Don't eagerly monomorphize drop for types that are impossible to instantiate

tests/ui/codegen/mono-impossible-drop.rs

@@ -0,0 +1,18 @@
+//@ compile-flags: -Clink-dead-code=on --crate-type=lib
+//@ build-pass
+
+#![feature(trivial_bounds)]
+#![allow(trivial_bounds)]
+
+// Make sure we don't monomorphize the drop impl for `Baz`, since it has predicates
+// that don't hold under a reveal-all param env.
+
+trait Foo {
+    type Assoc;
+}
+
+struct Bar;
+
+struct Baz(<Bar as Foo>::Assoc)
+where
+    Bar: Foo;