| //! This file declares the `ScopeTree` type, which describes |
| //! the parent links in the region hierarchy. |
| //! |
| //! For more information about how MIR-based region-checking works, |
| //! see the [rustc dev guide]. |
| //! |
| //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html |
| |
| use crate::ich::{NodeIdHashingMode, StableHashingContext}; |
| use crate::ty::TyCtxt; |
| use rustc_hir as hir; |
| use rustc_hir::Node; |
| |
| use rustc_data_structures::fx::FxHashMap; |
| use rustc_data_structures::stable_hasher::{HashStable, StableHasher}; |
| use rustc_macros::HashStable; |
| use rustc_span::{Span, DUMMY_SP}; |
| |
| use std::fmt; |
| |
| /// Represents a statically-describable scope that can be used to |
| /// bound the lifetime/region for values. |
| /// |
| /// `Node(node_id)`: Any AST node that has any scope at all has the |
| /// `Node(node_id)` scope. Other variants represent special cases not |
| /// immediately derivable from the abstract syntax tree structure. |
| /// |
| /// `DestructionScope(node_id)` represents the scope of destructors |
| /// implicitly-attached to `node_id` that run immediately after the |
| /// expression for `node_id` itself. Not every AST node carries a |
| /// `DestructionScope`, but those that are `terminating_scopes` do; |
| /// see discussion with `ScopeTree`. |
| /// |
| /// `Remainder { block, statement_index }` represents |
| /// the scope of user code running immediately after the initializer |
| /// expression for the indexed statement, until the end of the block. |
| /// |
| /// So: the following code can be broken down into the scopes beneath: |
| /// |
| /// ```text |
| /// let a = f().g( 'b: { let x = d(); let y = d(); x.h(y) } ) ; |
| /// |
| /// +-+ (D12.) |
| /// +-+ (D11.) |
| /// +---------+ (R10.) |
| /// +-+ (D9.) |
| /// +----------+ (M8.) |
| /// +----------------------+ (R7.) |
| /// +-+ (D6.) |
| /// +----------+ (M5.) |
| /// +-----------------------------------+ (M4.) |
| /// +--------------------------------------------------+ (M3.) |
| /// +--+ (M2.) |
| /// +-----------------------------------------------------------+ (M1.) |
| /// |
| /// (M1.): Node scope of the whole `let a = ...;` statement. |
| /// (M2.): Node scope of the `f()` expression. |
| /// (M3.): Node scope of the `f().g(..)` expression. |
| /// (M4.): Node scope of the block labeled `'b:`. |
| /// (M5.): Node scope of the `let x = d();` statement |
| /// (D6.): DestructionScope for temporaries created during M5. |
| /// (R7.): Remainder scope for block `'b:`, stmt 0 (let x = ...). |
| /// (M8.): Node scope of the `let y = d();` statement. |
| /// (D9.): DestructionScope for temporaries created during M8. |
| /// (R10.): Remainder scope for block `'b:`, stmt 1 (let y = ...). |
| /// (D11.): DestructionScope for temporaries and bindings from block `'b:`. |
| /// (D12.): DestructionScope for temporaries created during M1 (e.g., f()). |
| /// ``` |
| /// |
| /// Note that while the above picture shows the destruction scopes |
| /// as following their corresponding node scopes, in the internal |
| /// data structures of the compiler the destruction scopes are |
| /// represented as enclosing parents. This is sound because we use the |
| /// enclosing parent relationship just to ensure that referenced |
| /// values live long enough; phrased another way, the starting point |
| /// of each range is not really the important thing in the above |
| /// picture, but rather the ending point. |
| // |
| // FIXME(pnkfelix): this currently derives `PartialOrd` and `Ord` to |
| // placate the same deriving in `ty::FreeRegion`, but we may want to |
| // actually attach a more meaningful ordering to scopes than the one |
| // generated via deriving here. |
| #[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Copy, RustcEncodable, RustcDecodable)] |
| #[derive(HashStable)] |
| pub struct Scope { |
| pub id: hir::ItemLocalId, |
| pub data: ScopeData, |
| } |
| |
| impl fmt::Debug for Scope { |
| fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { |
| match self.data { |
| ScopeData::Node => write!(fmt, "Node({:?})", self.id), |
| ScopeData::CallSite => write!(fmt, "CallSite({:?})", self.id), |
| ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.id), |
| ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.id), |
| ScopeData::Remainder(fsi) => write!( |
| fmt, |
| "Remainder {{ block: {:?}, first_statement_index: {}}}", |
| self.id, |
| fsi.as_u32(), |
| ), |
| } |
| } |
| } |
| |
| #[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Debug, Copy, RustcEncodable, RustcDecodable)] |
| #[derive(HashStable)] |
| pub enum ScopeData { |
| Node, |
| |
| /// Scope of the call-site for a function or closure |
| /// (outlives the arguments as well as the body). |
| CallSite, |
| |
| /// Scope of arguments passed to a function or closure |
| /// (they outlive its body). |
| Arguments, |
| |
| /// Scope of destructors for temporaries of node-id. |
| Destruction, |
| |
| /// Scope following a `let id = expr;` binding in a block. |
| Remainder(FirstStatementIndex), |
| } |
| |
| rustc_index::newtype_index! { |
| /// Represents a subscope of `block` for a binding that is introduced |
| /// by `block.stmts[first_statement_index]`. Such subscopes represent |
| /// a suffix of the block. Note that each subscope does not include |
| /// the initializer expression, if any, for the statement indexed by |
| /// `first_statement_index`. |
| /// |
| /// For example, given `{ let (a, b) = EXPR_1; let c = EXPR_2; ... }`: |
| /// |
| /// * The subscope with `first_statement_index == 0` is scope of both |
| /// `a` and `b`; it does not include EXPR_1, but does include |
| /// everything after that first `let`. (If you want a scope that |
| /// includes EXPR_1 as well, then do not use `Scope::Remainder`, |
| /// but instead another `Scope` that encompasses the whole block, |
| /// e.g., `Scope::Node`. |
| /// |
| /// * The subscope with `first_statement_index == 1` is scope of `c`, |
| /// and thus does not include EXPR_2, but covers the `...`. |
| pub struct FirstStatementIndex { |
| derive [HashStable] |
| } |
| } |
| |
| // compilation error if size of `ScopeData` is not the same as a `u32` |
| static_assert_size!(ScopeData, 4); |
| |
| impl Scope { |
| /// Returns a item-local ID associated with this scope. |
| /// |
| /// N.B., likely to be replaced as API is refined; e.g., pnkfelix |
| /// anticipates `fn entry_node_id` and `fn each_exit_node_id`. |
| pub fn item_local_id(&self) -> hir::ItemLocalId { |
| self.id |
| } |
| |
| pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<hir::HirId> { |
| scope_tree |
| .root_body |
| .map(|hir_id| hir::HirId { owner: hir_id.owner, local_id: self.item_local_id() }) |
| } |
| |
| /// Returns the span of this `Scope`. Note that in general the |
| /// returned span may not correspond to the span of any `NodeId` in |
| /// the AST. |
| pub fn span(&self, tcx: TyCtxt<'_>, scope_tree: &ScopeTree) -> Span { |
| let hir_id = match self.hir_id(scope_tree) { |
| Some(hir_id) => hir_id, |
| None => return DUMMY_SP, |
| }; |
| let span = tcx.hir().span(hir_id); |
| if let ScopeData::Remainder(first_statement_index) = self.data { |
| if let Node::Block(ref blk) = tcx.hir().get(hir_id) { |
| // Want span for scope starting after the |
| // indexed statement and ending at end of |
| // `blk`; reuse span of `blk` and shift `lo` |
| // forward to end of indexed statement. |
| // |
| // (This is the special case alluded to in the |
| // doc-comment for this method) |
| |
| let stmt_span = blk.stmts[first_statement_index.index()].span; |
| |
| // To avoid issues with macro-generated spans, the span |
| // of the statement must be nested in that of the block. |
| if span.lo() <= stmt_span.lo() && stmt_span.lo() <= span.hi() { |
| return Span::new(stmt_span.lo(), span.hi(), span.ctxt()); |
| } |
| } |
| } |
| span |
| } |
| } |
| |
| pub type ScopeDepth = u32; |
| |
| /// The region scope tree encodes information about region relationships. |
| #[derive(Default, Debug)] |
| pub struct ScopeTree { |
| /// If not empty, this body is the root of this region hierarchy. |
| pub root_body: Option<hir::HirId>, |
| |
| /// The parent of the root body owner, if the latter is an |
| /// an associated const or method, as impls/traits can also |
| /// have lifetime parameters free in this body. |
| pub root_parent: Option<hir::HirId>, |
| |
| /// Maps from a scope ID to the enclosing scope id; |
| /// this is usually corresponding to the lexical nesting, though |
| /// in the case of closures the parent scope is the innermost |
| /// conditional expression or repeating block. (Note that the |
| /// enclosing scope ID for the block associated with a closure is |
| /// the closure itself.) |
| pub parent_map: FxHashMap<Scope, (Scope, ScopeDepth)>, |
| |
| /// Maps from a variable or binding ID to the block in which that |
| /// variable is declared. |
| var_map: FxHashMap<hir::ItemLocalId, Scope>, |
| |
| /// Maps from a `NodeId` to the associated destruction scope (if any). |
| destruction_scopes: FxHashMap<hir::ItemLocalId, Scope>, |
| |
| /// `rvalue_scopes` includes entries for those expressions whose |
| /// cleanup scope is larger than the default. The map goes from the |
| /// expression ID to the cleanup scope id. For rvalues not present in |
| /// this table, the appropriate cleanup scope is the innermost |
| /// enclosing statement, conditional expression, or repeating |
| /// block (see `terminating_scopes`). |
| /// In constants, None is used to indicate that certain expressions |
| /// escape into 'static and should have no local cleanup scope. |
| rvalue_scopes: FxHashMap<hir::ItemLocalId, Option<Scope>>, |
| |
| /// Encodes the hierarchy of fn bodies. Every fn body (including |
| /// closures) forms its own distinct region hierarchy, rooted in |
| /// the block that is the fn body. This map points from the ID of |
| /// that root block to the ID of the root block for the enclosing |
| /// fn, if any. Thus the map structures the fn bodies into a |
| /// hierarchy based on their lexical mapping. This is used to |
| /// handle the relationships between regions in a fn and in a |
| /// closure defined by that fn. See the "Modeling closures" |
| /// section of the README in infer::region_constraints for |
| /// more details. |
| closure_tree: FxHashMap<hir::ItemLocalId, hir::ItemLocalId>, |
| |
| /// If there are any `yield` nested within a scope, this map |
| /// stores the `Span` of the last one and its index in the |
| /// postorder of the Visitor traversal on the HIR. |
| /// |
| /// HIR Visitor postorder indexes might seem like a peculiar |
| /// thing to care about. but it turns out that HIR bindings |
| /// and the temporary results of HIR expressions are never |
| /// storage-live at the end of HIR nodes with postorder indexes |
| /// lower than theirs, and therefore don't need to be suspended |
| /// at yield-points at these indexes. |
| /// |
| /// For an example, suppose we have some code such as: |
| /// ```rust,ignore (example) |
| /// foo(f(), yield y, bar(g())) |
| /// ``` |
| /// |
| /// With the HIR tree (calls numbered for expository purposes) |
| /// ``` |
| /// Call#0(foo, [Call#1(f), Yield(y), Call#2(bar, Call#3(g))]) |
| /// ``` |
| /// |
| /// Obviously, the result of `f()` was created before the yield |
| /// (and therefore needs to be kept valid over the yield) while |
| /// the result of `g()` occurs after the yield (and therefore |
| /// doesn't). If we want to infer that, we can look at the |
| /// postorder traversal: |
| /// ```plain,ignore |
| /// `foo` `f` Call#1 `y` Yield `bar` `g` Call#3 Call#2 Call#0 |
| /// ``` |
| /// |
| /// In which we can easily see that `Call#1` occurs before the yield, |
| /// and `Call#3` after it. |
| /// |
| /// To see that this method works, consider: |
| /// |
| /// Let `D` be our binding/temporary and `U` be our other HIR node, with |
| /// `HIR-postorder(U) < HIR-postorder(D)` (in our example, U would be |
| /// the yield and D would be one of the calls). Let's show that |
| /// `D` is storage-dead at `U`. |
| /// |
| /// Remember that storage-live/storage-dead refers to the state of |
| /// the *storage*, and does not consider moves/drop flags. |
| /// |
| /// Then: |
| /// 1. From the ordering guarantee of HIR visitors (see |
| /// `rustc_hir::intravisit`), `D` does not dominate `U`. |
| /// 2. Therefore, `D` is *potentially* storage-dead at `U` (because |
| /// we might visit `U` without ever getting to `D`). |
| /// 3. However, we guarantee that at each HIR point, each |
| /// binding/temporary is always either always storage-live |
| /// or always storage-dead. This is what is being guaranteed |
| /// by `terminating_scopes` including all blocks where the |
| /// count of executions is not guaranteed. |
| /// 4. By `2.` and `3.`, `D` is *statically* storage-dead at `U`, |
| /// QED. |
| /// |
| /// This property ought to not on (3) in an essential way -- it |
| /// is probably still correct even if we have "unrestricted" terminating |
| /// scopes. However, why use the complicated proof when a simple one |
| /// works? |
| /// |
| /// A subtle thing: `box` expressions, such as `box (&x, yield 2, &y)`. It |
| /// might seem that a `box` expression creates a `Box<T>` temporary |
| /// when it *starts* executing, at `HIR-preorder(BOX-EXPR)`. That might |
| /// be true in the MIR desugaring, but it is not important in the semantics. |
| /// |
| /// The reason is that semantically, until the `box` expression returns, |
| /// the values are still owned by their containing expressions. So |
| /// we'll see that `&x`. |
| pub yield_in_scope: FxHashMap<Scope, YieldData>, |
| |
| /// The number of visit_expr and visit_pat calls done in the body. |
| /// Used to sanity check visit_expr/visit_pat call count when |
| /// calculating generator interiors. |
| pub body_expr_count: FxHashMap<hir::BodyId, usize>, |
| } |
| |
| #[derive(Debug, Copy, Clone, RustcEncodable, RustcDecodable, HashStable)] |
| pub struct YieldData { |
| /// The `Span` of the yield. |
| pub span: Span, |
| /// The number of expressions and patterns appearing before the `yield` in the body plus one. |
| pub expr_and_pat_count: usize, |
| pub source: hir::YieldSource, |
| } |
| |
| impl ScopeTree { |
| pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) { |
| debug!("{:?}.parent = {:?}", child, parent); |
| |
| if let Some(p) = parent { |
| let prev = self.parent_map.insert(child, p); |
| assert!(prev.is_none()); |
| } |
| |
| // Record the destruction scopes for later so we can query them. |
| if let ScopeData::Destruction = child.data { |
| self.destruction_scopes.insert(child.item_local_id(), child); |
| } |
| } |
| |
| pub fn opt_destruction_scope(&self, n: hir::ItemLocalId) -> Option<Scope> { |
| self.destruction_scopes.get(&n).cloned() |
| } |
| |
| /// Records that `sub_closure` is defined within `sup_closure`. These IDs |
| /// should be the ID of the block that is the fn body, which is |
| /// also the root of the region hierarchy for that fn. |
| pub fn record_closure_parent( |
| &mut self, |
| sub_closure: hir::ItemLocalId, |
| sup_closure: hir::ItemLocalId, |
| ) { |
| debug!( |
| "record_closure_parent(sub_closure={:?}, sup_closure={:?})", |
| sub_closure, sup_closure |
| ); |
| assert!(sub_closure != sup_closure); |
| let previous = self.closure_tree.insert(sub_closure, sup_closure); |
| assert!(previous.is_none()); |
| } |
| |
| pub fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) { |
| debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime); |
| assert!(var != lifetime.item_local_id()); |
| self.var_map.insert(var, lifetime); |
| } |
| |
| pub fn record_rvalue_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) { |
| debug!("record_rvalue_scope(sub={:?}, sup={:?})", var, lifetime); |
| if let Some(lifetime) = lifetime { |
| assert!(var != lifetime.item_local_id()); |
| } |
| self.rvalue_scopes.insert(var, lifetime); |
| } |
| |
| /// Returns the narrowest scope that encloses `id`, if any. |
| pub fn opt_encl_scope(&self, id: Scope) -> Option<Scope> { |
| self.parent_map.get(&id).cloned().map(|(p, _)| p) |
| } |
| |
| /// Returns the lifetime of the local variable `var_id` |
| pub fn var_scope(&self, var_id: hir::ItemLocalId) -> Scope { |
| self.var_map |
| .get(&var_id) |
| .cloned() |
| .unwrap_or_else(|| bug!("no enclosing scope for id {:?}", var_id)) |
| } |
| |
| /// Returns the scope when the temp created by `expr_id` will be cleaned up. |
| pub fn temporary_scope(&self, expr_id: hir::ItemLocalId) -> Option<Scope> { |
| // Check for a designated rvalue scope. |
| if let Some(&s) = self.rvalue_scopes.get(&expr_id) { |
| debug!("temporary_scope({:?}) = {:?} [custom]", expr_id, s); |
| return s; |
| } |
| |
| // Otherwise, locate the innermost terminating scope |
| // if there's one. Static items, for instance, won't |
| // have an enclosing scope, hence no scope will be |
| // returned. |
| let mut id = Scope { id: expr_id, data: ScopeData::Node }; |
| |
| while let Some(&(p, _)) = self.parent_map.get(&id) { |
| match p.data { |
| ScopeData::Destruction => { |
| debug!("temporary_scope({:?}) = {:?} [enclosing]", expr_id, id); |
| return Some(id); |
| } |
| _ => id = p, |
| } |
| } |
| |
| debug!("temporary_scope({:?}) = None", expr_id); |
| None |
| } |
| |
| /// Returns `true` if `subscope` is equal to or is lexically nested inside `superscope`, and |
| /// `false` otherwise. |
| pub fn is_subscope_of(&self, subscope: Scope, superscope: Scope) -> bool { |
| let mut s = subscope; |
| debug!("is_subscope_of({:?}, {:?})", subscope, superscope); |
| while superscope != s { |
| match self.opt_encl_scope(s) { |
| None => { |
| debug!("is_subscope_of({:?}, {:?}, s={:?})=false", subscope, superscope, s); |
| return false; |
| } |
| Some(scope) => s = scope, |
| } |
| } |
| |
| debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope); |
| |
| true |
| } |
| |
| /// Checks whether the given scope contains a `yield`. If so, |
| /// returns `Some((span, expr_count))` with the span of a yield we found and |
| /// the number of expressions and patterns appearing before the `yield` in the body + 1. |
| /// If there a are multiple yields in a scope, the one with the highest number is returned. |
| pub fn yield_in_scope(&self, scope: Scope) -> Option<YieldData> { |
| self.yield_in_scope.get(&scope).cloned() |
| } |
| |
| /// Gives the number of expressions visited in a body. |
| /// Used to sanity check visit_expr call count when |
| /// calculating generator interiors. |
| pub fn body_expr_count(&self, body_id: hir::BodyId) -> Option<usize> { |
| self.body_expr_count.get(&body_id).copied() |
| } |
| } |
| |
| impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree { |
| fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) { |
| let ScopeTree { |
| root_body, |
| root_parent, |
| ref body_expr_count, |
| ref parent_map, |
| ref var_map, |
| ref destruction_scopes, |
| ref rvalue_scopes, |
| ref closure_tree, |
| ref yield_in_scope, |
| } = *self; |
| |
| hcx.with_node_id_hashing_mode(NodeIdHashingMode::HashDefPath, |hcx| { |
| root_body.hash_stable(hcx, hasher); |
| root_parent.hash_stable(hcx, hasher); |
| }); |
| |
| body_expr_count.hash_stable(hcx, hasher); |
| parent_map.hash_stable(hcx, hasher); |
| var_map.hash_stable(hcx, hasher); |
| destruction_scopes.hash_stable(hcx, hasher); |
| rvalue_scopes.hash_stable(hcx, hasher); |
| closure_tree.hash_stable(hcx, hasher); |
| yield_in_scope.hash_stable(hcx, hasher); |
| } |
| } |
