compiler/rustc_middle/src/middle/region.rs - rust - Git at Google

 //! 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 std::fmt;

 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::unord::UnordMap;
 use rustc_hir as hir;
 use rustc_hir::{HirId, HirIdMap, Node};
 use rustc_macros::{HashStable, TyDecodable, TyEncodable};
 use rustc_span::{DUMMY_SP, Span};
 use tracing::debug;

 use crate::ty::TyCtxt;

 /// 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::LateParamRegion`, 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 local_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.local_id),
             ScopeData::CallSite => write!(fmt, "CallSite({:?})", self.local_id),
             ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.local_id),
             ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.local_id),
             ScopeData::IfThen => write!(fmt, "IfThen({:?})", self.local_id),
             ScopeData::IfThenRescope => write!(fmt, "IfThen[edition2024]({:?})", self.local_id),
             ScopeData::MatchGuard => write!(fmt, "MatchGuard({:?})", self.local_id),
             ScopeData::Remainder(fsi) => write!(
                 fmt,
                 "Remainder {{ block: {:?}, first_statement_index: {}}}",
                 self.local_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 of the condition and then block of an if expression
     /// Used for variables introduced in an if-let expression,
     /// whose lifetimes do not cross beyond this scope.
     IfThenRescope,

     /// Scope of the condition and body of a match arm with a guard
     /// Used for variables introduced in an if-let guard,
     /// whose lifetimes do not cross beyond this scope.
     MatchGuard,

     /// 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 `...`.
     #[derive(HashStable)]
     #[encodable]
     #[orderable]
     pub struct FirstStatementIndex {}
 }

 // compilation error if size of `ScopeData` is not the same as a `u32`
 rustc_data_structures::static_assert_size!(ScopeData, 4);

 impl Scope {
     pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<HirId> {
         scope_tree.root_body.map(|hir_id| HirId { owner: hir_id.owner, local_id: self.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
             // 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 Node::Block(blk) = tcx.hir_node(hir_id)
         {
             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
     }
 }

 /// The region scope tree encodes information about region relationships.
 #[derive(Default, Debug, HashStable)]
 pub struct ScopeTree {
     /// If not empty, this body is the root of this region hierarchy.
     pub root_body: Option<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>,

     /// Maps from a variable or binding ID to the block in which that
     /// variable is declared.
     var_map: 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: HirIdMap<RvalueCandidate>,

     /// Backwards incompatible scoping that will be introduced in future editions.
     /// This information is used later for linting to identify locals and
     /// temporary values that will receive backwards-incompatible drop orders.
     pub backwards_incompatible_scope: UnordMap<hir::ItemLocalId, Scope>,
 }

 /// See the `rvalue_candidates` field for more information on rvalue
 /// candidates in general.
 /// The `lifetime` field is None to indicate that certain expressions escape
 /// into 'static and should have no local cleanup scope.
 #[derive(Debug, Copy, Clone, HashStable)]
 pub struct RvalueCandidate {
     pub target: hir::ItemLocalId,
     pub lifetime: Option<Scope>,
 }

 impl ScopeTree {
     pub fn record_scope_parent(&mut self, child: Scope, parent: Option<Scope>) {
         debug!("{:?}.parent = {:?}", child, parent);

         if let Some(p) = parent {
             let prev = self.parent_map.insert(child, p);
             assert!(prev.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.local_id);
         self.var_map.insert(var, lifetime);
     }

     pub fn record_rvalue_candidate(&mut self, var: HirId, candidate: RvalueCandidate) {
         debug!("record_rvalue_candidate(var={var:?}, candidate={candidate:?})");
         if let Some(lifetime) = &candidate.lifetime {
             assert!(var.local_id != lifetime.local_id)
         }
         self.rvalue_candidates.insert(var, candidate);
     }

     /// 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()
     }

     /// 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
     }
 }
	//! 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 std::fmt;

	use rustc_data_structures::fx::FxIndexMap;
	use rustc_data_structures::unord::UnordMap;
	use rustc_hir as hir;
	use rustc_hir::{HirId, HirIdMap, Node};
	use rustc_macros::{HashStable, TyDecodable, TyEncodable};
	use rustc_span::{DUMMY_SP, Span};
	use tracing::debug;

	use crate::ty::TyCtxt;

	/// 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::LateParamRegion`, 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 local_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.local_id),
	ScopeData::CallSite => write!(fmt, "CallSite({:?})", self.local_id),
	ScopeData::Arguments => write!(fmt, "Arguments({:?})", self.local_id),
	ScopeData::Destruction => write!(fmt, "Destruction({:?})", self.local_id),
	ScopeData::IfThen => write!(fmt, "IfThen({:?})", self.local_id),
	ScopeData::IfThenRescope => write!(fmt, "IfThen[edition2024]({:?})", self.local_id),
	ScopeData::MatchGuard => write!(fmt, "MatchGuard({:?})", self.local_id),
	ScopeData::Remainder(fsi) => write!(
	fmt,
	"Remainder {{ block: {:?}, first_statement_index: {}}}",
	self.local_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 of the condition and then block of an if expression
	/// Used for variables introduced in an if-let expression,
	/// whose lifetimes do not cross beyond this scope.
	IfThenRescope,

	/// Scope of the condition and body of a match arm with a guard
	/// Used for variables introduced in an if-let guard,
	/// whose lifetimes do not cross beyond this scope.
	MatchGuard,

	/// 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 `...`.
	#[derive(HashStable)]
	#[encodable]
	#[orderable]
	pub struct FirstStatementIndex {}
	}

	// compilation error if size of `ScopeData` is not the same as a `u32`
	rustc_data_structures::static_assert_size!(ScopeData, 4);

	impl Scope {
	pub fn hir_id(&self, scope_tree: &ScopeTree) -> Option<HirId> {
	scope_tree.root_body.map(\|hir_id\| HirId { owner: hir_id.owner, local_id: self.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
	// 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 Node::Block(blk) = tcx.hir_node(hir_id)
	{
	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
	}
	}

	/// The region scope tree encodes information about region relationships.
	#[derive(Default, Debug, HashStable)]
	pub struct ScopeTree {
	/// If not empty, this body is the root of this region hierarchy.
	pub root_body: Option<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>,

	/// Maps from a variable or binding ID to the block in which that
	/// variable is declared.
	var_map: 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: HirIdMap<RvalueCandidate>,

	/// Backwards incompatible scoping that will be introduced in future editions.
	/// This information is used later for linting to identify locals and
	/// temporary values that will receive backwards-incompatible drop orders.
	pub backwards_incompatible_scope: UnordMap<hir::ItemLocalId, Scope>,
	}

	/// See the `rvalue_candidates` field for more information on rvalue
	/// candidates in general.
	/// The `lifetime` field is None to indicate that certain expressions escape
	/// into 'static and should have no local cleanup scope.
	#[derive(Debug, Copy, Clone, HashStable)]
	pub struct RvalueCandidate {
	pub target: hir::ItemLocalId,
	pub lifetime: Option<Scope>,
	}

	impl ScopeTree {
	pub fn record_scope_parent(&mut self, child: Scope, parent: Option<Scope>) {
	debug!("{:?}.parent = {:?}", child, parent);

	if let Some(p) = parent {
	let prev = self.parent_map.insert(child, p);
	assert!(prev.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.local_id);
	self.var_map.insert(var, lifetime);
	}

	pub fn record_rvalue_candidate(&mut self, var: HirId, candidate: RvalueCandidate) {
	debug!("record_rvalue_candidate(var={var:?}, candidate={candidate:?})");
	if let Some(lifetime) = &candidate.lifetime {
	assert!(var.local_id != lifetime.local_id)
	}
	self.rvalue_candidates.insert(var, candidate);
	}

	/// 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()
	}

	/// 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
	}
	}