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Rollup merge of #67697 - cjgillot:passes-scope-tree, r=Zoxc

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Move the region_scope_tree query to librustc_passes.

Split out of #67688.

r? @Zoxc
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Dylan DPC 2019-12-31 18:14:03 +05:30 • committed by GitHub
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src/librustc/middle/region.rs
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@ -1,4 +1,4 @@
//! This file builds up the `ScopeTree`, which describes //! This file declares the `ScopeTree` type, which describes
//! the parent links in the region hierarchy. //! the parent links in the region hierarchy.
//! //!
//! For more information about how MIR-based region-checking works, //! For more information about how MIR-based region-checking works,
@ -8,22 +8,17 @@
use crate::hir; use crate::hir;
use crate::hir::def_id::DefId; use crate::hir::def_id::DefId;
use crate::hir::intravisit::{self, NestedVisitorMap, Visitor};
use crate::hir::Node; use crate::hir::Node;
use crate::hir::{Arm, Block, Expr, Local, Pat, PatKind, Stmt};
use crate::ich::{NodeIdHashingMode, StableHashingContext}; use crate::ich::{NodeIdHashingMode, StableHashingContext};
use crate::ty::query::Providers;
use crate::ty::{self, DefIdTree, TyCtxt}; use crate::ty::{self, DefIdTree, TyCtxt};
use crate::util::nodemap::{FxHashMap, FxHashSet}; use crate::util::nodemap::FxHashMap;
use rustc_data_structures::stable_hasher::{HashStable, StableHasher}; use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_index::vec::Idx; use rustc_index::vec::Idx;
use rustc_macros::HashStable; use rustc_macros::HashStable;
use syntax::source_map;
use syntax_pos::{Span, DUMMY_SP}; use syntax_pos::{Span, DUMMY_SP};
use std::fmt; use std::fmt;
use std::mem;
/// Represents a statically-describable scope that can be used to /// Represents a statically-describable scope that can be used to
/// bound the lifetime/region for values. /// bound the lifetime/region for values.
@ -232,12 +227,12 @@ pub type ScopeDepth = u32;
#[derive(Default, Debug)] #[derive(Default, Debug)]
pub struct ScopeTree { pub struct ScopeTree {
/// If not empty, this body is the root of this region hierarchy. /// If not empty, this body is the root of this region hierarchy.
root_body: Option<hir::HirId>, pub root_body: Option<hir::HirId>,
/// The parent of the root body owner, if the latter is an /// The parent of the root body owner, if the latter is an
/// an associated const or method, as impls/traits can also /// an associated const or method, as impls/traits can also
/// have lifetime parameters free in this body. /// have lifetime parameters free in this body.
root_parent: Option<hir::HirId>, pub root_parent: Option<hir::HirId>,
/// Maps from a scope ID to the enclosing scope id; /// Maps from a scope ID to the enclosing scope id;
/// this is usually corresponding to the lexical nesting, though /// this is usually corresponding to the lexical nesting, though
@ -245,7 +240,7 @@ pub struct ScopeTree {
/// conditional expression or repeating block. (Note that the /// conditional expression or repeating block. (Note that the
/// enclosing scope ID for the block associated with a closure is /// enclosing scope ID for the block associated with a closure is
/// the closure itself.) /// the closure itself.)
parent_map: FxHashMap<Scope, (Scope, ScopeDepth)>, pub parent_map: FxHashMap<Scope, (Scope, ScopeDepth)>,
/// Maps from a variable or binding ID to the block in which that /// Maps from a variable or binding ID to the block in which that
/// variable is declared. /// variable is declared.
@ -345,12 +340,12 @@ pub struct ScopeTree {
/// The reason is that semantically, until the `box` expression returns, /// The reason is that semantically, until the `box` expression returns,
/// the values are still owned by their containing expressions. So /// the values are still owned by their containing expressions. So
/// we'll see that `&x`. /// we'll see that `&x`.
yield_in_scope: FxHashMap<Scope, YieldData>, pub yield_in_scope: FxHashMap<Scope, YieldData>,
/// The number of visit_expr and visit_pat calls done in the body. /// The number of visit_expr and visit_pat calls done in the body.
/// Used to sanity check visit_expr/visit_pat call count when /// Used to sanity check visit_expr/visit_pat call count when
/// calculating generator interiors. /// calculating generator interiors.
body_expr_count: FxHashMap<hir::BodyId, usize>, pub body_expr_count: FxHashMap<hir::BodyId, usize>,
} }
#[derive(Debug, Copy, Clone, RustcEncodable, RustcDecodable, HashStable)] #[derive(Debug, Copy, Clone, RustcEncodable, RustcDecodable, HashStable)]
@ -362,101 +357,6 @@ pub struct YieldData {
pub source: hir::YieldSource, pub source: hir::YieldSource,
} }
#[derive(Debug, Copy, Clone)]
pub struct Context {
/// The root of the current region tree. This is typically the id
/// of the innermost fn body. Each fn forms its own disjoint tree
/// in the region hierarchy. These fn bodies are themselves
/// arranged into a tree. See the "Modeling closures" section of
/// the README in `infer::region_constraints` for more
/// details.
root_id: Option<hir::ItemLocalId>,
/// The scope that contains any new variables declared, plus its depth in
/// the scope tree.
var_parent: Option<(Scope, ScopeDepth)>,
/// Region parent of expressions, etc., plus its depth in the scope tree.
parent: Option<(Scope, ScopeDepth)>,
}
struct RegionResolutionVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
// The number of expressions and patterns visited in the current body.
expr_and_pat_count: usize,
// When this is `true`, we record the `Scopes` we encounter
// when processing a Yield expression. This allows us to fix
// up their indices.
pessimistic_yield: bool,
// Stores scopes when `pessimistic_yield` is `true`.
fixup_scopes: Vec<Scope>,
// The generated scope tree.
scope_tree: ScopeTree,
cx: Context,
/// `terminating_scopes` is a set containing the ids of each
/// statement, or conditional/repeating expression. These scopes
/// are calling "terminating scopes" because, when attempting to
/// find the scope of a temporary, by default we search up the
/// enclosing scopes until we encounter the terminating scope. A
/// conditional/repeating expression is one which is not
/// guaranteed to execute exactly once upon entering the parent
/// scope. This could be because the expression only executes
/// conditionally, such as the expression `b` in `a && b`, or
/// because the expression may execute many times, such as a loop
/// body. The reason that we distinguish such expressions is that,
/// upon exiting the parent scope, we cannot statically know how
/// many times the expression executed, and thus if the expression
/// creates temporaries we cannot know statically how many such
/// temporaries we would have to cleanup. Therefore, we ensure that
/// the temporaries never outlast the conditional/repeating
/// expression, preventing the need for dynamic checks and/or
/// arbitrary amounts of stack space. Terminating scopes end
/// up being contained in a DestructionScope that contains the
/// destructor's execution.
terminating_scopes: FxHashSet<hir::ItemLocalId>,
}
struct ExprLocatorVisitor {
hir_id: hir::HirId,
result: Option<usize>,
expr_and_pat_count: usize,
}
// This visitor has to have the same `visit_expr` calls as `RegionResolutionVisitor`
// since `expr_count` is compared against the results there.
impl<'tcx> Visitor<'tcx> for ExprLocatorVisitor {
fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
fn visit_pat(&mut self, pat: &'tcx Pat<'tcx>) {
intravisit::walk_pat(self, pat);
self.expr_and_pat_count += 1;
if pat.hir_id == self.hir_id {
self.result = Some(self.expr_and_pat_count);
}
}
fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
debug!("ExprLocatorVisitor - pre-increment {} expr = {:?}", self.expr_and_pat_count, expr);
intravisit::walk_expr(self, expr);
self.expr_and_pat_count += 1;
debug!("ExprLocatorVisitor - post-increment {} expr = {:?}", self.expr_and_pat_count, expr);
if expr.hir_id == self.hir_id {
self.result = Some(self.expr_and_pat_count);
}
}
}
impl<'tcx> ScopeTree { impl<'tcx> ScopeTree {
pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) { pub fn record_scope_parent(&mut self, child: Scope, parent: Option<(Scope, ScopeDepth)>) {
debug!("{:?}.parent = {:?}", child, parent); debug!("{:?}.parent = {:?}", child, parent);
@ -497,7 +397,7 @@ impl<'tcx> ScopeTree {
/// Records that `sub_closure` is defined within `sup_closure`. These IDs /// Records that `sub_closure` is defined within `sup_closure`. These IDs
/// should be the ID of the block that is the fn body, which is /// should be the ID of the block that is the fn body, which is
/// also the root of the region hierarchy for that fn. /// also the root of the region hierarchy for that fn.
fn record_closure_parent( pub fn record_closure_parent(
&mut self, &mut self,
sub_closure: hir::ItemLocalId, sub_closure: hir::ItemLocalId,
sup_closure: hir::ItemLocalId, sup_closure: hir::ItemLocalId,
@ -511,13 +411,13 @@ impl<'tcx> ScopeTree {
assert!(previous.is_none()); assert!(previous.is_none());
} }
fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) { pub fn record_var_scope(&mut self, var: hir::ItemLocalId, lifetime: Scope) {
debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime); debug!("record_var_scope(sub={:?}, sup={:?})", var, lifetime);
assert!(var != lifetime.item_local_id()); assert!(var != lifetime.item_local_id());
self.var_map.insert(var, lifetime); self.var_map.insert(var, lifetime);
} }
fn record_rvalue_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) { pub fn record_rvalue_scope(&mut self, var: hir::ItemLocalId, lifetime: Option<Scope>) {
debug!("record_rvalue_scope(sub={:?}, sup={:?})", var, lifetime); debug!("record_rvalue_scope(sub={:?}, sup={:?})", var, lifetime);
if let Some(lifetime) = lifetime { if let Some(lifetime) = lifetime {
assert!(var != lifetime.item_local_id()); assert!(var != lifetime.item_local_id());
@ -732,23 +632,6 @@ impl<'tcx> ScopeTree {
self.yield_in_scope.get(&scope).cloned() self.yield_in_scope.get(&scope).cloned()
} }
/// Checks whether the given scope contains a `yield` and if that yield could execute
/// after `expr`. If so, it returns the span of that `yield`.
/// `scope` must be inside the body.
pub fn yield_in_scope_for_expr(
&self,
scope: Scope,
expr_hir_id: hir::HirId,
body: &'tcx hir::Body<'tcx>,
) -> Option<Span> {
self.yield_in_scope(scope).and_then(|YieldData { span, expr_and_pat_count, .. }| {
let mut visitor =
ExprLocatorVisitor { hir_id: expr_hir_id, result: None, expr_and_pat_count: 0 };
visitor.visit_body(body);
if expr_and_pat_count >= visitor.result.unwrap() { Some(span) } else { None }
})
}
/// Gives the number of expressions visited in a body. /// Gives the number of expressions visited in a body.
/// Used to sanity check visit_expr call count when /// Used to sanity check visit_expr call count when
/// calculating generator interiors. /// calculating generator interiors.
@ -757,755 +640,6 @@ impl<'tcx> ScopeTree {
} }
} }
/// Records the lifetime of a local variable as `cx.var_parent`
fn record_var_lifetime(
visitor: &mut RegionResolutionVisitor<'_>,
var_id: hir::ItemLocalId,
_sp: Span,
) {
match visitor.cx.var_parent {
None => {
// this can happen in extern fn declarations like
//
// extern fn isalnum(c: c_int) -> c_int
}
Some((parent_scope, _)) => visitor.scope_tree.record_var_scope(var_id, parent_scope),
}
}
fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx hir::Block<'tcx>) {
debug!("resolve_block(blk.hir_id={:?})", blk.hir_id);
let prev_cx = visitor.cx;
// We treat the tail expression in the block (if any) somewhat
// differently from the statements. The issue has to do with
// temporary lifetimes. Consider the following:
//
// quux({
// let inner = ... (&bar()) ...;
//
// (... (&foo()) ...) // (the tail expression)
// }, other_argument());
//
// Each of the statements within the block is a terminating
// scope, and thus a temporary (e.g., the result of calling
// `bar()` in the initializer expression for `let inner = ...;`)
// will be cleaned up immediately after its corresponding
// statement (i.e., `let inner = ...;`) executes.
//
// On the other hand, temporaries associated with evaluating the
// tail expression for the block are assigned lifetimes so that
// they will be cleaned up as part of the terminating scope
// *surrounding* the block expression. Here, the terminating
// scope for the block expression is the `quux(..)` call; so
// those temporaries will only be cleaned up *after* both
// `other_argument()` has run and also the call to `quux(..)`
// itself has returned.
visitor.enter_node_scope_with_dtor(blk.hir_id.local_id);
visitor.cx.var_parent = visitor.cx.parent;
{
// This block should be kept approximately in sync with
// `intravisit::walk_block`. (We manually walk the block, rather
// than call `walk_block`, in order to maintain precise
// index information.)
for (i, statement) in blk.stmts.iter().enumerate() {
match statement.kind {
hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
// Each declaration introduces a subscope for bindings
// introduced by the declaration; this subscope covers a
// suffix of the block. Each subscope in a block has the
// previous subscope in the block as a parent, except for
// the first such subscope, which has the block itself as a
// parent.
visitor.enter_scope(Scope {
id: blk.hir_id.local_id,
data: ScopeData::Remainder(FirstStatementIndex::new(i)),
});
visitor.cx.var_parent = visitor.cx.parent;
}
hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
}
visitor.visit_stmt(statement)
}
walk_list!(visitor, visit_expr, &blk.expr);
}
visitor.cx = prev_cx;
}
fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
let prev_cx = visitor.cx;
visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node });
visitor.cx.var_parent = visitor.cx.parent;
visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
if let Some(hir::Guard::If(ref expr)) = arm.guard {
visitor.terminating_scopes.insert(expr.hir_id.local_id);
}
intravisit::walk_arm(visitor, arm);
visitor.cx = prev_cx;
}
fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node });
// If this is a binding then record the lifetime of that binding.
if let PatKind::Binding(..) = pat.kind {
record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
}
debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
intravisit::walk_pat(visitor, pat);
visitor.expr_and_pat_count += 1;
debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
}
fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
let stmt_id = stmt.hir_id.local_id;
debug!("resolve_stmt(stmt.id={:?})", stmt_id);
// Every statement will clean up the temporaries created during
// execution of that statement. Therefore each statement has an
// associated destruction scope that represents the scope of the
// statement plus its destructors, and thus the scope for which
// regions referenced by the destructors need to survive.
visitor.terminating_scopes.insert(stmt_id);
let prev_parent = visitor.cx.parent;
visitor.enter_node_scope_with_dtor(stmt_id);
intravisit::walk_stmt(visitor, stmt);
visitor.cx.parent = prev_parent;
}
fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
let prev_cx = visitor.cx;
visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
{
let terminating_scopes = &mut visitor.terminating_scopes;
let mut terminating = |id: hir::ItemLocalId| {
terminating_scopes.insert(id);
};
match expr.kind {
// Conditional or repeating scopes are always terminating
// scopes, meaning that temporaries cannot outlive them.
// This ensures fixed size stacks.
hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::And, .. },
_,
ref r,
)
| hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::Or, .. },
_,
ref r,
) => {
// For shortcircuiting operators, mark the RHS as a terminating
// scope since it only executes conditionally.
terminating(r.hir_id.local_id);
}
hir::ExprKind::Loop(ref body, _, _) => {
terminating(body.hir_id.local_id);
}
hir::ExprKind::DropTemps(ref expr) => {
// `DropTemps(expr)` does not denote a conditional scope.
// Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
terminating(expr.hir_id.local_id);
}
hir::ExprKind::AssignOp(..)
| hir::ExprKind::Index(..)
| hir::ExprKind::Unary(..)
| hir::ExprKind::Call(..)
| hir::ExprKind::MethodCall(..) => {
// FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
//
// The lifetimes for a call or method call look as follows:
//
// call.id
// - arg0.id
// - ...
// - argN.id
// - call.callee_id
//
// The idea is that call.callee_id represents *the time when
// the invoked function is actually running* and call.id
// represents *the time to prepare the arguments and make the
// call*. See the section "Borrows in Calls" borrowck/README.md
// for an extended explanation of why this distinction is
// important.
//
// record_superlifetime(new_cx, expr.callee_id);
}
_ => {}
}
}
let prev_pessimistic = visitor.pessimistic_yield;
// Ordinarily, we can rely on the visit order of HIR intravisit
// to correspond to the actual execution order of statements.
// However, there's a weird corner case with compund assignment
// operators (e.g. `a += b`). The evaluation order depends on whether
// or not the operator is overloaded (e.g. whether or not a trait
// like AddAssign is implemented).
// For primitive types (which, despite having a trait impl, don't actually
// end up calling it), the evluation order is right-to-left. For example,
// the following code snippet:
//
// let y = &mut 0;
// *{println!("LHS!"); y} += {println!("RHS!"); 1};
//
// will print:
//
// RHS!
// LHS!
//
// However, if the operator is used on a non-primitive type,
// the evaluation order will be left-to-right, since the operator
// actually get desugared to a method call. For example, this
// nearly identical code snippet:
//
// let y = &mut String::new();
// *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
//
// will print:
// LHS String
// RHS String
//
// To determine the actual execution order, we need to perform
// trait resolution. Unfortunately, we need to be able to compute
// yield_in_scope before type checking is even done, as it gets
// used by AST borrowcheck.
//
// Fortunately, we don't need to know the actual execution order.
// It suffices to know the 'worst case' order with respect to yields.
// Specifically, we need to know the highest 'expr_and_pat_count'
// that we could assign to the yield expression. To do this,
// we pick the greater of the two values from the left-hand
// and right-hand expressions. This makes us overly conservative
// about what types could possibly live across yield points,
// but we will never fail to detect that a type does actually
// live across a yield point. The latter part is critical -
// we're already overly conservative about what types will live
// across yield points, as the generated MIR will determine
// when things are actually live. However, for typecheck to work
// properly, we can't miss any types.
match expr.kind {
// Manually recurse over closures, because they are the only
// case of nested bodies that share the parent environment.
hir::ExprKind::Closure(.., body, _, _) => {
let body = visitor.tcx.hir().body(body);
visitor.visit_body(body);
}
hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
debug!(
"resolve_expr - enabling pessimistic_yield, was previously {}",
prev_pessimistic
);
let start_point = visitor.fixup_scopes.len();
visitor.pessimistic_yield = true;
// If the actual execution order turns out to be right-to-left,
// then we're fine. However, if the actual execution order is left-to-right,
// then we'll assign too low a count to any `yield` expressions
// we encounter in 'right_expression' - they should really occur after all of the
// expressions in 'left_expression'.
visitor.visit_expr(&right_expr);
visitor.pessimistic_yield = prev_pessimistic;
debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
visitor.visit_expr(&left_expr);
debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
// Remove and process any scopes pushed by the visitor
let target_scopes = visitor.fixup_scopes.drain(start_point..);
for scope in target_scopes {
let mut yield_data = visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap();
let count = yield_data.expr_and_pat_count;
let span = yield_data.span;
// expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
// before walking the left-hand side, it should be impossible for the recorded
// count to be greater than the left-hand side count.
if count > visitor.expr_and_pat_count {
bug!(
"Encountered greater count {} at span {:?} - expected no greater than {}",
count,
span,
visitor.expr_and_pat_count
);
}
let new_count = visitor.expr_and_pat_count;
debug!(
"resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
scope, count, new_count, span
);
yield_data.expr_and_pat_count = new_count;
}
}
_ => intravisit::walk_expr(visitor, expr),
}
visitor.expr_and_pat_count += 1;
debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
if let hir::ExprKind::Yield(_, source) = &expr.kind {
// Mark this expr's scope and all parent scopes as containing `yield`.
let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
loop {
let data = YieldData {
span: expr.span,
expr_and_pat_count: visitor.expr_and_pat_count,
source: *source,
};
visitor.scope_tree.yield_in_scope.insert(scope, data);
if visitor.pessimistic_yield {
debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
visitor.fixup_scopes.push(scope);
}
// Keep traversing up while we can.
match visitor.scope_tree.parent_map.get(&scope) {
// Don't cross from closure bodies to their parent.
Some(&(superscope, _)) => match superscope.data {
ScopeData::CallSite => break,
_ => scope = superscope,
},
None => break,
}
}
}
visitor.cx = prev_cx;
}
fn resolve_local<'tcx>(
visitor: &mut RegionResolutionVisitor<'tcx>,
pat: Option<&'tcx hir::Pat<'tcx>>,
init: Option<&'tcx hir::Expr<'tcx>>,
) {
debug!("resolve_local(pat={:?}, init={:?})", pat, init);
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.
///
/// 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, ..)
| PatKind::Binding(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:
///
/// 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`:
///
/// 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::UnDeref, 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 nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
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_string(body.value.span),
body_id,
self.cx.parent
);
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);
self.terminating_scopes.insert(body.value.hir_id.local_id);
if let Some(root_id) = self.cx.root_id {
self.scope_tree.record_closure_parent(body.value.hir_id.local_id, root_id);
}
self.cx.root_id = Some(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(&param.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;
}
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.as_ref().map(|e| &**e));
}
}
fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
let closure_base_def_id = tcx.closure_base_def_id(def_id);
if closure_base_def_id != def_id {
return tcx.region_scope_tree(closure_base_def_id);
}
let id = tcx.hir().as_local_hir_id(def_id).unwrap();
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 { root_id: None, 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);
// If the item is an associated const or a method,
// record its impl/trait parent, as it can also have
// lifetime parameters free in this body.
match tcx.hir().get(id) {
Node::ImplItem(_) | Node::TraitItem(_) => {
visitor.scope_tree.root_parent = Some(tcx.hir().get_parent_item(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 };
}
impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree { impl<'a> HashStable<StableHashingContext<'a>> for ScopeTree {
fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) { fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
let ScopeTree { let ScopeTree {

1
src/librustc_interface/passes.rs
View file

@ -686,7 +686,6 @@ pub fn default_provide(providers: &mut ty::query::Providers<'_>) {
stability::provide(providers); stability::provide(providers);
rustc_passes::provide(providers); rustc_passes::provide(providers);
rustc_traits::provide(providers); rustc_traits::provide(providers);
middle::region::provide(providers);
rustc_metadata::provide(providers); rustc_metadata::provide(providers);
lint::provide(providers); lint::provide(providers);
rustc_lint::provide(providers); rustc_lint::provide(providers);

2
src/librustc_passes/lib.rs
View file

@ -31,6 +31,7 @@ mod lib_features;
mod liveness; mod liveness;
pub mod loops; pub mod loops;
mod reachable; mod reachable;
mod region;
pub fn provide(providers: &mut Providers<'_>) { pub fn provide(providers: &mut Providers<'_>) {
check_const::provide(providers); check_const::provide(providers);
@ -41,4 +42,5 @@ pub fn provide(providers: &mut Providers<'_>) {
liveness::provide(providers); liveness::provide(providers);
intrinsicck::provide(providers); intrinsicck::provide(providers);
reachable::provide(providers); reachable::provide(providers);
region::provide(providers);
} }

835
src/librustc_passes/region.rs Normal file
View file

@ -0,0 +1,835 @@
//! This file builds up the `ScopeTree`, which describes
//! the parent links in the region hierarchy.
//!
//! For more information about how MIR-based region-checking works,
//! see the [rustc guide].
//!
//! [rustc guide]: https://rust-lang.github.io/rustc-guide/mir/borrowck.html
use rustc::hir;
use rustc::hir::def_id::DefId;
use rustc::hir::intravisit::{self, NestedVisitorMap, Visitor};
use rustc::hir::Node;
use rustc::hir::{Arm, Block, Expr, Local, Pat, PatKind, Stmt};
use rustc::middle::region::*;
use rustc::ty::query::Providers;
use rustc::ty::TyCtxt;
use rustc::util::nodemap::FxHashSet;
use rustc_index::vec::Idx;
use syntax::source_map;
use syntax_pos::Span;
use std::mem;
#[derive(Debug, Copy, Clone)]
pub struct Context {
/// The root of the current region tree. This is typically the id
/// of the innermost fn body. Each fn forms its own disjoint tree
/// in the region hierarchy. These fn bodies are themselves
/// arranged into a tree. See the "Modeling closures" section of
/// the README in `infer::region_constraints` for more
/// details.
root_id: Option<hir::ItemLocalId>,
/// The scope that contains any new variables declared, plus its depth in
/// the scope tree.
var_parent: Option<(Scope, ScopeDepth)>,
/// Region parent of expressions, etc., plus its depth in the scope tree.
parent: Option<(Scope, ScopeDepth)>,
}
struct RegionResolutionVisitor<'tcx> {
tcx: TyCtxt<'tcx>,
// The number of expressions and patterns visited in the current body.
expr_and_pat_count: usize,
// When this is `true`, we record the `Scopes` we encounter
// when processing a Yield expression. This allows us to fix
// up their indices.
pessimistic_yield: bool,
// Stores scopes when `pessimistic_yield` is `true`.
fixup_scopes: Vec<Scope>,
// The generated scope tree.
scope_tree: ScopeTree,
cx: Context,
/// `terminating_scopes` is a set containing the ids of each
/// statement, or conditional/repeating expression. These scopes
/// are calling "terminating scopes" because, when attempting to
/// find the scope of a temporary, by default we search up the
/// enclosing scopes until we encounter the terminating scope. A
/// conditional/repeating expression is one which is not
/// guaranteed to execute exactly once upon entering the parent
/// scope. This could be because the expression only executes
/// conditionally, such as the expression `b` in `a && b`, or
/// because the expression may execute many times, such as a loop
/// body. The reason that we distinguish such expressions is that,
/// upon exiting the parent scope, we cannot statically know how
/// many times the expression executed, and thus if the expression
/// creates temporaries we cannot know statically how many such
/// temporaries we would have to cleanup. Therefore, we ensure that
/// the temporaries never outlast the conditional/repeating
/// expression, preventing the need for dynamic checks and/or
/// arbitrary amounts of stack space. Terminating scopes end
/// up being contained in a DestructionScope that contains the
/// destructor's execution.
terminating_scopes: FxHashSet<hir::ItemLocalId>,
}
/// Records the lifetime of a local variable as `cx.var_parent`
fn record_var_lifetime(
visitor: &mut RegionResolutionVisitor<'_>,
var_id: hir::ItemLocalId,
_sp: Span,
) {
match visitor.cx.var_parent {
None => {
// this can happen in extern fn declarations like
//
// extern fn isalnum(c: c_int) -> c_int
}
Some((parent_scope, _)) => visitor.scope_tree.record_var_scope(var_id, parent_scope),
}
}
fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx hir::Block<'tcx>) {
debug!("resolve_block(blk.hir_id={:?})", blk.hir_id);
let prev_cx = visitor.cx;
// We treat the tail expression in the block (if any) somewhat
// differently from the statements. The issue has to do with
// temporary lifetimes. Consider the following:
//
// quux({
// let inner = ... (&bar()) ...;
//
// (... (&foo()) ...) // (the tail expression)
// }, other_argument());
//
// Each of the statements within the block is a terminating
// scope, and thus a temporary (e.g., the result of calling
// `bar()` in the initializer expression for `let inner = ...;`)
// will be cleaned up immediately after its corresponding
// statement (i.e., `let inner = ...;`) executes.
//
// On the other hand, temporaries associated with evaluating the
// tail expression for the block are assigned lifetimes so that
// they will be cleaned up as part of the terminating scope
// *surrounding* the block expression. Here, the terminating
// scope for the block expression is the `quux(..)` call; so
// those temporaries will only be cleaned up *after* both
// `other_argument()` has run and also the call to `quux(..)`
// itself has returned.
visitor.enter_node_scope_with_dtor(blk.hir_id.local_id);
visitor.cx.var_parent = visitor.cx.parent;
{
// This block should be kept approximately in sync with
// `intravisit::walk_block`. (We manually walk the block, rather
// than call `walk_block`, in order to maintain precise
// index information.)
for (i, statement) in blk.stmts.iter().enumerate() {
match statement.kind {
hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
// Each declaration introduces a subscope for bindings
// introduced by the declaration; this subscope covers a
// suffix of the block. Each subscope in a block has the
// previous subscope in the block as a parent, except for
// the first such subscope, which has the block itself as a
// parent.
visitor.enter_scope(Scope {
id: blk.hir_id.local_id,
data: ScopeData::Remainder(FirstStatementIndex::new(i)),
});
visitor.cx.var_parent = visitor.cx.parent;
}
hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
}
visitor.visit_stmt(statement)
}
walk_list!(visitor, visit_expr, &blk.expr);
}
visitor.cx = prev_cx;
}
fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
let prev_cx = visitor.cx;
visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node });
visitor.cx.var_parent = visitor.cx.parent;
visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
if let Some(hir::Guard::If(ref expr)) = arm.guard {
visitor.terminating_scopes.insert(expr.hir_id.local_id);
}
intravisit::walk_arm(visitor, arm);
visitor.cx = prev_cx;
}
fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node });
// If this is a binding then record the lifetime of that binding.
if let PatKind::Binding(..) = pat.kind {
record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
}
debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
intravisit::walk_pat(visitor, pat);
visitor.expr_and_pat_count += 1;
debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
}
fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
let stmt_id = stmt.hir_id.local_id;
debug!("resolve_stmt(stmt.id={:?})", stmt_id);
// Every statement will clean up the temporaries created during
// execution of that statement. Therefore each statement has an
// associated destruction scope that represents the scope of the
// statement plus its destructors, and thus the scope for which
// regions referenced by the destructors need to survive.
visitor.terminating_scopes.insert(stmt_id);
let prev_parent = visitor.cx.parent;
visitor.enter_node_scope_with_dtor(stmt_id);
intravisit::walk_stmt(visitor, stmt);
visitor.cx.parent = prev_parent;
}
fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
let prev_cx = visitor.cx;
visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
{
let terminating_scopes = &mut visitor.terminating_scopes;
let mut terminating = |id: hir::ItemLocalId| {
terminating_scopes.insert(id);
};
match expr.kind {
// Conditional or repeating scopes are always terminating
// scopes, meaning that temporaries cannot outlive them.
// This ensures fixed size stacks.
hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::And, .. },
_,
ref r,
)
| hir::ExprKind::Binary(
source_map::Spanned { node: hir::BinOpKind::Or, .. },
_,
ref r,
) => {
// For shortcircuiting operators, mark the RHS as a terminating
// scope since it only executes conditionally.
terminating(r.hir_id.local_id);
}
hir::ExprKind::Loop(ref body, _, _) => {
terminating(body.hir_id.local_id);
}
hir::ExprKind::DropTemps(ref expr) => {
// `DropTemps(expr)` does not denote a conditional scope.
// Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
terminating(expr.hir_id.local_id);
}
hir::ExprKind::AssignOp(..)
| hir::ExprKind::Index(..)
| hir::ExprKind::Unary(..)
| hir::ExprKind::Call(..)
| hir::ExprKind::MethodCall(..) => {
// FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
//
// The lifetimes for a call or method call look as follows:
//
// call.id
// - arg0.id
// - ...
// - argN.id
// - call.callee_id
//
// The idea is that call.callee_id represents *the time when
// the invoked function is actually running* and call.id
// represents *the time to prepare the arguments and make the
// call*. See the section "Borrows in Calls" borrowck/README.md
// for an extended explanation of why this distinction is
// important.
//
// record_superlifetime(new_cx, expr.callee_id);
}
_ => {}
}
}
let prev_pessimistic = visitor.pessimistic_yield;
// Ordinarily, we can rely on the visit order of HIR intravisit
// to correspond to the actual execution order of statements.
// However, there's a weird corner case with compund assignment
// operators (e.g. `a += b`). The evaluation order depends on whether
// or not the operator is overloaded (e.g. whether or not a trait
// like AddAssign is implemented).
// For primitive types (which, despite having a trait impl, don't actually
// end up calling it), the evluation order is right-to-left. For example,
// the following code snippet:
//
// let y = &mut 0;
// *{println!("LHS!"); y} += {println!("RHS!"); 1};
//
// will print:
//
// RHS!
// LHS!
//
// However, if the operator is used on a non-primitive type,
// the evaluation order will be left-to-right, since the operator
// actually get desugared to a method call. For example, this
// nearly identical code snippet:
//
// let y = &mut String::new();
// *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
//
// will print:
// LHS String
// RHS String
//
// To determine the actual execution order, we need to perform
// trait resolution. Unfortunately, we need to be able to compute
// yield_in_scope before type checking is even done, as it gets
// used by AST borrowcheck.
//
// Fortunately, we don't need to know the actual execution order.
// It suffices to know the 'worst case' order with respect to yields.
// Specifically, we need to know the highest 'expr_and_pat_count'
// that we could assign to the yield expression. To do this,
// we pick the greater of the two values from the left-hand
// and right-hand expressions. This makes us overly conservative
// about what types could possibly live across yield points,
// but we will never fail to detect that a type does actually
// live across a yield point. The latter part is critical -
// we're already overly conservative about what types will live
// across yield points, as the generated MIR will determine
// when things are actually live. However, for typecheck to work
// properly, we can't miss any types.
match expr.kind {
// Manually recurse over closures, because they are the only
// case of nested bodies that share the parent environment.
hir::ExprKind::Closure(.., body, _, _) => {
let body = visitor.tcx.hir().body(body);
visitor.visit_body(body);
}
hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
debug!(
"resolve_expr - enabling pessimistic_yield, was previously {}",
prev_pessimistic
);
let start_point = visitor.fixup_scopes.len();
visitor.pessimistic_yield = true;
// If the actual execution order turns out to be right-to-left,
// then we're fine. However, if the actual execution order is left-to-right,
// then we'll assign too low a count to any `yield` expressions
// we encounter in 'right_expression' - they should really occur after all of the
// expressions in 'left_expression'.
visitor.visit_expr(&right_expr);
visitor.pessimistic_yield = prev_pessimistic;
debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
visitor.visit_expr(&left_expr);
debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
// Remove and process any scopes pushed by the visitor
let target_scopes = visitor.fixup_scopes.drain(start_point..);
for scope in target_scopes {
let mut yield_data = visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap();
let count = yield_data.expr_and_pat_count;
let span = yield_data.span;
// expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
// before walking the left-hand side, it should be impossible for the recorded
// count to be greater than the left-hand side count.
if count > visitor.expr_and_pat_count {
bug!(
"Encountered greater count {} at span {:?} - expected no greater than {}",
count,
span,
visitor.expr_and_pat_count
);
}
let new_count = visitor.expr_and_pat_count;
debug!(
"resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
scope, count, new_count, span
);
yield_data.expr_and_pat_count = new_count;
}
}
_ => intravisit::walk_expr(visitor, expr),
}
visitor.expr_and_pat_count += 1;
debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
if let hir::ExprKind::Yield(_, source) = &expr.kind {
// Mark this expr's scope and all parent scopes as containing `yield`.
let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
loop {
let data = YieldData {
span: expr.span,
expr_and_pat_count: visitor.expr_and_pat_count,
source: *source,
};
visitor.scope_tree.yield_in_scope.insert(scope, data);
if visitor.pessimistic_yield {
debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
visitor.fixup_scopes.push(scope);
}
// Keep traversing up while we can.
match visitor.scope_tree.parent_map.get(&scope) {
// Don't cross from closure bodies to their parent.
Some(&(superscope, _)) => match superscope.data {
ScopeData::CallSite => break,
_ => scope = superscope,
},
None => break,
}
}
}
visitor.cx = prev_cx;
}
fn resolve_local<'tcx>(
visitor: &mut RegionResolutionVisitor<'tcx>,
pat: Option<&'tcx hir::Pat<'tcx>>,
init: Option<&'tcx hir::Expr<'tcx>>,
) {
debug!("resolve_local(pat={:?}, init={:?})", pat, init);
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, ..)
| PatKind::Binding(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::UnDeref, 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 nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
NestedVisitorMap::None
}
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_string(body.value.span),
body_id,
self.cx.parent
);
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);
self.terminating_scopes.insert(body.value.hir_id.local_id);
if let Some(root_id) = self.cx.root_id {
self.scope_tree.record_closure_parent(body.value.hir_id.local_id, root_id);
}
self.cx.root_id = Some(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(&param.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;
}
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.as_ref().map(|e| &**e));
}
}
fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
let closure_base_def_id = tcx.closure_base_def_id(def_id);
if closure_base_def_id != def_id {
return tcx.region_scope_tree(closure_base_def_id);
}
let id = tcx.hir().as_local_hir_id(def_id).unwrap();
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 { root_id: None, 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);
// If the item is an associated const or a method,
// record its impl/trait parent, as it can also have
// lifetime parameters free in this body.
match tcx.hir().get(id) {
Node::ImplItem(_) | Node::TraitItem(_) => {
visitor.scope_tree.root_parent = Some(tcx.hir().get_parent_item(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 };
}