856 lines
34 KiB
Rust
856 lines
34 KiB
Rust
//! This file builds up the `ScopeTree`, 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 rustc_ast::walk_list;
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use rustc_data_structures::fx::FxHashSet;
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use rustc_hir as hir;
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use rustc_hir::def_id::DefId;
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use rustc_hir::intravisit::{self, Visitor};
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use rustc_hir::{Arm, Block, Expr, Local, Pat, PatKind, Stmt};
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use rustc_index::vec::Idx;
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use rustc_middle::middle::region::*;
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use rustc_middle::ty::query::Providers;
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use rustc_middle::ty::TyCtxt;
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use rustc_span::source_map;
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use rustc_span::Span;
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use std::mem;
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#[derive(Debug, Copy, Clone)]
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pub struct Context {
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/// The scope that contains any new variables declared, plus its depth in
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/// the scope tree.
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var_parent: Option<(Scope, ScopeDepth)>,
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/// Region parent of expressions, etc., plus its depth in the scope tree.
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parent: Option<(Scope, ScopeDepth)>,
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}
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struct RegionResolutionVisitor<'tcx> {
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tcx: TyCtxt<'tcx>,
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// The number of expressions and patterns visited in the current body.
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expr_and_pat_count: usize,
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// When this is `true`, we record the `Scopes` we encounter
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// when processing a Yield expression. This allows us to fix
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// up their indices.
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pessimistic_yield: bool,
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// Stores scopes when `pessimistic_yield` is `true`.
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fixup_scopes: Vec<Scope>,
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// The generated scope tree.
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scope_tree: ScopeTree,
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cx: Context,
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/// `terminating_scopes` is a set containing the ids of each
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/// statement, or conditional/repeating expression. These scopes
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/// are calling "terminating scopes" because, when attempting to
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/// find the scope of a temporary, by default we search up the
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/// enclosing scopes until we encounter the terminating scope. A
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/// conditional/repeating expression is one which is not
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/// guaranteed to execute exactly once upon entering the parent
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/// scope. This could be because the expression only executes
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/// conditionally, such as the expression `b` in `a && b`, or
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/// because the expression may execute many times, such as a loop
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/// body. The reason that we distinguish such expressions is that,
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/// upon exiting the parent scope, we cannot statically know how
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/// many times the expression executed, and thus if the expression
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/// creates temporaries we cannot know statically how many such
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/// temporaries we would have to cleanup. Therefore, we ensure that
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/// the temporaries never outlast the conditional/repeating
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/// expression, preventing the need for dynamic checks and/or
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/// arbitrary amounts of stack space. Terminating scopes end
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/// up being contained in a DestructionScope that contains the
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/// destructor's execution.
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terminating_scopes: FxHashSet<hir::ItemLocalId>,
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}
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/// Records the lifetime of a local variable as `cx.var_parent`
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fn record_var_lifetime(
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visitor: &mut RegionResolutionVisitor<'_>,
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var_id: hir::ItemLocalId,
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_sp: Span,
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) {
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match visitor.cx.var_parent {
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None => {
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// this can happen in extern fn declarations like
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//
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// extern fn isalnum(c: c_int) -> c_int
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}
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Some((parent_scope, _)) => visitor.scope_tree.record_var_scope(var_id, parent_scope),
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}
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}
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fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx hir::Block<'tcx>) {
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debug!("resolve_block(blk.hir_id={:?})", blk.hir_id);
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let prev_cx = visitor.cx;
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// We treat the tail expression in the block (if any) somewhat
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// differently from the statements. The issue has to do with
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// temporary lifetimes. Consider the following:
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//
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// quux({
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// let inner = ... (&bar()) ...;
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//
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// (... (&foo()) ...) // (the tail expression)
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// }, other_argument());
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//
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// Each of the statements within the block is a terminating
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// scope, and thus a temporary (e.g., the result of calling
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// `bar()` in the initializer expression for `let inner = ...;`)
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// will be cleaned up immediately after its corresponding
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// statement (i.e., `let inner = ...;`) executes.
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//
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// On the other hand, temporaries associated with evaluating the
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// tail expression for the block are assigned lifetimes so that
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// they will be cleaned up as part of the terminating scope
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// *surrounding* the block expression. Here, the terminating
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// scope for the block expression is the `quux(..)` call; so
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// those temporaries will only be cleaned up *after* both
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// `other_argument()` has run and also the call to `quux(..)`
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// itself has returned.
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visitor.enter_node_scope_with_dtor(blk.hir_id.local_id);
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visitor.cx.var_parent = visitor.cx.parent;
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{
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// This block should be kept approximately in sync with
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// `intravisit::walk_block`. (We manually walk the block, rather
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// than call `walk_block`, in order to maintain precise
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// index information.)
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for (i, statement) in blk.stmts.iter().enumerate() {
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match statement.kind {
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hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
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// Each declaration introduces a subscope for bindings
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// introduced by the declaration; this subscope covers a
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// suffix of the block. Each subscope in a block has the
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// previous subscope in the block as a parent, except for
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// the first such subscope, which has the block itself as a
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// parent.
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visitor.enter_scope(Scope {
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id: blk.hir_id.local_id,
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data: ScopeData::Remainder(FirstStatementIndex::new(i)),
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});
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visitor.cx.var_parent = visitor.cx.parent;
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}
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hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
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}
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visitor.visit_stmt(statement)
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}
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walk_list!(visitor, visit_expr, &blk.expr);
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}
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visitor.cx = prev_cx;
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}
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fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
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let prev_cx = visitor.cx;
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visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node });
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visitor.cx.var_parent = visitor.cx.parent;
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visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
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if let Some(hir::Guard::If(ref expr)) = arm.guard {
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visitor.terminating_scopes.insert(expr.hir_id.local_id);
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}
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intravisit::walk_arm(visitor, arm);
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visitor.cx = prev_cx;
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}
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fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
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visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node });
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// If this is a binding then record the lifetime of that binding.
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if let PatKind::Binding(..) = pat.kind {
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record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
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}
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debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
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intravisit::walk_pat(visitor, pat);
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visitor.expr_and_pat_count += 1;
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debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
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}
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fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
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let stmt_id = stmt.hir_id.local_id;
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debug!("resolve_stmt(stmt.id={:?})", stmt_id);
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// Every statement will clean up the temporaries created during
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// execution of that statement. Therefore each statement has an
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// associated destruction scope that represents the scope of the
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// statement plus its destructors, and thus the scope for which
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// regions referenced by the destructors need to survive.
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visitor.terminating_scopes.insert(stmt_id);
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let prev_parent = visitor.cx.parent;
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visitor.enter_node_scope_with_dtor(stmt_id);
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intravisit::walk_stmt(visitor, stmt);
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visitor.cx.parent = prev_parent;
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}
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fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
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debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
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let prev_cx = visitor.cx;
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visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
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{
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let terminating_scopes = &mut visitor.terminating_scopes;
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let mut terminating = |id: hir::ItemLocalId| {
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terminating_scopes.insert(id);
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};
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match expr.kind {
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// Conditional or repeating scopes are always terminating
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// scopes, meaning that temporaries cannot outlive them.
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// This ensures fixed size stacks.
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hir::ExprKind::Binary(
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source_map::Spanned { node: hir::BinOpKind::And, .. },
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_,
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ref r,
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)
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| hir::ExprKind::Binary(
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source_map::Spanned { node: hir::BinOpKind::Or, .. },
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_,
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ref r,
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) => {
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// For shortcircuiting operators, mark the RHS as a terminating
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// scope since it only executes conditionally.
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terminating(r.hir_id.local_id);
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}
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hir::ExprKind::If(_, ref then, Some(ref otherwise)) => {
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terminating(then.hir_id.local_id);
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terminating(otherwise.hir_id.local_id);
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}
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hir::ExprKind::If(_, ref then, None) => {
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terminating(then.hir_id.local_id);
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}
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hir::ExprKind::Loop(ref body, _, _, _) => {
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terminating(body.hir_id.local_id);
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}
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hir::ExprKind::DropTemps(ref expr) => {
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// `DropTemps(expr)` does not denote a conditional scope.
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// Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
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terminating(expr.hir_id.local_id);
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}
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hir::ExprKind::AssignOp(..)
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| hir::ExprKind::Index(..)
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| hir::ExprKind::Unary(..)
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| hir::ExprKind::Call(..)
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| hir::ExprKind::MethodCall(..) => {
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// FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
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//
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// The lifetimes for a call or method call look as follows:
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//
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// call.id
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// - arg0.id
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// - ...
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// - argN.id
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// - call.callee_id
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//
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// The idea is that call.callee_id represents *the time when
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// the invoked function is actually running* and call.id
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// represents *the time to prepare the arguments and make the
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// call*. See the section "Borrows in Calls" borrowck/README.md
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// for an extended explanation of why this distinction is
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// important.
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//
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// record_superlifetime(new_cx, expr.callee_id);
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}
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_ => {}
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}
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}
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let prev_pessimistic = visitor.pessimistic_yield;
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// Ordinarily, we can rely on the visit order of HIR intravisit
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// to correspond to the actual execution order of statements.
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// However, there's a weird corner case with compound assignment
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// operators (e.g. `a += b`). The evaluation order depends on whether
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// or not the operator is overloaded (e.g. whether or not a trait
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// like AddAssign is implemented).
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// For primitive types (which, despite having a trait impl, don't actually
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// end up calling it), the evluation order is right-to-left. For example,
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// the following code snippet:
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//
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// let y = &mut 0;
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// *{println!("LHS!"); y} += {println!("RHS!"); 1};
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//
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// will print:
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//
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// RHS!
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// LHS!
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//
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// However, if the operator is used on a non-primitive type,
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// the evaluation order will be left-to-right, since the operator
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// actually get desugared to a method call. For example, this
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// nearly identical code snippet:
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//
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// let y = &mut String::new();
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// *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
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//
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// will print:
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// LHS String
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// RHS String
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//
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// To determine the actual execution order, we need to perform
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// trait resolution. Unfortunately, we need to be able to compute
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// yield_in_scope before type checking is even done, as it gets
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// used by AST borrowcheck.
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//
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// Fortunately, we don't need to know the actual execution order.
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// It suffices to know the 'worst case' order with respect to yields.
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// Specifically, we need to know the highest 'expr_and_pat_count'
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// that we could assign to the yield expression. To do this,
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// we pick the greater of the two values from the left-hand
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// and right-hand expressions. This makes us overly conservative
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// about what types could possibly live across yield points,
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// but we will never fail to detect that a type does actually
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// live across a yield point. The latter part is critical -
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// we're already overly conservative about what types will live
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// across yield points, as the generated MIR will determine
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// when things are actually live. However, for typecheck to work
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// properly, we can't miss any types.
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match expr.kind {
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// Manually recurse over closures and inline consts, because they are the only
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// case of nested bodies that share the parent environment.
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hir::ExprKind::Closure(.., body, _, _)
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| hir::ExprKind::ConstBlock(hir::AnonConst { body, .. }) => {
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let body = visitor.tcx.hir().body(body);
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visitor.visit_body(body);
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}
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hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
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debug!(
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"resolve_expr - enabling pessimistic_yield, was previously {}",
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prev_pessimistic
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);
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let start_point = visitor.fixup_scopes.len();
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visitor.pessimistic_yield = true;
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// If the actual execution order turns out to be right-to-left,
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// then we're fine. However, if the actual execution order is left-to-right,
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// then we'll assign too low a count to any `yield` expressions
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// we encounter in 'right_expression' - they should really occur after all of the
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// expressions in 'left_expression'.
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visitor.visit_expr(&right_expr);
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visitor.pessimistic_yield = prev_pessimistic;
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debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
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visitor.visit_expr(&left_expr);
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debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
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// Remove and process any scopes pushed by the visitor
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let target_scopes = visitor.fixup_scopes.drain(start_point..);
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for scope in target_scopes {
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let mut yield_data =
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visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap().last_mut().unwrap();
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let count = yield_data.expr_and_pat_count;
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let span = yield_data.span;
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// expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
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// before walking the left-hand side, it should be impossible for the recorded
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// count to be greater than the left-hand side count.
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if count > visitor.expr_and_pat_count {
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bug!(
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"Encountered greater count {} at span {:?} - expected no greater than {}",
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count,
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span,
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visitor.expr_and_pat_count
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);
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}
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let new_count = visitor.expr_and_pat_count;
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debug!(
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"resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
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scope, count, new_count, span
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);
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yield_data.expr_and_pat_count = new_count;
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}
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}
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hir::ExprKind::If(ref cond, ref then, Some(ref otherwise)) => {
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let expr_cx = visitor.cx;
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visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
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visitor.cx.var_parent = visitor.cx.parent;
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visitor.visit_expr(cond);
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visitor.visit_expr(then);
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visitor.cx = expr_cx;
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visitor.visit_expr(otherwise);
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}
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hir::ExprKind::If(ref cond, ref then, None) => {
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let expr_cx = visitor.cx;
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visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
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visitor.cx.var_parent = visitor.cx.parent;
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visitor.visit_expr(cond);
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visitor.visit_expr(then);
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visitor.cx = expr_cx;
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}
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_ => intravisit::walk_expr(visitor, expr),
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}
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visitor.expr_and_pat_count += 1;
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debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
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if let hir::ExprKind::Yield(_, source) = &expr.kind {
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// Mark this expr's scope and all parent scopes as containing `yield`.
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let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
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loop {
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let span = match expr.kind {
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hir::ExprKind::Yield(expr, hir::YieldSource::Await { .. }) => {
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expr.span.shrink_to_hi().to(expr.span)
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}
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_ => expr.span,
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};
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let data =
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YieldData { span, expr_and_pat_count: visitor.expr_and_pat_count, source: *source };
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match visitor.scope_tree.yield_in_scope.get_mut(&scope) {
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Some(yields) => yields.push(data),
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None => {
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visitor.scope_tree.yield_in_scope.insert(scope, vec![data]);
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}
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}
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if visitor.pessimistic_yield {
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debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
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visitor.fixup_scopes.push(scope);
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}
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// Keep traversing up while we can.
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match visitor.scope_tree.parent_map.get(&scope) {
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// Don't cross from closure bodies to their parent.
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Some(&(superscope, _)) => match superscope.data {
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ScopeData::CallSite => break,
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_ => scope = superscope,
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},
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None => break,
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}
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}
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}
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visitor.cx = prev_cx;
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}
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fn resolve_local<'tcx>(
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visitor: &mut RegionResolutionVisitor<'tcx>,
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pat: Option<&'tcx hir::Pat<'tcx>>,
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init: Option<&'tcx hir::Expr<'tcx>>,
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) {
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debug!("resolve_local(pat={:?}, init={:?})", pat, init);
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|
|
let blk_scope = visitor.cx.var_parent.map(|(p, _)| p);
|
|
|
|
// As an exception to the normal rules governing temporary
|
|
// lifetimes, initializers in a let have a temporary lifetime
|
|
// of the enclosing block. This means that e.g., a program
|
|
// like the following is legal:
|
|
//
|
|
// let ref x = HashMap::new();
|
|
//
|
|
// Because the hash map will be freed in the enclosing block.
|
|
//
|
|
// We express the rules more formally based on 3 grammars (defined
|
|
// fully in the helpers below that implement them):
|
|
//
|
|
// 1. `E&`, which matches expressions like `&<rvalue>` that
|
|
// own a pointer into the stack.
|
|
//
|
|
// 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
|
|
// y)` that produce ref bindings into the value they are
|
|
// matched against or something (at least partially) owned by
|
|
// the value they are matched against. (By partially owned,
|
|
// I mean that creating a binding into a ref-counted or managed value
|
|
// would still count.)
|
|
//
|
|
// 3. `ET`, which matches both rvalues like `foo()` as well as places
|
|
// based on rvalues like `foo().x[2].y`.
|
|
//
|
|
// A subexpression `<rvalue>` that appears in a let initializer
|
|
// `let pat [: ty] = expr` has an extended temporary lifetime if
|
|
// any of the following conditions are met:
|
|
//
|
|
// A. `pat` matches `P&` and `expr` matches `ET`
|
|
// (covers cases where `pat` creates ref bindings into an rvalue
|
|
// produced by `expr`)
|
|
// B. `ty` is a borrowed pointer and `expr` matches `ET`
|
|
// (covers cases where coercion creates a borrow)
|
|
// C. `expr` matches `E&`
|
|
// (covers cases `expr` borrows an rvalue that is then assigned
|
|
// to memory (at least partially) owned by the binding)
|
|
//
|
|
// Here are some examples hopefully giving an intuition where each
|
|
// rule comes into play and why:
|
|
//
|
|
// Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
|
|
// would have an extended lifetime, but not `foo()`.
|
|
//
|
|
// Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
|
|
// lifetime.
|
|
//
|
|
// In some cases, multiple rules may apply (though not to the same
|
|
// rvalue). For example:
|
|
//
|
|
// let ref x = [&a(), &b()];
|
|
//
|
|
// Here, the expression `[...]` has an extended lifetime due to rule
|
|
// A, but the inner rvalues `a()` and `b()` have an extended lifetime
|
|
// due to rule C.
|
|
|
|
if let Some(expr) = init {
|
|
record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope);
|
|
|
|
if let Some(pat) = pat {
|
|
if is_binding_pat(pat) {
|
|
record_rvalue_scope(visitor, &expr, blk_scope);
|
|
}
|
|
}
|
|
}
|
|
|
|
// Make sure we visit the initializer first, so expr_and_pat_count remains correct
|
|
if let Some(expr) = init {
|
|
visitor.visit_expr(expr);
|
|
}
|
|
if let Some(pat) = pat {
|
|
visitor.visit_pat(pat);
|
|
}
|
|
|
|
/// Returns `true` if `pat` match the `P&` non-terminal.
|
|
///
|
|
/// ```text
|
|
/// P& = ref X
|
|
/// | StructName { ..., P&, ... }
|
|
/// | VariantName(..., P&, ...)
|
|
/// | [ ..., P&, ... ]
|
|
/// | ( ..., P&, ... )
|
|
/// | ... "|" P& "|" ...
|
|
/// | box P&
|
|
/// ```
|
|
fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
|
|
// Note that the code below looks for *explicit* refs only, that is, it won't
|
|
// know about *implicit* refs as introduced in #42640.
|
|
//
|
|
// This is not a problem. For example, consider
|
|
//
|
|
// let (ref x, ref y) = (Foo { .. }, Bar { .. });
|
|
//
|
|
// Due to the explicit refs on the left hand side, the below code would signal
|
|
// that the temporary value on the right hand side should live until the end of
|
|
// the enclosing block (as opposed to being dropped after the let is complete).
|
|
//
|
|
// To create an implicit ref, however, you must have a borrowed value on the RHS
|
|
// already, as in this example (which won't compile before #42640):
|
|
//
|
|
// let Foo { x, .. } = &Foo { x: ..., ... };
|
|
//
|
|
// in place of
|
|
//
|
|
// let Foo { ref x, .. } = Foo { ... };
|
|
//
|
|
// In the former case (the implicit ref version), the temporary is created by the
|
|
// & expression, and its lifetime would be extended to the end of the block (due
|
|
// to a different rule, not the below code).
|
|
match pat.kind {
|
|
PatKind::Binding(hir::BindingAnnotation::Ref, ..)
|
|
| PatKind::Binding(hir::BindingAnnotation::RefMut, ..) => true,
|
|
|
|
PatKind::Struct(_, ref field_pats, _) => {
|
|
field_pats.iter().any(|fp| is_binding_pat(&fp.pat))
|
|
}
|
|
|
|
PatKind::Slice(ref pats1, ref pats2, ref pats3) => {
|
|
pats1.iter().any(|p| is_binding_pat(&p))
|
|
|| pats2.iter().any(|p| is_binding_pat(&p))
|
|
|| pats3.iter().any(|p| is_binding_pat(&p))
|
|
}
|
|
|
|
PatKind::Or(ref subpats)
|
|
| PatKind::TupleStruct(_, ref subpats, _)
|
|
| PatKind::Tuple(ref subpats, _) => subpats.iter().any(|p| is_binding_pat(&p)),
|
|
|
|
PatKind::Box(ref subpat) => is_binding_pat(&subpat),
|
|
|
|
PatKind::Ref(_, _)
|
|
| PatKind::Binding(
|
|
hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
|
|
..,
|
|
)
|
|
| PatKind::Wild
|
|
| PatKind::Path(_)
|
|
| PatKind::Lit(_)
|
|
| PatKind::Range(_, _, _) => false,
|
|
}
|
|
}
|
|
|
|
/// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
|
|
///
|
|
/// ```text
|
|
/// E& = & ET
|
|
/// | StructName { ..., f: E&, ... }
|
|
/// | [ ..., E&, ... ]
|
|
/// | ( ..., E&, ... )
|
|
/// | {...; E&}
|
|
/// | box E&
|
|
/// | E& as ...
|
|
/// | ( E& )
|
|
/// ```
|
|
fn record_rvalue_scope_if_borrow_expr<'tcx>(
|
|
visitor: &mut RegionResolutionVisitor<'tcx>,
|
|
expr: &hir::Expr<'_>,
|
|
blk_id: Option<Scope>,
|
|
) {
|
|
match expr.kind {
|
|
hir::ExprKind::AddrOf(_, _, ref subexpr) => {
|
|
record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
|
|
record_rvalue_scope(visitor, &subexpr, blk_id);
|
|
}
|
|
hir::ExprKind::Struct(_, fields, _) => {
|
|
for field in fields {
|
|
record_rvalue_scope_if_borrow_expr(visitor, &field.expr, blk_id);
|
|
}
|
|
}
|
|
hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
|
|
for subexpr in subexprs {
|
|
record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
|
|
}
|
|
}
|
|
hir::ExprKind::Cast(ref subexpr, _) => {
|
|
record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id)
|
|
}
|
|
hir::ExprKind::Block(ref block, _) => {
|
|
if let Some(ref subexpr) = block.expr {
|
|
record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
|
|
}
|
|
}
|
|
_ => {}
|
|
}
|
|
}
|
|
|
|
/// Applied to an expression `expr` if `expr` -- or something owned or partially owned by
|
|
/// `expr` -- is going to be indirectly referenced by a variable in a let statement. In that
|
|
/// case, the "temporary lifetime" or `expr` is extended to be the block enclosing the `let`
|
|
/// statement.
|
|
///
|
|
/// More formally, if `expr` matches the grammar `ET`, record the rvalue scope of the matching
|
|
/// `<rvalue>` as `blk_id`:
|
|
///
|
|
/// ```text
|
|
/// ET = *ET
|
|
/// | ET[...]
|
|
/// | ET.f
|
|
/// | (ET)
|
|
/// | <rvalue>
|
|
/// ```
|
|
///
|
|
/// Note: ET is intended to match "rvalues or places based on rvalues".
|
|
fn record_rvalue_scope<'tcx>(
|
|
visitor: &mut RegionResolutionVisitor<'tcx>,
|
|
expr: &hir::Expr<'_>,
|
|
blk_scope: Option<Scope>,
|
|
) {
|
|
let mut expr = expr;
|
|
loop {
|
|
// Note: give all the expressions matching `ET` with the
|
|
// extended temporary lifetime, not just the innermost rvalue,
|
|
// because in codegen if we must compile e.g., `*rvalue()`
|
|
// into a temporary, we request the temporary scope of the
|
|
// outer expression.
|
|
visitor.scope_tree.record_rvalue_scope(expr.hir_id.local_id, blk_scope);
|
|
|
|
match expr.kind {
|
|
hir::ExprKind::AddrOf(_, _, ref subexpr)
|
|
| hir::ExprKind::Unary(hir::UnOp::Deref, ref subexpr)
|
|
| hir::ExprKind::Field(ref subexpr, _)
|
|
| hir::ExprKind::Index(ref subexpr, _) => {
|
|
expr = &subexpr;
|
|
}
|
|
_ => {
|
|
return;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<'tcx> RegionResolutionVisitor<'tcx> {
|
|
/// Records the current parent (if any) as the parent of `child_scope`.
|
|
/// Returns the depth of `child_scope`.
|
|
fn record_child_scope(&mut self, child_scope: Scope) -> ScopeDepth {
|
|
let parent = self.cx.parent;
|
|
self.scope_tree.record_scope_parent(child_scope, parent);
|
|
// If `child_scope` has no parent, it must be the root node, and so has
|
|
// a depth of 1. Otherwise, its depth is one more than its parent's.
|
|
parent.map_or(1, |(_p, d)| d + 1)
|
|
}
|
|
|
|
/// Records the current parent (if any) as the parent of `child_scope`,
|
|
/// and sets `child_scope` as the new current parent.
|
|
fn enter_scope(&mut self, child_scope: Scope) {
|
|
let child_depth = self.record_child_scope(child_scope);
|
|
self.cx.parent = Some((child_scope, child_depth));
|
|
}
|
|
|
|
fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) {
|
|
// If node was previously marked as a terminating scope during the
|
|
// recursive visit of its parent node in the AST, then we need to
|
|
// account for the destruction scope representing the scope of
|
|
// the destructors that run immediately after it completes.
|
|
if self.terminating_scopes.contains(&id) {
|
|
self.enter_scope(Scope { id, data: ScopeData::Destruction });
|
|
}
|
|
self.enter_scope(Scope { id, data: ScopeData::Node });
|
|
}
|
|
}
|
|
|
|
impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
|
|
fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
|
|
resolve_block(self, b);
|
|
}
|
|
|
|
fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
|
|
let body_id = body.id();
|
|
let owner_id = self.tcx.hir().body_owner(body_id);
|
|
|
|
debug!(
|
|
"visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
|
|
owner_id,
|
|
self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
|
|
body_id,
|
|
self.cx.parent
|
|
);
|
|
|
|
// Save all state that is specific to the outer function
|
|
// body. These will be restored once down below, once we've
|
|
// visited the body.
|
|
let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
|
|
let outer_cx = self.cx;
|
|
let outer_ts = mem::take(&mut self.terminating_scopes);
|
|
// The 'pessimistic yield' flag is set to true when we are
|
|
// processing a `+=` statement and have to make pessimistic
|
|
// control flow assumptions. This doesn't apply to nested
|
|
// bodies within the `+=` statements. See #69307.
|
|
let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false);
|
|
self.terminating_scopes.insert(body.value.hir_id.local_id);
|
|
|
|
self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite });
|
|
self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments });
|
|
|
|
// The arguments and `self` are parented to the fn.
|
|
self.cx.var_parent = self.cx.parent.take();
|
|
for param in body.params {
|
|
self.visit_pat(¶m.pat);
|
|
}
|
|
|
|
// The body of the every fn is a root scope.
|
|
self.cx.parent = self.cx.var_parent;
|
|
if self.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() {
|
|
self.visit_expr(&body.value)
|
|
} else {
|
|
// Only functions have an outer terminating (drop) scope, while
|
|
// temporaries in constant initializers may be 'static, but only
|
|
// according to rvalue lifetime semantics, using the same
|
|
// syntactical rules used for let initializers.
|
|
//
|
|
// e.g., in `let x = &f();`, the temporary holding the result from
|
|
// the `f()` call lives for the entirety of the surrounding block.
|
|
//
|
|
// Similarly, `const X: ... = &f();` would have the result of `f()`
|
|
// live for `'static`, implying (if Drop restrictions on constants
|
|
// ever get lifted) that the value *could* have a destructor, but
|
|
// it'd get leaked instead of the destructor running during the
|
|
// evaluation of `X` (if at all allowed by CTFE).
|
|
//
|
|
// However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
|
|
// would *not* let the `f()` temporary escape into an outer scope
|
|
// (i.e., `'static`), which means that after `g` returns, it drops,
|
|
// and all the associated destruction scope rules apply.
|
|
self.cx.var_parent = None;
|
|
resolve_local(self, None, Some(&body.value));
|
|
}
|
|
|
|
if body.generator_kind.is_some() {
|
|
self.scope_tree.body_expr_count.insert(body_id, self.expr_and_pat_count);
|
|
}
|
|
|
|
// Restore context we had at the start.
|
|
self.expr_and_pat_count = outer_ec;
|
|
self.cx = outer_cx;
|
|
self.terminating_scopes = outer_ts;
|
|
self.pessimistic_yield = outer_pessimistic_yield;
|
|
}
|
|
|
|
fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
|
|
resolve_arm(self, a);
|
|
}
|
|
fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
|
|
resolve_pat(self, p);
|
|
}
|
|
fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
|
|
resolve_stmt(self, s);
|
|
}
|
|
fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
|
|
resolve_expr(self, ex);
|
|
}
|
|
fn visit_local(&mut self, l: &'tcx Local<'tcx>) {
|
|
resolve_local(self, Some(&l.pat), l.init);
|
|
}
|
|
}
|
|
|
|
fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
|
|
let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
|
|
if typeck_root_def_id != def_id {
|
|
return tcx.region_scope_tree(typeck_root_def_id);
|
|
}
|
|
|
|
let id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
|
|
let scope_tree = if let Some(body_id) = tcx.hir().maybe_body_owned_by(id) {
|
|
let mut visitor = RegionResolutionVisitor {
|
|
tcx,
|
|
scope_tree: ScopeTree::default(),
|
|
expr_and_pat_count: 0,
|
|
cx: Context { parent: None, var_parent: None },
|
|
terminating_scopes: Default::default(),
|
|
pessimistic_yield: false,
|
|
fixup_scopes: vec![],
|
|
};
|
|
|
|
let body = tcx.hir().body(body_id);
|
|
visitor.scope_tree.root_body = Some(body.value.hir_id);
|
|
visitor.visit_body(body);
|
|
visitor.scope_tree
|
|
} else {
|
|
ScopeTree::default()
|
|
};
|
|
|
|
tcx.arena.alloc(scope_tree)
|
|
}
|
|
|
|
pub fn provide(providers: &mut Providers) {
|
|
*providers = Providers { region_scope_tree, ..*providers };
|
|
}
|