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use rustc_data_structures::fx::FxIndexMap;
use rustc_middle::mir::*;
use rustc_middle::ty::Ty;
use rustc_span::Span;
use tracing::debug;
use crate::build::expr::as_place::PlaceBase;
use crate::build::matches::{Binding, Candidate, FlatPat, MatchPairTree, TestCase};
use crate::build::Builder;
impl<'a, 'tcx> Builder<'a, 'tcx> {
/// Creates a false edge to `imaginary_target` and a real edge to
/// real_target. If `imaginary_target` is none, or is the same as the real
/// target, a Goto is generated instead to simplify the generated MIR.
pub(crate) fn false_edges(
&mut self,
from_block: BasicBlock,
real_target: BasicBlock,
imaginary_target: BasicBlock,
source_info: SourceInfo,
) {
if imaginary_target != real_target {
self.cfg.terminate(
from_block,
source_info,
TerminatorKind::FalseEdge { real_target, imaginary_target },
);
} else {
self.cfg.goto(from_block, source_info, real_target)
}
}
}
/// Determine the set of places that have to be stable across match guards.
///
/// Returns a list of places that need a fake borrow along with a local to store it.
///
/// Match exhaustiveness checking is not able to handle the case where the place being matched on is
/// mutated in the guards. We add "fake borrows" to the guards that prevent any mutation of the
/// place being matched. There are a some subtleties:
///
/// 1. Borrowing `*x` doesn't prevent assigning to `x`. If `x` is a shared reference, the borrow
/// isn't even tracked. As such we have to add fake borrows of any prefixes of a place.
/// 2. We don't want `match x { (Some(_), _) => (), .. }` to conflict with mutable borrows of `x.1`, so we
/// only add fake borrows for places which are bound or tested by the match.
/// 3. We don't want `match x { Some(_) => (), .. }` to conflict with mutable borrows of `(x as
/// Some).0`, so the borrows are a special shallow borrow that only affects the place and not its
/// projections.
/// ```rust
/// let mut x = (Some(0), true);
/// match x {
/// (Some(_), false) => {}
/// _ if { if let Some(ref mut y) = x.0 { *y += 1 }; true } => {}
/// _ => {}
/// }
/// ```
/// 4. The fake borrows may be of places in inactive variants, e.g. here we need to fake borrow `x`
/// and `(x as Some).0`, but when we reach the guard `x` may not be `Some`.
/// ```rust
/// let mut x = (Some(Some(0)), true);
/// match x {
/// (Some(Some(_)), false) => {}
/// _ if { if let Some(Some(ref mut y)) = x.0 { *y += 1 }; true } => {}
/// _ => {}
/// }
/// ```
/// So it would be UB to generate code for the fake borrows. They therefore have to be removed by
/// a MIR pass run after borrow checking.
pub(super) fn collect_fake_borrows<'tcx>(
cx: &mut Builder<'_, 'tcx>,
candidates: &[Candidate<'_, 'tcx>],
temp_span: Span,
scrutinee_base: PlaceBase,
) -> Vec<(Place<'tcx>, Local, FakeBorrowKind)> {
if candidates.iter().all(|candidate| !candidate.has_guard) {
// Fake borrows are only used when there is a guard.
return Vec::new();
}
let mut collector =
FakeBorrowCollector { cx, scrutinee_base, fake_borrows: FxIndexMap::default() };
for candidate in candidates.iter() {
collector.visit_candidate(candidate);
}
let fake_borrows = collector.fake_borrows;
debug!("add_fake_borrows fake_borrows = {:?}", fake_borrows);
let tcx = cx.tcx;
fake_borrows
.iter()
.map(|(matched_place, borrow_kind)| {
let fake_borrow_deref_ty = matched_place.ty(&cx.local_decls, tcx).ty;
let fake_borrow_ty =
Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, fake_borrow_deref_ty);
let mut fake_borrow_temp = LocalDecl::new(fake_borrow_ty, temp_span);
fake_borrow_temp.local_info = ClearCrossCrate::Set(Box::new(LocalInfo::FakeBorrow));
let fake_borrow_temp = cx.local_decls.push(fake_borrow_temp);
(*matched_place, fake_borrow_temp, *borrow_kind)
})
.collect()
}
pub(super) struct FakeBorrowCollector<'a, 'b, 'tcx> {
cx: &'a mut Builder<'b, 'tcx>,
/// Base of the scrutinee place. Used to distinguish bindings inside the scrutinee place from
/// bindings inside deref patterns.
scrutinee_base: PlaceBase,
/// Store for each place the kind of borrow to take. In case of conflicts, we take the strongest
/// borrow (i.e. Deep > Shallow).
/// Invariant: for any place in `fake_borrows`, all the prefixes of this place that are
/// dereferences are also borrowed with the same of stronger borrow kind.
fake_borrows: FxIndexMap<Place<'tcx>, FakeBorrowKind>,
}
impl<'a, 'b, 'tcx> FakeBorrowCollector<'a, 'b, 'tcx> {
// Fake borrow this place and its dereference prefixes.
fn fake_borrow(&mut self, place: Place<'tcx>, kind: FakeBorrowKind) {
if self.fake_borrows.get(&place).is_some_and(|k| *k >= kind) {
return;
}
self.fake_borrows.insert(place, kind);
// Also fake borrow the prefixes of any fake borrow.
self.fake_borrow_deref_prefixes(place, kind);
}
// Fake borrow the prefixes of this place that are dereferences.
fn fake_borrow_deref_prefixes(&mut self, place: Place<'tcx>, kind: FakeBorrowKind) {
for (place_ref, elem) in place.as_ref().iter_projections().rev() {
if let ProjectionElem::Deref = elem {
// Insert a shallow borrow after a deref. For other projections the borrow of
// `place_ref` will conflict with any mutation of `place.base`.
let place = place_ref.to_place(self.cx.tcx);
if self.fake_borrows.get(&place).is_some_and(|k| *k >= kind) {
return;
}
self.fake_borrows.insert(place, kind);
}
}
}
fn visit_candidate(&mut self, candidate: &Candidate<'_, 'tcx>) {
for binding in &candidate.extra_data.bindings {
self.visit_binding(binding);
}
for match_pair in &candidate.match_pairs {
self.visit_match_pair(match_pair);
}
}
fn visit_flat_pat(&mut self, flat_pat: &FlatPat<'_, 'tcx>) {
for binding in &flat_pat.extra_data.bindings {
self.visit_binding(binding);
}
for match_pair in &flat_pat.match_pairs {
self.visit_match_pair(match_pair);
}
}
fn visit_match_pair(&mut self, match_pair: &MatchPairTree<'_, 'tcx>) {
if let TestCase::Or { pats, .. } = &match_pair.test_case {
for flat_pat in pats.iter() {
self.visit_flat_pat(flat_pat)
}
} else if matches!(match_pair.test_case, TestCase::Deref { .. }) {
// The subpairs of a deref pattern are all places relative to the deref temporary, so we
// don't fake borrow them. Problem is, if we only shallowly fake-borrowed
// `match_pair.place`, this would allow:
// ```
// let mut b = Box::new(false);
// match b {
// deref!(true) => {} // not reached because `*b == false`
// _ if { *b = true; false } => {} // not reached because the guard is `false`
// deref!(false) => {} // not reached because the guard changed it
// // UB because we reached the unreachable.
// }
// ```
// Hence we fake borrow using a deep borrow.
if let Some(place) = match_pair.place {
self.fake_borrow(place, FakeBorrowKind::Deep);
}
} else {
// Insert a Shallow borrow of any place that is switched on.
if let Some(place) = match_pair.place {
self.fake_borrow(place, FakeBorrowKind::Shallow);
}
for subpair in &match_pair.subpairs {
self.visit_match_pair(subpair);
}
}
}
fn visit_binding(&mut self, Binding { source, .. }: &Binding<'tcx>) {
if let PlaceBase::Local(l) = self.scrutinee_base
&& l != source.local
{
// The base of this place is a temporary created for deref patterns. We don't emit fake
// borrows for these as they are not initialized in all branches.
return;
}
// Insert a borrows of prefixes of places that are bound and are
// behind a dereference projection.
//
// These borrows are taken to avoid situations like the following:
//
// match x[10] {
// _ if { x = &[0]; false } => (),
// y => (), // Out of bounds array access!
// }
//
// match *x {
// // y is bound by reference in the guard and then by copy in the
// // arm, so y is 2 in the arm!
// y if { y == 1 && (x = &2) == () } => y,
// _ => 3,
// }
//
// We don't just fake borrow the whole place because this is allowed:
// match u {
// _ if { u = true; false } => (),
// x => (),
// }
self.fake_borrow_deref_prefixes(*source, FakeBorrowKind::Shallow);
}
}
#[must_use]
pub(crate) fn ref_pat_borrow_kind(ref_mutability: Mutability) -> BorrowKind {
match ref_mutability {
Mutability::Mut => BorrowKind::Mut { kind: MutBorrowKind::Default },
Mutability::Not => BorrowKind::Shared,
}
}