1 //! This file declares the `ScopeTree` type, which describes
2 //! the parent links in the region hierarchy.
4 //! For more information about how MIR-based region-checking works,
5 //! see the [rustc dev guide].
7 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
9 use crate::ich::{NodeIdHashingMode, StableHashingContext};
10 use crate::ty::TyCtxt;
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};
21 /// Represents a statically-describable scope that can be used to
22 /// bound the lifetime/region for values.
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.
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`.
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.
38 /// So: the following code can be broken down into the scopes beneath:
41 /// let a = f().g( 'b: { let x = d(); let y = d(); x.h(y) } ) ;
45 /// +---------+ (R10.)
47 /// +----------+ (M8.)
48 /// +----------------------+ (R7.)
50 /// +----------+ (M5.)
51 /// +-----------------------------------+ (M4.)
52 /// +--------------------------------------------------+ (M3.)
54 /// +-----------------------------------------------------------+ (M1.)
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()).
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.
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)]
86 pub id: hir::ItemLocalId,
90 impl fmt::Debug for Scope {
91 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
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::IfThen => write!(fmt, "IfThen({:?})", self.id),
98 ScopeData::Remainder(fsi) => write!(
100 "Remainder {{ block: {:?}, first_statement_index: {}}}",
108 #[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Debug, Copy, TyEncodable, TyDecodable)]
109 #[derive(HashStable)]
113 /// Scope of the call-site for a function or closure
114 /// (outlives the arguments as well as the body).
117 /// Scope of arguments passed to a function or closure
118 /// (they outlive its body).
121 /// Scope of destructors for temporaries of node-id.
124 /// Scope of the condition and then block of an if expression
125 /// Used for variables introduced in an if-let expression.
128 /// Scope following a `let id = expr;` binding in a block.
129 Remainder(FirstStatementIndex),
132 rustc_index::newtype_index! {
133 /// Represents a subscope of `block` for a binding that is introduced
134 /// by `block.stmts[first_statement_index]`. Such subscopes represent
135 /// a suffix of the block. Note that each subscope does not include
136 /// the initializer expression, if any, for the statement indexed by
137 /// `first_statement_index`.
139 /// For example, given `{ let (a, b) = EXPR_1; let c = EXPR_2; ... }`:
141 /// * The subscope with `first_statement_index == 0` is scope of both
142 /// `a` and `b`; it does not include EXPR_1, but does include
143 /// everything after that first `let`. (If you want a scope that
144 /// includes EXPR_1 as well, then do not use `Scope::Remainder`,
145 /// but instead another `Scope` that encompasses the whole block,
146 /// e.g., `Scope::Node`.
148 /// * The subscope with `first_statement_index == 1` is scope of `c`,
149 /// and thus does not include EXPR_2, but covers the `...`.
150 pub struct FirstStatementIndex {
155 // compilation error if size of `ScopeData` is not the same as a `u32`
156 static_assert_size!(ScopeData, 4);
159 /// Returns an item-local ID associated with this scope.
161 /// N.B., likely to be replaced as API is refined; e.g., pnkfelix
162 /// anticipates `fn entry_node_id` and `fn each_exit_node_id`.
163 pub fn item_local_id(&self) -> hir::ItemLocalId {
167 pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<hir::HirId> {
170 .map(|hir_id| hir::HirId { owner: hir_id.owner, local_id: self.item_local_id() })
173 /// Returns the span of this `Scope`. Note that in general the
174 /// returned span may not correspond to the span of any `NodeId` in
176 pub fn span(&self, tcx: TyCtxt<'_>, scope_tree: &ScopeTree) -> Span {
177 let hir_id = match self.hir_id(scope_tree) {
178 Some(hir_id) => hir_id,
179 None => return DUMMY_SP,
181 let span = tcx.hir().span(hir_id);
182 if let ScopeData::Remainder(first_statement_index) = self.data {
183 if let Node::Block(ref blk) = tcx.hir().get(hir_id) {
184 // Want span for scope starting after the
185 // indexed statement and ending at end of
186 // `blk`; reuse span of `blk` and shift `lo`
187 // forward to end of indexed statement.
189 // (This is the special case alluded to in the
190 // doc-comment for this method)
192 let stmt_span = blk.stmts[first_statement_index.index()].span;
194 // To avoid issues with macro-generated spans, the span
195 // of the statement must be nested in that of the block.
196 if span.lo() <= stmt_span.lo() && stmt_span.lo() <= span.hi() {
197 return span.with_lo(stmt_span.lo());
205 pub type ScopeDepth = u32;
207 /// The region scope tree encodes information about region relationships.
208 #[derive(Default, Debug)]
209 pub struct ScopeTree {
210 /// If not empty, this body is the root of this region hierarchy.
211 pub root_body: Option<hir::HirId>,
213 /// The parent of the root body owner, if the latter is an
214 /// an associated const or method, as impls/traits can also
215 /// have lifetime parameters free in this body.
216 pub root_parent: Option<hir::HirId>,
218 /// Maps from a scope ID to the enclosing scope id;
219 /// this is usually corresponding to the lexical nesting, though
220 /// in the case of closures the parent scope is the innermost
221 /// conditional expression or repeating block. (Note that the
222 /// enclosing scope ID for the block associated with a closure is
223 /// the closure itself.)
224 pub parent_map: FxHashMap<Scope, (Scope, ScopeDepth)>,
226 /// Maps from a variable or binding ID to the block in which that
227 /// variable is declared.
228 var_map: FxHashMap<hir::ItemLocalId, Scope>,
230 /// Maps from a `NodeId` to the associated destruction scope (if any).
231 destruction_scopes: FxHashMap<hir::ItemLocalId, Scope>,
233 /// `rvalue_scopes` includes entries for those expressions whose
234 /// cleanup scope is larger than the default. The map goes from the
235 /// expression ID to the cleanup scope id. For rvalues not present in
236 /// this table, the appropriate cleanup scope is the innermost
237 /// enclosing statement, conditional expression, or repeating
238 /// block (see `terminating_scopes`).
239 /// In constants, None is used to indicate that certain expressions
240 /// escape into 'static and should have no local cleanup scope.
241 rvalue_scopes: FxHashMap<hir::ItemLocalId, Option<Scope>>,
243 /// If there are any `yield` nested within a scope, this map
244 /// stores the `Span` of the last one and its index in the
245 /// postorder of the Visitor traversal on the HIR.
247 /// HIR Visitor postorder indexes might seem like a peculiar
248 /// thing to care about. but it turns out that HIR bindings
249 /// and the temporary results of HIR expressions are never
250 /// storage-live at the end of HIR nodes with postorder indexes
251 /// lower than theirs, and therefore don't need to be suspended
252 /// at yield-points at these indexes.
254 /// For an example, suppose we have some code such as:
255 /// ```rust,ignore (example)
256 /// foo(f(), yield y, bar(g()))
259 /// With the HIR tree (calls numbered for expository purposes)
261 /// Call#0(foo, [Call#1(f), Yield(y), Call#2(bar, Call#3(g))])
264 /// Obviously, the result of `f()` was created before the yield
265 /// (and therefore needs to be kept valid over the yield) while
266 /// the result of `g()` occurs after the yield (and therefore
267 /// doesn't). If we want to infer that, we can look at the
268 /// postorder traversal:
270 /// `foo` `f` Call#1 `y` Yield `bar` `g` Call#3 Call#2 Call#0
273 /// In which we can easily see that `Call#1` occurs before the yield,
274 /// and `Call#3` after it.
276 /// To see that this method works, consider:
278 /// Let `D` be our binding/temporary and `U` be our other HIR node, with
279 /// `HIR-postorder(U) < HIR-postorder(D)`. Suppose, as in our example,
280 /// U is the yield and D is one of the calls.
281 /// Let's show that `D` is storage-dead at `U`.
283 /// Remember that storage-live/storage-dead refers to the state of
284 /// the *storage*, and does not consider moves/drop flags.
288 /// 1. From the ordering guarantee of HIR visitors (see
289 /// `rustc_hir::intravisit`), `D` does not dominate `U`.
291 /// 2. Therefore, `D` is *potentially* storage-dead at `U` (because
292 /// we might visit `U` without ever getting to `D`).
294 /// 3. However, we guarantee that at each HIR point, each
295 /// binding/temporary is always either always storage-live
296 /// or always storage-dead. This is what is being guaranteed
297 /// by `terminating_scopes` including all blocks where the
298 /// count of executions is not guaranteed.
300 /// 4. By `2.` and `3.`, `D` is *statically* storage-dead at `U`,
303 /// This property ought to not on (3) in an essential way -- it
304 /// is probably still correct even if we have "unrestricted" terminating
305 /// scopes. However, why use the complicated proof when a simple one
308 /// A subtle thing: `box` expressions, such as `box (&x, yield 2, &y)`. It
309 /// might seem that a `box` expression creates a `Box<T>` temporary
310 /// when it *starts* executing, at `HIR-preorder(BOX-EXPR)`. That might
311 /// be true in the MIR desugaring, but it is not important in the semantics.
313 /// The reason is that semantically, until the `box` expression returns,
314 /// the values are still owned by their containing expressions. So
315 /// we'll see that `&x`.
316 pub yield_in_scope: FxHashMap<Scope, YieldData>,
318 /// The number of visit_expr and visit_pat calls done in the body.
319 /// Used to sanity check visit_expr/visit_pat call count when
320 /// calculating generator interiors.
321 pub body_expr_count: FxHashMap<hir::BodyId, usize>,
324 #[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
325 pub struct YieldData {
326 /// The `Span` of the yield.
328 /// The number of expressions and patterns appearing before the `yield` in the body, plus one.
329 pub expr_and_pat_count: usize,
330 pub source: hir::YieldSource,
334 pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) {
335 debug!("{:?}.parent = {:?}", child, parent);
337 if let Some(p) = parent {
338 let prev = self.parent_map.insert(child, p);
339 assert!(prev.is_none());
342 // Record the destruction scopes for later so we can query them.
343 if let ScopeData::Destruction = child.data {
344 self.destruction_scopes.insert(child.item_local_id(), child);
348 pub fn opt_destruction_scope(&self, n: hir::ItemLocalId) -> Option<Scope> {
349 self.destruction_scopes.get(&n).cloned()
352 pub fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) {
353 debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime);
354 assert!(var != lifetime.item_local_id());
355 self.var_map.insert(var, lifetime);
358 pub fn record_rvalue_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) {
359 debug!("record_rvalue_scope(sub={:?}, sup={:?})", var, lifetime);
360 if let Some(lifetime) = lifetime {
361 assert!(var != lifetime.item_local_id());
363 self.rvalue_scopes.insert(var, lifetime);
366 /// Returns the narrowest scope that encloses `id`, if any.
367 pub fn opt_encl_scope(&self, id: Scope) -> Option<Scope> {
368 self.parent_map.get(&id).cloned().map(|(p, _)| p)
371 /// Returns the lifetime of the local variable `var_id`
372 pub fn var_scope(&self, var_id: hir::ItemLocalId) -> Scope {
376 .unwrap_or_else(|| bug!("no enclosing scope for id {:?}", var_id))
379 /// Returns the scope when the temp created by `expr_id` will be cleaned up.
380 pub fn temporary_scope(&self, expr_id: hir::ItemLocalId) -> Option<Scope> {
381 // Check for a designated rvalue scope.
382 if let Some(&s) = self.rvalue_scopes.get(&expr_id) {
383 debug!("temporary_scope({:?}) = {:?} [custom]", expr_id, s);
387 // Otherwise, locate the innermost terminating scope
388 // if there's one. Static items, for instance, won't
389 // have an enclosing scope, hence no scope will be
391 let mut id = Scope { id: expr_id, data: ScopeData::Node };
393 while let Some(&(p, _)) = self.parent_map.get(&id) {
395 ScopeData::Destruction => {
396 debug!("temporary_scope({:?}) = {:?} [enclosing]", expr_id, id);
403 debug!("temporary_scope({:?}) = None", expr_id);
407 /// Returns `true` if `subscope` is equal to or is lexically nested inside `superscope`, and
408 /// `false` otherwise.
411 pub fn is_subscope_of(&self, subscope: Scope, superscope: Scope) -> bool {
412 let mut s = subscope;
413 debug!("is_subscope_of({:?}, {:?})", subscope, superscope);
414 while superscope != s {
415 match self.opt_encl_scope(s) {
417 debug!("is_subscope_of({:?}, {:?}, s={:?})=false", subscope, superscope, s);
420 Some(scope) => s = scope,
424 debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope);
429 /// Checks whether the given scope contains a `yield`. If so,
430 /// returns `Some(YieldData)`. If not, returns `None`.
431 pub fn yield_in_scope(&self, scope: Scope) -> Option<YieldData> {
432 self.yield_in_scope.get(&scope).cloned()
435 /// Gives the number of expressions visited in a body.
436 /// Used to sanity check visit_expr call count when
437 /// calculating generator interiors.
438 pub fn body_expr_count(&self, body_id: hir::BodyId) -> Option<usize> {
439 self.body_expr_count.get(&body_id).copied()
443 impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree {
444 fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
451 ref destruction_scopes,
456 hcx.with_node_id_hashing_mode(NodeIdHashingMode::HashDefPath, |hcx| {
457 root_body.hash_stable(hcx, hasher);
458 root_parent.hash_stable(hcx, hasher);
461 body_expr_count.hash_stable(hcx, hasher);
462 parent_map.hash_stable(hcx, hasher);
463 var_map.hash_stable(hcx, hasher);
464 destruction_scopes.hash_stable(hcx, hasher);
465 rvalue_scopes.hash_stable(hcx, hasher);
466 yield_in_scope.hash_stable(hcx, hasher);