Don't run Drop terminators on types that do not have drop glue in c…

…onst eval

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -119,11 +119,20 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Drop { place, target, unwind } => {
+                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)?;
-                let ty = place.layout.ty;
                 trace!("TerminatorKind::drop: {:?}, type {}", place, ty);
-
-                let instance = Instance::resolve_drop_in_place(*self.tcx, ty);
                 self.drop_in_place(&place, instance, target, unwind)?;
             }
 

compiler/rustc_middle/src/mir/syntax.rs

@@ -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:

src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.rs

@@ -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/