1 files changed, 123 insertions, 0 deletions
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/post_drop_elaboration.rs b/compiler/rustc_const_eval/src/transform/check_consts/post_drop_elaboration.rs
new file mode 100644
index 000000000..4e210f663
--- /dev/null
+++ b/compiler/rustc_const_eval/src/transform/check_consts/post_drop_elaboration.rs
@@ -0,0 +1,123 @@
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::{self, BasicBlock, Location};
+use rustc_middle::ty::TyCtxt;
+use rustc_span::{symbol::sym, Span};
+
+use super::check::Qualifs;
+use super::ops::{self, NonConstOp};
+use super::qualifs::{NeedsNonConstDrop, Qualif};
+use super::ConstCx;
+
+/// Returns `true` if we should use the more precise live drop checker that runs after drop
+/// elaboration.
+pub fn checking_enabled(ccx: &ConstCx<'_, '_>) -> bool {
+    // Const-stable functions must always use the stable live drop checker.
+    if ccx.is_const_stable_const_fn() {
+        return false;
+    }
+
+    ccx.tcx.features().const_precise_live_drops
+}
+
+/// Look for live drops in a const context.
+///
+/// This is separate from the rest of the const checking logic because it must run after drop
+/// elaboration.
+pub fn check_live_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mir::Body<'tcx>) {
+    let def_id = body.source.def_id().expect_local();
+    let const_kind = tcx.hir().body_const_context(def_id);
+    if const_kind.is_none() {
+        return;
+    }
+
+    if tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
+        return;
+    }
+
+    let ccx = ConstCx { body, tcx, const_kind, param_env: tcx.param_env(def_id) };
+    if !checking_enabled(&ccx) {
+        return;
+    }
+
+    let mut visitor = CheckLiveDrops { ccx: &ccx, qualifs: Qualifs::default() };
+
+    visitor.visit_body(body);
+}
+
+struct CheckLiveDrops<'mir, 'tcx> {
+    ccx: &'mir ConstCx<'mir, 'tcx>,
+    qualifs: Qualifs<'mir, 'tcx>,
+}
+
+// So we can access `body` and `tcx`.
+impl<'mir, 'tcx> std::ops::Deref for CheckLiveDrops<'mir, 'tcx> {
+    type Target = ConstCx<'mir, 'tcx>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.ccx
+    }
+}
+
+impl CheckLiveDrops<'_, '_> {
+    fn check_live_drop(&self, span: Span) {
+        ops::LiveDrop { dropped_at: None }.build_error(self.ccx, span).emit();
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for CheckLiveDrops<'_, 'tcx> {
+    fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &mir::BasicBlockData<'tcx>) {
+        trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
+
+        // Ignore drop terminators in cleanup blocks.
+        if block.is_cleanup {
+            return;
+        }
+
+        self.super_basic_block_data(bb, block);
+    }
+
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'tcx>, location: Location) {
+        trace!("visit_terminator: terminator={:?} location={:?}", terminator, location);
+
+        match &terminator.kind {
+            mir::TerminatorKind::Drop { place: dropped_place, .. }
+            | mir::TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
+                let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
+                if !NeedsNonConstDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
+                    // Instead of throwing a bug, we just return here. This is because we have to
+                    // run custom `const Drop` impls.
+                    return;
+                }
+
+                if dropped_place.is_indirect() {
+                    self.check_live_drop(terminator.source_info.span);
+                    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_non_const_drop(self.ccx, dropped_place.local, location) {
+                    // Use the span where the dropped local was declared for the error.
+                    let span = self.body.local_decls[dropped_place.local].source_info.span;
+                    self.check_live_drop(span);
+                }
+            }
+
+            mir::TerminatorKind::Abort
+            | mir::TerminatorKind::Call { .. }
+            | mir::TerminatorKind::Assert { .. }
+            | mir::TerminatorKind::FalseEdge { .. }
+            | mir::TerminatorKind::FalseUnwind { .. }
+            | mir::TerminatorKind::GeneratorDrop
+            | mir::TerminatorKind::Goto { .. }
+            | mir::TerminatorKind::InlineAsm { .. }
+            | mir::TerminatorKind::Resume
+            | mir::TerminatorKind::Return
+            | mir::TerminatorKind::SwitchInt { .. }
+            | mir::TerminatorKind::Unreachable
+            | mir::TerminatorKind::Yield { .. } => {}
+        }
+    }
+}