6044fbe462
remove region_scope_tree from RegionCtxt Apply suggestions from code review Co-authored-by: Niko Matsakis <niko@alum.mit.edu>
442 lines
18 KiB
Rust
442 lines
18 KiB
Rust
//! This file declares the `ScopeTree` type, which describes
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//! the parent links in the region hierarchy.
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//!
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//! For more information about how MIR-based region-checking works,
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//! see the [rustc dev guide].
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//!
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//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
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use crate::ty::TyCtxt;
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use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
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use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
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use rustc_hir as hir;
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use rustc_hir::Node;
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use rustc_macros::HashStable;
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use rustc_query_system::ich::StableHashingContext;
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use rustc_span::{Span, DUMMY_SP};
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use std::fmt;
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/// Represents a statically-describable scope that can be used to
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/// bound the lifetime/region for values.
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///
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/// `Node(node_id)`: Any AST node that has any scope at all has the
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/// `Node(node_id)` scope. Other variants represent special cases not
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/// immediately derivable from the abstract syntax tree structure.
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///
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/// `DestructionScope(node_id)` represents the scope of destructors
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/// implicitly-attached to `node_id` that run immediately after the
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/// expression for `node_id` itself. Not every AST node carries a
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/// `DestructionScope`, but those that are `terminating_scopes` do;
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/// see discussion with `ScopeTree`.
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///
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/// `Remainder { block, statement_index }` represents
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/// the scope of user code running immediately after the initializer
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/// expression for the indexed statement, until the end of the block.
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///
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/// So: the following code can be broken down into the scopes beneath:
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///
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/// ```text
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/// let a = f().g( 'b: { let x = d(); let y = d(); x.h(y) } ) ;
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///
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/// +-+ (D12.)
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/// +-+ (D11.)
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/// +---------+ (R10.)
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/// +-+ (D9.)
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/// +----------+ (M8.)
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/// +----------------------+ (R7.)
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/// +-+ (D6.)
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/// +----------+ (M5.)
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/// +-----------------------------------+ (M4.)
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/// +--------------------------------------------------+ (M3.)
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/// +--+ (M2.)
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/// +-----------------------------------------------------------+ (M1.)
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///
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/// (M1.): Node scope of the whole `let a = ...;` statement.
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/// (M2.): Node scope of the `f()` expression.
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/// (M3.): Node scope of the `f().g(..)` expression.
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/// (M4.): Node scope of the block labeled `'b:`.
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/// (M5.): Node scope of the `let x = d();` statement
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/// (D6.): DestructionScope for temporaries created during M5.
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/// (R7.): Remainder scope for block `'b:`, stmt 0 (let x = ...).
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/// (M8.): Node scope of the `let y = d();` statement.
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/// (D9.): DestructionScope for temporaries created during M8.
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/// (R10.): Remainder scope for block `'b:`, stmt 1 (let y = ...).
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/// (D11.): DestructionScope for temporaries and bindings from block `'b:`.
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/// (D12.): DestructionScope for temporaries created during M1 (e.g., f()).
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/// ```
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///
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/// Note that while the above picture shows the destruction scopes
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/// as following their corresponding node scopes, in the internal
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/// data structures of the compiler the destruction scopes are
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/// represented as enclosing parents. This is sound because we use the
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/// enclosing parent relationship just to ensure that referenced
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/// values live long enough; phrased another way, the starting point
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/// of each range is not really the important thing in the above
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/// picture, but rather the ending point.
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//
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// FIXME(pnkfelix): this currently derives `PartialOrd` and `Ord` to
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// placate the same deriving in `ty::FreeRegion`, but we may want to
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// actually attach a more meaningful ordering to scopes than the one
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// generated via deriving here.
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#[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Copy, TyEncodable, TyDecodable)]
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#[derive(HashStable)]
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pub struct Scope {
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pub id: hir::ItemLocalId,
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pub data: ScopeData,
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}
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impl fmt::Debug for Scope {
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fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self.data {
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ScopeData::Node => write!(fmt, "Node({:?})", self.id),
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ScopeData::CallSite => write!(fmt, "CallSite({:?})", self.id),
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ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.id),
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ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.id),
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ScopeData::IfThen => write!(fmt, "IfThen({:?})", self.id),
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ScopeData::Remainder(fsi) => write!(
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fmt,
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"Remainder {{ block: {:?}, first_statement_index: {}}}",
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self.id,
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fsi.as_u32(),
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),
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}
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}
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}
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#[derive(Clone, PartialEq, PartialOrd, Eq, Ord, Hash, Debug, Copy, TyEncodable, TyDecodable)]
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#[derive(HashStable)]
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pub enum ScopeData {
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Node,
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/// Scope of the call-site for a function or closure
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/// (outlives the arguments as well as the body).
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CallSite,
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/// Scope of arguments passed to a function or closure
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/// (they outlive its body).
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Arguments,
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/// Scope of destructors for temporaries of node-id.
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Destruction,
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/// Scope of the condition and then block of an if expression
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/// Used for variables introduced in an if-let expression.
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IfThen,
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/// Scope following a `let id = expr;` binding in a block.
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Remainder(FirstStatementIndex),
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}
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rustc_index::newtype_index! {
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/// Represents a subscope of `block` for a binding that is introduced
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/// by `block.stmts[first_statement_index]`. Such subscopes represent
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/// a suffix of the block. Note that each subscope does not include
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/// the initializer expression, if any, for the statement indexed by
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/// `first_statement_index`.
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///
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/// For example, given `{ let (a, b) = EXPR_1; let c = EXPR_2; ... }`:
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///
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/// * The subscope with `first_statement_index == 0` is scope of both
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/// `a` and `b`; it does not include EXPR_1, but does include
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/// everything after that first `let`. (If you want a scope that
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/// includes EXPR_1 as well, then do not use `Scope::Remainder`,
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/// but instead another `Scope` that encompasses the whole block,
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/// e.g., `Scope::Node`.
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///
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/// * The subscope with `first_statement_index == 1` is scope of `c`,
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/// and thus does not include EXPR_2, but covers the `...`.
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pub struct FirstStatementIndex {
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derive [HashStable]
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}
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}
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// compilation error if size of `ScopeData` is not the same as a `u32`
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static_assert_size!(ScopeData, 4);
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impl Scope {
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/// Returns an item-local ID associated with this scope.
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///
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/// N.B., likely to be replaced as API is refined; e.g., pnkfelix
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/// anticipates `fn entry_node_id` and `fn each_exit_node_id`.
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pub fn item_local_id(&self) -> hir::ItemLocalId {
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self.id
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}
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pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<hir::HirId> {
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scope_tree
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.root_body
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.map(|hir_id| hir::HirId { owner: hir_id.owner, local_id: self.item_local_id() })
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}
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/// Returns the span of this `Scope`. Note that in general the
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/// returned span may not correspond to the span of any `NodeId` in
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/// the AST.
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pub fn span(&self, tcx: TyCtxt<'_>, scope_tree: &ScopeTree) -> Span {
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let Some(hir_id) = self.hir_id(scope_tree) else {
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return DUMMY_SP;
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};
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let span = tcx.hir().span(hir_id);
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if let ScopeData::Remainder(first_statement_index) = self.data {
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if let Node::Block(ref blk) = tcx.hir().get(hir_id) {
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// Want span for scope starting after the
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// indexed statement and ending at end of
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// `blk`; reuse span of `blk` and shift `lo`
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// forward to end of indexed statement.
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//
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// (This is the special case alluded to in the
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// doc-comment for this method)
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let stmt_span = blk.stmts[first_statement_index.index()].span;
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// To avoid issues with macro-generated spans, the span
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// of the statement must be nested in that of the block.
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if span.lo() <= stmt_span.lo() && stmt_span.lo() <= span.hi() {
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return span.with_lo(stmt_span.lo());
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}
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}
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}
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span
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}
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}
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pub type ScopeDepth = u32;
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/// The region scope tree encodes information about region relationships.
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#[derive(TyEncodable, TyDecodable, Default, Debug)]
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pub struct ScopeTree {
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/// If not empty, this body is the root of this region hierarchy.
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pub root_body: Option<hir::HirId>,
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/// Maps from a scope ID to the enclosing scope id;
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/// this is usually corresponding to the lexical nesting, though
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/// in the case of closures the parent scope is the innermost
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/// conditional expression or repeating block. (Note that the
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/// enclosing scope ID for the block associated with a closure is
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/// the closure itself.)
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pub parent_map: FxIndexMap<Scope, (Scope, ScopeDepth)>,
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/// Maps from a variable or binding ID to the block in which that
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/// variable is declared.
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var_map: FxIndexMap<hir::ItemLocalId, Scope>,
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/// Maps from a `NodeId` to the associated destruction scope (if any).
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destruction_scopes: FxIndexMap<hir::ItemLocalId, Scope>,
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/// Identifies expressions which, if captured into a temporary, ought to
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/// have a temporary whose lifetime extends to the end of the enclosing *block*,
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/// and not the enclosing *statement*. Expressions that are not present in this
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/// table are not rvalue candidates. The set of rvalue candidates is computed
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/// during type check based on a traversal of the AST.
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pub rvalue_candidates: FxHashMap<hir::HirId, RvalueCandidateType>,
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/// If there are any `yield` nested within a scope, this map
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/// stores the `Span` of the last one and its index in the
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/// postorder of the Visitor traversal on the HIR.
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///
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/// HIR Visitor postorder indexes might seem like a peculiar
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/// thing to care about. but it turns out that HIR bindings
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/// and the temporary results of HIR expressions are never
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/// storage-live at the end of HIR nodes with postorder indexes
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/// lower than theirs, and therefore don't need to be suspended
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/// at yield-points at these indexes.
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///
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/// For an example, suppose we have some code such as:
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/// ```rust,ignore (example)
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/// foo(f(), yield y, bar(g()))
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/// ```
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///
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/// With the HIR tree (calls numbered for expository purposes)
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///
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/// ```text
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/// Call#0(foo, [Call#1(f), Yield(y), Call#2(bar, Call#3(g))])
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/// ```
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///
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/// Obviously, the result of `f()` was created before the yield
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/// (and therefore needs to be kept valid over the yield) while
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/// the result of `g()` occurs after the yield (and therefore
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/// doesn't). If we want to infer that, we can look at the
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/// postorder traversal:
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/// ```plain,ignore
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/// `foo` `f` Call#1 `y` Yield `bar` `g` Call#3 Call#2 Call#0
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/// ```
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///
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/// In which we can easily see that `Call#1` occurs before the yield,
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/// and `Call#3` after it.
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///
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/// To see that this method works, consider:
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///
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/// Let `D` be our binding/temporary and `U` be our other HIR node, with
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/// `HIR-postorder(U) < HIR-postorder(D)`. Suppose, as in our example,
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/// U is the yield and D is one of the calls.
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/// Let's show that `D` is storage-dead at `U`.
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///
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/// Remember that storage-live/storage-dead refers to the state of
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/// the *storage*, and does not consider moves/drop flags.
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///
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/// Then:
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///
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/// 1. From the ordering guarantee of HIR visitors (see
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/// `rustc_hir::intravisit`), `D` does not dominate `U`.
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///
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/// 2. Therefore, `D` is *potentially* storage-dead at `U` (because
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/// we might visit `U` without ever getting to `D`).
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///
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/// 3. However, we guarantee that at each HIR point, each
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/// binding/temporary is always either always storage-live
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/// or always storage-dead. This is what is being guaranteed
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/// by `terminating_scopes` including all blocks where the
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/// count of executions is not guaranteed.
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///
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/// 4. By `2.` and `3.`, `D` is *statically* storage-dead at `U`,
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/// QED.
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///
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/// This property ought to not on (3) in an essential way -- it
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/// is probably still correct even if we have "unrestricted" terminating
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/// scopes. However, why use the complicated proof when a simple one
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/// works?
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///
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/// A subtle thing: `box` expressions, such as `box (&x, yield 2, &y)`. It
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/// might seem that a `box` expression creates a `Box<T>` temporary
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/// when it *starts* executing, at `HIR-preorder(BOX-EXPR)`. That might
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/// be true in the MIR desugaring, but it is not important in the semantics.
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///
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/// The reason is that semantically, until the `box` expression returns,
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/// the values are still owned by their containing expressions. So
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/// we'll see that `&x`.
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pub yield_in_scope: FxHashMap<Scope, Vec<YieldData>>,
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/// The number of visit_expr and visit_pat calls done in the body.
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/// Used to sanity check visit_expr/visit_pat call count when
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/// calculating generator interiors.
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pub body_expr_count: FxHashMap<hir::BodyId, usize>,
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}
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/// Identifies the reason that a given expression is an rvalue candidate
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/// (see the `rvalue_candidates` field for more information what rvalue
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/// candidates in general). In constants, the `lifetime` field is None
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/// to indicate that certain expressions escape into 'static and
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/// should have no local cleanup scope.
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#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
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pub enum RvalueCandidateType {
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Borrow { target: hir::ItemLocalId, lifetime: Option<Scope> },
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Pattern { target: hir::ItemLocalId, lifetime: Option<Scope> },
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}
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#[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
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pub struct YieldData {
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/// The `Span` of the yield.
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pub span: Span,
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/// The number of expressions and patterns appearing before the `yield` in the body, plus one.
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pub expr_and_pat_count: usize,
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pub source: hir::YieldSource,
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}
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impl ScopeTree {
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pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) {
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debug!("{:?}.parent = {:?}", child, parent);
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if let Some(p) = parent {
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let prev = self.parent_map.insert(child, p);
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assert!(prev.is_none());
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}
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// Record the destruction scopes for later so we can query them.
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if let ScopeData::Destruction = child.data {
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self.destruction_scopes.insert(child.item_local_id(), child);
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}
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}
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pub fn opt_destruction_scope(&self, n: hir::ItemLocalId) -> Option<Scope> {
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self.destruction_scopes.get(&n).cloned()
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}
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pub fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) {
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debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime);
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assert!(var != lifetime.item_local_id());
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self.var_map.insert(var, lifetime);
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}
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pub fn record_rvalue_candidate(
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&mut self,
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var: hir::HirId,
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candidate_type: RvalueCandidateType,
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) {
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debug!("record_rvalue_candidate(var={var:?}, type={candidate_type:?})");
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match &candidate_type {
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RvalueCandidateType::Borrow { lifetime: Some(lifetime), .. }
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| RvalueCandidateType::Pattern { lifetime: Some(lifetime), .. } => {
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assert!(var.local_id != lifetime.item_local_id())
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}
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_ => {}
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}
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self.rvalue_candidates.insert(var, candidate_type);
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}
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/// Returns the narrowest scope that encloses `id`, if any.
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pub fn opt_encl_scope(&self, id: Scope) -> Option<Scope> {
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self.parent_map.get(&id).cloned().map(|(p, _)| p)
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}
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/// Returns the lifetime of the local variable `var_id`, if any.
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pub fn var_scope(&self, var_id: hir::ItemLocalId) -> Option<Scope> {
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self.var_map.get(&var_id).cloned()
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}
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/// Returns `true` if `subscope` is equal to or is lexically nested inside `superscope`, and
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/// `false` otherwise.
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///
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/// Used by clippy.
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pub fn is_subscope_of(&self, subscope: Scope, superscope: Scope) -> bool {
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let mut s = subscope;
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debug!("is_subscope_of({:?}, {:?})", subscope, superscope);
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while superscope != s {
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match self.opt_encl_scope(s) {
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None => {
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debug!("is_subscope_of({:?}, {:?}, s={:?})=false", subscope, superscope, s);
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return false;
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}
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Some(scope) => s = scope,
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}
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}
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debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope);
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true
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}
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/// Checks whether the given scope contains a `yield`. If so,
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/// returns `Some(YieldData)`. If not, returns `None`.
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pub fn yield_in_scope(&self, scope: Scope) -> Option<&Vec<YieldData>> {
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self.yield_in_scope.get(&scope)
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}
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/// Gives the number of expressions visited in a body.
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/// Used to sanity check visit_expr call count when
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/// calculating generator interiors.
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pub fn body_expr_count(&self, body_id: hir::BodyId) -> Option<usize> {
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self.body_expr_count.get(&body_id).copied()
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}
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}
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impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree {
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fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
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let ScopeTree {
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root_body,
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ref body_expr_count,
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ref parent_map,
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ref var_map,
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ref destruction_scopes,
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ref rvalue_candidates,
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ref yield_in_scope,
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} = *self;
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root_body.hash_stable(hcx, hasher);
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body_expr_count.hash_stable(hcx, hasher);
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parent_map.hash_stable(hcx, hasher);
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var_map.hash_stable(hcx, hasher);
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destruction_scopes.hash_stable(hcx, hasher);
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rvalue_candidates.hash_stable(hcx, hasher);
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yield_in_scope.hash_stable(hcx, hasher);
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}
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}
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