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Rollup merge of #83351 - RalfJung:precise-const-drop, r=oli-obk

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post-drop-elab check-const: explain why we still check qualifs

r? `@oli-obk`
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Dylan DPC 2021-03-22 15:21:29 +01:00 • committed by GitHub
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8
compiler/rustc_mir/src/transform/check_consts/post_drop_elaboration.rs
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@ -79,7 +79,9 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
mir::TerminatorKind::Drop { place: dropped_place, .. } => { mir::TerminatorKind::Drop { place: dropped_place, .. } => {
let dropped_ty = dropped_place.ty(self.body, self.tcx).ty; let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
if !NeedsDrop::in_any_value_of_ty(self.ccx, dropped_ty) { if !NeedsDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
return; bug!(
"Drop elaboration left behind a Drop for a type that does not need dropping"
);
} }
if dropped_place.is_indirect() { if dropped_place.is_indirect() {
@ -87,6 +89,10 @@ impl Visitor<'tcx> for CheckLiveDrops<'mir, 'tcx> {
return; return;
} }
// Drop elaboration is not precise enough to accept code like
// `src/test/ui/consts/control-flow/drop-pass.rs`; e.g., when an `Option<Vec<T>>` is
// initialized with `None` and never changed, it still emits drop glue.
// Hence we additionally check the qualifs here to allow more code to pass.
if self.qualifs.needs_drop(self.ccx, dropped_place.local, location) { if self.qualifs.needs_drop(self.ccx, dropped_place.local, location) {
// Use the span where the dropped local was declared for the error. // Use the span where the dropped local was declared for the error.
let span = self.body.local_decls[dropped_place.local].source_info.span; let span = self.body.local_decls[dropped_place.local].source_info.span;