rustc_middle/middle/

region.rs

//! 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, ItemLocalMap, Node};
use rustc_macros::{StableHash, TyDecodable, TyEncodable};
use rustc_span::{DUMMY_SP, Span};
use tracing::debug;

use crate::ty::{self, 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(StableHash)]
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(StableHash)]
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 `...`.
    #[stable_hash]
    #[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, StableHash)]
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>,

    /// Tracks expressions with extended temporary scopes, based on the syntactic rules for
    /// temporary lifetime extension. Further details may be found in
    /// `rustc_hir_analysis::check::region` and in the [Reference].
    ///
    /// [Reference]: https://doc.rust-lang.org/nightly/reference/destructors.html#temporary-lifetime-extension
    extended_temp_scopes: ItemLocalMap<Option<Scope>>,

    /// 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>,
}

/// Temporary lifetime information for expressions, used when lowering to MIR.
#[derive(Clone, Copy, Debug, StableHash)]
pub struct TempLifetime {
    /// The scope in which a temporary should be dropped. If `None`, no drop is scheduled; this is
    /// the case for lifetime-extended temporaries extended by a const/static item or const block.
    pub temp_lifetime: Option<Scope>,
    /// If `Some(lt)`, indicates that the lifetime of this temporary will change to `lt` in a future edition.
    /// If `None`, then no changes are expected, or lints are disabled.
    pub backwards_incompatible: 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);
    }

    /// Make an association between a sub-expression and an extended lifetime
    pub fn record_extended_temp_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) {
        debug!(?var, ?lifetime);
        if let Some(lifetime) = lifetime {
            assert!(var != lifetime.local_id);
        }
        self.extended_temp_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()
    }

    /// 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
    }

    /// Returns the scope of non-lifetime-extended temporaries within a given scope, as well as
    /// whether we've recorded a potential backwards-incompatible change to lint on.
    /// Panics if no enclosing temporary scope is found.
    pub fn default_temporary_scope(&self, inner: Scope) -> (Scope, Option<Scope>) {
        let mut id = inner;
        let mut backwards_incompatible = None;

        while let Some(&p) = self.parent_map.get(&id) {
            match p.data {
                ScopeData::Destruction => {
                    debug!("temporary_scope({inner:?}) = {id:?} [enclosing]");
                    return (id, backwards_incompatible);
                }
                ScopeData::IfThenRescope | ScopeData::MatchGuard => {
                    debug!("temporary_scope({inner:?}) = {p:?} [enclosing]");
                    return (p, backwards_incompatible);
                }
                ScopeData::Node
                | ScopeData::CallSite
                | ScopeData::Arguments
                | ScopeData::IfThen
                | ScopeData::Remainder(_) => {
                    // If we haven't already passed through a backwards-incompatible node,
                    // then check if we are passing through one now and record it if so.
                    // This is for now only working for cases where a temporary lifetime is
                    // *shortened*.
                    if backwards_incompatible.is_none() {
                        backwards_incompatible =
                            self.backwards_incompatible_scope.get(&p.local_id).copied();
                    }
                    id = p
                }
            }
        }

        span_bug!(ty::tls::with(|tcx| inner.span(tcx, self)), "no enclosing temporary scope")
    }

    /// Returns the scope when the temp created by `expr_id` will be cleaned up.
    /// It also emits a lint on potential backwards incompatible change to the temporary scope
    /// which is *for now* always shortening.
    pub fn temporary_scope(&self, expr_id: hir::ItemLocalId) -> TempLifetime {
        // Check for a designated extended temporary scope.
        if let Some(&s) = self.extended_temp_scopes.get(&expr_id) {
            debug!("temporary_scope({expr_id:?}) = {s:?} [custom]");
            return TempLifetime { temp_lifetime: s, backwards_incompatible: None };
        }

        // Otherwise, locate the innermost terminating scope.
        let (scope, backwards_incompatible) =
            self.default_temporary_scope(Scope { local_id: expr_id, data: ScopeData::Node });
        TempLifetime { temp_lifetime: Some(scope), backwards_incompatible }
    }
}