Don't allow using const fn arguments as "args_required_const"

src/librustc_mir/transform/qualify_consts.rs

@@ -1304,7 +1304,9 @@ impl<'a, 'tcx> Visitor<'tcx> for Checker<'a, 'tcx> {
                 }
             }
 
-            if self.mode == Mode::Fn {
+            // No need to do anything in constants and statics, as everything is "constant" anyway
+            // so promotion would be useless.
+            if self.mode != Mode::Static && self.mode != Mode::Const {
                 let constant_args = callee_def_id.and_then(|id| {
                     args_required_const(self.tcx, id)
                 }).unwrap_or_default();

src/test/ui/consts/const_arg_promotable2.rs

@@ -0,0 +1,18 @@
+// 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
+}
+
+#[rustc_args_required_const(0)]
+const fn bar(_: i32) {}
+
+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

@@ -0,0 +1,8 @@
+error: argument 1 is required to be a constant
+  --> $DIR/const_arg_promotable2.rs:8:5
+   |
+LL |     bar(a);
+   |     ^^^^^^
+
+error: aborting due to previous error
+