Add comments to explain the test case and the special treatment

src/test/ui/statics/static-promotion.rs

@@ -1,4 +1,12 @@
-// run-pass
+// check-pass
+
+// Use of global static variables in literal values should be allowed for
+// promotion.
+// This test is to demonstrate the issue raised in
+// https://github.com/rust-lang/rust/issues/70584
+
+// Literal values were previously promoted into local static values when
+// other global static variables are used.
 
 struct A<T: 'static>(&'static T);
 struct B<T: 'static + ?Sized> {