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Diffstat (limited to 'compiler/rustc_middle/src/middle/region.rs')
| -rw-r--r-- | compiler/rustc_middle/src/middle/region.rs | 443 |
1 files changed, 443 insertions, 0 deletions
diff --git a/compiler/rustc_middle/src/middle/region.rs b/compiler/rustc_middle/src/middle/region.rs new file mode 100644 index 000000000..c886175c6 --- /dev/null +++ b/compiler/rustc_middle/src/middle/region.rs @@ -0,0 +1,443 @@ +//! 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::ty::TyCtxt; +use rustc_data_structures::fx::{FxHashMap, FxIndexMap}; +use rustc_data_structures::stable_hasher::{HashStable, StableHasher}; +use rustc_hir as hir; +use rustc_hir::Node; +use rustc_macros::HashStable; +use rustc_query_system::ich::StableHashingContext; +use rustc_span::{Span, DUMMY_SP}; + +use std::fmt; +use std::ops::Deref; + +/// 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, TyEncodable, TyDecodable)] +#[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::IfThen => write!(fmt, "IfThen({:?})", 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, TyEncodable, TyDecodable)] +#[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 of the condition and then block of an if expression + /// Used for variables introduced in an if-let expression. + IfThen, + + /// 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 an 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 Some(hir_id) = self.hir_id(scope_tree) else { + 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.with_lo(stmt_span.lo()); + } + } + } + span + } +} + +pub type ScopeDepth = u32; + +/// The region scope tree encodes information about region relationships. +#[derive(TyEncodable, TyDecodable, Default, Debug)] +pub struct ScopeTree { + /// If not empty, this body is the root of this region hierarchy. + pub root_body: 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: FxIndexMap<Scope, (Scope, ScopeDepth)>, + + /// Maps from a variable or binding ID to the block in which that + /// variable is declared. + var_map: FxIndexMap<hir::ItemLocalId, Scope>, + + /// Maps from a `NodeId` to the associated destruction scope (if any). + destruction_scopes: FxIndexMap<hir::ItemLocalId, Scope>, + + /// Identifies expressions which, if captured into a temporary, ought to + /// have a temporary whose lifetime extends to the end of the enclosing *block*, + /// and not the enclosing *statement*. Expressions that are not present in this + /// table are not rvalue candidates. The set of rvalue candidates is computed + /// during type check based on a traversal of the AST. + pub rvalue_candidates: FxHashMap<hir::HirId, RvalueCandidateType>, + + /// 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) + /// + /// ```text + /// 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)`. Suppose, as in our example, + /// U is the yield and D is 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, Vec<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>, +} + +/// Identifies the reason that a given expression is an rvalue candidate +/// (see the `rvalue_candidates` field for more information what rvalue +/// candidates in general). In constants, the `lifetime` field is None +/// to indicate that certain expressions escape into 'static and +/// should have no local cleanup scope. +#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)] +pub enum RvalueCandidateType { + Borrow { target: hir::ItemLocalId, lifetime: Option<Scope> }, + Pattern { target: hir::ItemLocalId, lifetime: Option<Scope> }, +} + +#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, 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() + } + + 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_candidate( + &mut self, + var: hir::HirId, + candidate_type: RvalueCandidateType, + ) { + debug!("record_rvalue_candidate(var={var:?}, type={candidate_type:?})"); + match &candidate_type { + RvalueCandidateType::Borrow { lifetime: Some(lifetime), .. } + | RvalueCandidateType::Pattern { lifetime: Some(lifetime), .. } => { + assert!(var.local_id != lifetime.item_local_id()) + } + _ => {} + } + self.rvalue_candidates.insert(var, candidate_type); + } + + /// 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`, if any. + pub fn var_scope(&self, var_id: hir::ItemLocalId) -> Option<Scope> { + self.var_map.get(&var_id).cloned() + } + + /// Returns `true` if `subscope` is equal to or is lexically nested inside `superscope`, and + /// `false` otherwise. + /// + /// Used by clippy. + 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(YieldData)`. If not, returns `None`. + pub fn yield_in_scope(&self, scope: Scope) -> Option<&[YieldData]> { + self.yield_in_scope.get(&scope).map(Deref::deref) + } + + /// 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, + ref body_expr_count, + ref parent_map, + ref var_map, + ref destruction_scopes, + ref rvalue_candidates, + ref yield_in_scope, + } = *self; + + root_body.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_candidates.hash_stable(hcx, hasher); + yield_in_scope.hash_stable(hcx, hasher); + } +} |