fix MIR validation around impl Trait

src/librustc_mir/transform/validate.rs

@@ -120,7 +120,8 @@ pub fn equal_up_to_regions(
     }
 
     // Instantiate and run relation.
-    let mut relator: LifetimeIgnoreRelation<'tcx> = LifetimeIgnoreRelation { tcx: tcx, param_env };
+    let param_env = param_env.with_reveal_all_normalized(tcx);
+    let mut relator: LifetimeIgnoreRelation<'tcx> = LifetimeIgnoreRelation { tcx, param_env };
     relator.relate(src, dest).is_ok()
 }
 

src/test/ui/mir/validation_impl_trait_lifetime.rs

@@ -0,0 +1,9 @@
+// check-pass
+//! This used to fail MIR validation due to the types on both sides of an assignment not being equal.
+fn iter_slice<'a, T>(xs: &'a [T]) -> impl Iterator<Item = &'a T> {
+    xs.iter()
+}
+
+fn main() {
+    iter_slice::<()> as fn(_) -> _;
+}