwise.
 433     pub fn is_subscope_of(&self, subscope: Scope, superscope: Scope) -> bool {
 434         let mut s = subscope;
 435         debug!("is_subscope_of({:?}, {:?})", subscope, superscope);
 436         while superscope != s {
 437             match self.opt_encl_scope(s) {
 438                 None => {
 439                     debug!("is_subscope_of({:?}, {:?}, s={:?})=false", subscope, superscope, s);
 440                     return false;
 441                 }
 442                 Some(scope) => s = scope,
 443             }
 444         }
 445
 446         debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope);
 447
 448         true
 449     }
 450
 451     /// Checks whether the given scope contains a `yield`. If so,
 452     /// returns `Some(YieldData)`. If not, returns `None`.
 453     pub fn yield_in_scope(&self, scope: Scope) -> Option<YieldData> {
 454         self.yield_in_scope.get(&scope).cloned()
 455     }
 456
 457     /// Gives the number of expressions visited in a body.
 458     /// Used to sanity check visit_expr call count when
 459     /// calculating generator interiors.
 460     pub fn body_expr_count(&self, body_id: hir::BodyId) -> Option<usize> {
 461         self.body_expr_count.get(&body_id).copied()
 462     }
 463 }
 464
 465 impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree {
 466     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
 467         let ScopeTree {
 468             root_body,
 469             root_parent,
 470             ref body_expr_count,
 471             ref parent_map,
 472             ref var_map,
 473             ref destruction_scopes,
 474             ref rvalue_scopes,
 475             ref closure_tree,
 476             ref yield_in_scope,
 477         } = *self;
 478
 479         hcx.with_node_id_hashing_mode(NodeIdHashingMode::HashDefPath, |hcx| {
 480             root_body.hash_stable(hcx, hasher);
 481             root_parent.hash_stable(hcx, hasher);
 482         });
 483
 484         body_expr_count.hash_stable(hcx, hasher);
 485         parent_map.hash_stable(hcx, hasher);
 486         var_map.hash_stable(hcx, hasher);
 487         destruction_scopes.hash_stable(hcx, hasher);
 488         rvalue_scopes.hash_stable(hcx, hasher);
 489         closure_tree.hash_stable(hcx, hasher);
 490         yield_in_scope.hash_stable(hcx, hasher);
 491     }
 492 }