CFI: Handle dyn with no principal

In user-facing Rust, `dyn` always has at least one predicate following
it. Unfortunately, because we filter out marker traits and `dyn Sync`
is, for example, legal, this results in us having `dyn` types with no
predicates on occasion. This patch handles cases where there are no
predicates in a `dyn` type.

compiler/rustc_middle/src/ty/predicate.rs

@@ -290,7 +290,7 @@ impl<'tcx> ty::List<ty::PolyExistentialPredicate<'tcx>> {
     /// have a "trivial" vtable consisting of just the size, alignment,
     /// and destructor.
     pub fn principal(&self) -> Option<ty::Binder<'tcx, ExistentialTraitRef<'tcx>>> {
-        self[0]
+        self.get(0)?
             .map_bound(|this| match this {
                 ExistentialPredicate::Trait(tr) => Some(tr),
                 _ => None,

compiler/rustc_symbol_mangling/src/typeid/typeid_itanium_cxx_abi.rs

@@ -759,7 +759,11 @@ fn transform_predicates<'tcx>(
             ty::ExistentialPredicate::AutoTrait(..) => Some(predicate),
         })
         .collect();
-    tcx.mk_poly_existential_predicates(&predicates)
+    if predicates.len() == 0 {
+        List::empty()
+    } else {
+        tcx.mk_poly_existential_predicates(&predicates)
+    }
 }
 
 /// Transforms args for being encoded and used in the substitution dictionary.

tests/ui/sanitizer/cfi-drop-no-principal.rs

@@ -0,0 +1,16 @@
+// Check that dropping a trait object without a principal trait succeeds
+
+//@ needs-sanitizer-cfi
+//@ compile-flags: --crate-type=bin -Cprefer-dynamic=off -Clto -Zsanitizer=cfi
+//@ compile-flags: -C codegen-units=1 -C opt-level=0
+//@ run-pass
+// Check that trait objects without a principal can be dropped.
+
+struct CustomDrop;
+impl Drop for CustomDrop {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let _ = Box::new(CustomDrop) as Box<dyn Send>;
+}