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Rollup merge of #94087 - tmiasko:rm-ignore-borrow-on-drop, r=jackh726

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Remove unused `unsound_ignore_borrow_on_drop`
This commit is contained in:
Matthias Krüger 2022-02-26 07:52:42 +01:00 committed by GitHub
commit 734b924d05
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2 changed files with 13 additions and 38 deletions
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45
compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
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@ -10,38 +10,11 @@ use rustc_middle::mir::*;
/// At present, this is used as a very limited form of alias analysis. For example,
/// `MaybeBorrowedLocals` is used to compute which locals are live during a yield expression for
/// immovable generators.
pub struct MaybeBorrowedLocals {
ignore_borrow_on_drop: bool,
}
pub struct MaybeBorrowedLocals;
impl MaybeBorrowedLocals {
/// A dataflow analysis that records whether a pointer or reference exists that may alias the
/// given local.
pub fn all_borrows() -> Self {
MaybeBorrowedLocals { ignore_borrow_on_drop: false }
}
}
impl MaybeBorrowedLocals {
/// During dataflow analysis, ignore the borrow that may occur when a place is dropped.
///
/// Drop terminators may call custom drop glue (`Drop::drop`), which takes `&mut self` as a
/// parameter. In the general case, a drop impl could launder that reference into the
/// surrounding environment through a raw pointer, thus creating a valid `*mut` pointing to the
/// dropped local. We are not yet willing to declare this particular case UB, so we must treat
/// all dropped locals as mutably borrowed for now. See discussion on [#61069].
///
/// In some contexts, we know that this borrow will never occur. For example, during
/// const-eval, custom drop glue cannot be run. Code that calls this should document the
/// assumptions that justify ignoring `Drop` terminators in this way.
///
/// [#61069]: https://github.com/rust-lang/rust/pull/61069
pub fn unsound_ignore_borrow_on_drop(self) -> Self {
MaybeBorrowedLocals { ignore_borrow_on_drop: true, ..self }
}
fn transfer_function<'a, T>(&'a self, trans: &'a mut T) -> TransferFunction<'a, T> {
TransferFunction { trans, ignore_borrow_on_drop: self.ignore_borrow_on_drop }
TransferFunction { trans }
}
}
@ -92,7 +65,6 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeBorrowedLocals {
/// A `Visitor` that defines the transfer function for `MaybeBorrowedLocals`.
struct TransferFunction<'a, T> {
trans: &'a mut T,
ignore_borrow_on_drop: bool,
}
impl<'tcx, T> Visitor<'tcx> for TransferFunction<'_, T>
@ -146,10 +118,15 @@ where
match terminator.kind {
mir::TerminatorKind::Drop { place: dropped_place, .. }
| mir::TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
// See documentation for `unsound_ignore_borrow_on_drop` for an explanation.
if !self.ignore_borrow_on_drop {
self.trans.gen(dropped_place.local);
}
// Drop terminators may call custom drop glue (`Drop::drop`), which takes `&mut
// self` as a parameter. In the general case, a drop impl could launder that
// reference into the surrounding environment through a raw pointer, thus creating
// a valid `*mut` pointing to the dropped local. We are not yet willing to declare
// this particular case UB, so we must treat all dropped locals as mutably borrowed
// for now. See discussion on [#61069].
//
// [#61069]: https://github.com/rust-lang/rust/pull/61069
self.trans.gen(dropped_place.local);
}
TerminatorKind::Abort

6
compiler/rustc_mir_transform/src/generator.rs
View File

@ -463,10 +463,8 @@ fn locals_live_across_suspend_points<'tcx>(
// Calculate the MIR locals which have been previously
// borrowed (even if they are still active).
let borrowed_locals_results = MaybeBorrowedLocals::all_borrows()
.into_engine(tcx, body_ref)
.pass_name("generator")
.iterate_to_fixpoint();
let borrowed_locals_results =
MaybeBorrowedLocals.into_engine(tcx, body_ref).pass_name("generator").iterate_to_fixpoint();
let mut borrowed_locals_cursor =
rustc_mir_dataflow::ResultsCursor::new(body_ref, &borrowed_locals_results);