Add `CheckLiveDrops` pass

3 files changed, 126 insertions, 0 deletions

diff --git a/src/librustc_mir/transform/check_consts/mod.rs b/src/librustc_mir/transform/check_consts/mod.rs
index 7c439f80ef6..e4aa88e3c20 100644
--- a/src/librustc_mir/transform/check_consts/mod.rs
+++ b/src/librustc_mir/transform/check_consts/mod.rs
@@ -12,6 +12,7 @@ use rustc_middle::ty::{self, TyCtxt};
 pub use self::qualifs::Qualif;
 
 mod ops;
+pub mod post_drop_elaboration;
 pub mod qualifs;
 mod resolver;
 pub mod validation;
diff --git a/src/librustc_mir/transform/check_consts/post_drop_elaboration.rs b/src/librustc_mir/transform/check_consts/post_drop_elaboration.rs
new file mode 100644
index 00000000000..226e0e2049e
--- /dev/null
+++ b/src/librustc_mir/transform/check_consts/post_drop_elaboration.rs
@@ -0,0 +1,119 @@
+use rustc_hir::def_id::LocalDefId;
+use rustc_middle::mir::visit::Visitor;
+use rustc_middle::mir::{self, BasicBlock, Location};
+use rustc_middle::ty::TyCtxt;
+use rustc_span::Span;
+
+use super::ops;
+use super::qualifs::{NeedsDrop, Qualif};
+use super::validation::Qualifs;
+use super::ConstCx;
+
+/// Returns `true` if we should use the more precise live drop checker that runs after drop
+/// elaboration.
+pub fn checking_enabled(tcx: TyCtxt<'tcx>) -> bool {
+    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: TyCtxt<'tcx>, def_id: LocalDefId, body: &mir::Body<'tcx>) {
+    let const_kind = tcx.hir().body_const_context(def_id);
+    if const_kind.is_none() {
+        return;
+    }
+
+    if !checking_enabled(tcx) {
+        return;
+    }
+
+    let ccx = ConstCx {
+        body,
+        tcx,
+        def_id: def_id.to_def_id(),
+        const_kind,
+        param_env: tcx.param_env(def_id),
+    };
+
+    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 std::ops::Deref for CheckLiveDrops<'mir, 'tcx> {
+    type Target = ConstCx<'mir, 'tcx>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.ccx
+    }
+}
+
+impl CheckLiveDrops<'mir, 'tcx> {
+    fn check_live_drop(&self, span: Span) {
+        ops::non_const(self.ccx, ops::LiveDrop, span);
+    }
+}
+
+impl Visitor<'tcx> for CheckLiveDrops<'mir, '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 { location: dropped_place, .. } => {
+                let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
+                if !NeedsDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
+                    return;
+                }
+
+                if dropped_place.is_indirect() {
+                    self.check_live_drop(terminator.source_info.span);
+                    return;
+                }
+
+                if self.qualifs.needs_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::DropAndReplace { .. } => span_bug!(
+                terminator.source_info.span,
+                "`DropAndReplace` should be removed by drop elaboration",
+            ),
+
+            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 { .. } => {}
+        }
+    }
+}
diff --git a/src/librustc_mir/transform/check_consts/validation.rs b/src/librustc_mir/transform/check_consts/validation.rs
index cc9b593b7ab..428a74bcdcb 100644
--- a/src/librustc_mir/transform/check_consts/validation.rs
+++ b/src/librustc_mir/transform/check_consts/validation.rs
@@ -562,6 +562,12 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
             // projections that cannot be `NeedsDrop`.
             TerminatorKind::Drop { location: dropped_place, .. }
             | TerminatorKind::DropAndReplace { location: dropped_place, .. } => {
+                // If we are checking live drops after drop-elaboration, don't emit duplicate
+                // errors here.
+                if super::post_drop_elaboration::checking_enabled(self.tcx) {
+                    return;
+                }
+
                 let mut err_span = self.span;
 
                 // Check to see if the type of this place can ever have a drop impl. If not, this