Explain the existience of the regression test

src/test/ui/consts/const_arg_promotable2.rs

@@ -1,3 +1,8 @@
+// This test is a regression test for a bug where we only checked function calls in no-const
+// functions for `rustc_args_required_const` arguments. This meant that even though `bar` needs its
+// argument to be const, inside a const fn (callable at runtime), the value for it may come from a
+// non-constant (namely an argument to the const fn).
+
 #![feature(rustc_attrs)]
 const fn foo(a: i32) {
     bar(a); //~ ERROR argument 1 is required to be a constant
@@ -6,4 +11,8 @@ const fn foo(a: i32) {
 #[rustc_args_required_const(0)]
 const fn bar(_: i32) {}
 
-fn main() {}
+fn main() {
+    // this function call will pass a runtime-value (number of program arguments) to `foo`, which
+    // will in turn forward it to `bar`, which expects a compile-time argument
+    foo(std::env::args().count() as i32);
+}

src/test/ui/consts/const_arg_promotable2.stderr

@@ -1,5 +1,5 @@
 error: argument 1 is required to be a constant
-  --> $DIR/const_arg_promotable2.rs:3:5
+  --> $DIR/const_arg_promotable2.rs:8:5
    |
 LL |     bar(a);
    |     ^^^^^^