Convert impl-trait/issue-42479.rs to compile-pass

src/test/run-pass/impl-trait/issue-42479.stderr

@@ -1,14 +0,0 @@
-warning: struct is never constructed: `Foo`
-  --> $DIR/issue-42479.rs:15:1
-   |
-LL | struct Foo {
-   | ^^^^^^^^^^
-   |
-   = note: #[warn(dead_code)] on by default
-
-warning: method is never used: `inside`
-  --> $DIR/issue-42479.rs:20:5
-   |
-LL |     fn inside(&self) -> impl Iterator<Item = &i32> {
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-

src/test/ui/impl-trait/issue-42479.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// run-pass
+// compile-pass
 
 use std::iter::once;