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Don't run Drop terminators on types that do not have drop glue in const eval

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This commit is contained in:
Jakob Degen 2022-12-21 15:22:17 -08:00
parent 37e00165e4
commit 0229281d03
3 changed files with 21 additions and 13 deletions
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17
compiler/rustc_const_eval/src/interpret/terminator.rs
View File

@ -119,11 +119,20 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
Drop { place, target, unwind } => {
let place = self.eval_place(place)?;
let ty = place.layout.ty;
trace!("TerminatorKind::drop: {:?}, type {}", place, ty);
let frame = self.frame();
let ty = place.ty(&frame.body.local_decls, *self.tcx).ty;
let ty = self.subst_from_frame_and_normalize_erasing_regions(frame, ty)?;
let instance = Instance::resolve_drop_in_place(*self.tcx, ty);
if let ty::InstanceDef::DropGlue(_, None) = instance.def {
// This is the branch we enter if and only if the dropped type has no drop glue
// whatsoever. This can happen as a result of monomorphizing a drop of a
// generic. In order to make sure that generic and non-generic code behaves
// roughly the same (and in keeping with Mir semantics) we do nothing here.
self.go_to_block(target);
return Ok(());
}
let place = self.eval_place(place)?;
trace!("TerminatorKind::drop: {:?}, type {}", place, ty);
self.drop_in_place(&place, instance, target, unwind)?;
}

13
compiler/rustc_middle/src/mir/syntax.rs
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@ -562,14 +562,13 @@ pub enum TerminatorKind<'tcx> {
Unreachable,
/// The behavior of this statement differs significantly before and after drop elaboration.
/// After drop elaboration, `Drop` executes the drop glue for the specified place, after which
/// it continues execution/unwinds at the given basic blocks. It is possible that executing drop
/// glue is special - this would be part of Rust's memory model. (**FIXME**: due we have an
/// issue tracking if drop glue has any interesting semantics in addition to those of a function
/// call?)
///
/// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically, the
/// `Drop` will be executed if...
/// After drop elaboration: `Drop` terminators are a complete nop for types that have no drop
/// glue. For other types, `Drop` terminators behave exactly like a call to
/// `core::mem::drop_in_place` with a pointer to the given place.
///
/// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically,
/// the `Drop` will be executed if...
///
/// **Needs clarification**: End of that sentence. This in effect should document the exact
/// behavior of drop elaboration. The following sounds vaguely right, but I'm not quite sure:

4
src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.rs
View File

@ -1,5 +1,5 @@
//! Test that drop_in_place mutably retags the entire place,
//! ensuring it is writeable
//! Test that drop_in_place mutably retags the entire place, even for a type that does not need
//! dropping, ensuring among other things that it is writeable
//@error-pattern: /retag .* for Unique permission .* only grants SharedReadOnly permission/