Work around opaque types hiding needs_drop

When building the MIR we sometimes try to unschedule drops. In this we
assert that the drop has already been scheduled. Opaque types however
may be initialized with an expression kind that we know doesn't have a
type that needs to be dropped. To fix this we don't panic if we can't
find the drop of a variable with an opaque type.

compiler/rustc_mir_build/src/build/scope.rs

@@ -90,6 +90,7 @@ use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::middle::region;
 use rustc_middle::mir::*;
 use rustc_middle::thir::{ExprId, LintLevel};
+use rustc_middle::ty::TypeVisitableExt;
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::Level;
 use rustc_span::{Span, DUMMY_SP};
@@ -1056,6 +1057,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     {
                         return;
                     }
+                    // Opaque type may not have been scheduled if its underlying
+                    // type does not need drop.
+                    None if self.local_decls[local].ty.has_opaque_types() => return,
                     _ => bug!(
                         "found wrong drop, expected value drop of {:?}, found {:?}",
                         local,

tests/ui/impl-trait/rpit/non-drop-type.rs

@@ -0,0 +1,22 @@
+//@ check-pass
+
+fn if_else(c: bool) -> impl Sized {
+    if c { () } else { () }
+}
+
+fn if_no_else(c: bool) -> impl Sized {
+    if c {}
+}
+
+fn matches(c: bool) -> impl Sized {
+    match c {
+        true => (),
+        _ => (),
+    }
+}
+
+fn tuple_tuple(c: bool) -> (impl Sized,) {
+    if c { ((),) } else { ((),) }
+}
+
+fn main() {}