Adjust as-precedence.rs

- Document `as-precedence.rs`
- Move `as-precedence.rs` under `tests/ui/parser/`

Author: jieyouxu

Diff for: tests/ui/as-precedence.rs

@@ -0,0 +1,18 @@
+//! Parser precedence test to help with [RFC 87 "Trait Bounds with Plus"][rfc-87], to check the
+//! precedence of the `as` operator in relation to some arithmetic bin-ops and parentheses.
+//!
+//! Editor's note: this test seems quite incomplete compared to what's possible nowadays. Maybe
+//! there's another set of tests whose coverage overshadows this test?
+//!
+//! [rfc-87]: https://rust-lang.github.io/rfcs/0087-trait-bounds-with-plus.html
+
+//@ run-pass
+
+#[allow(unused_parens)]
+fn main() {
+    assert_eq!(3 as usize * 3, 9);
+    assert_eq!(3 as (usize) * 3, 9);
+    assert_eq!(3 as (usize) / 3, 1);
+    assert_eq!(3 as usize + 3, 6);
+    assert_eq!(3 as (usize) + 3, 6);
+}