Adjust assoc-oddities-3.rs

- Include the original MCVE as reported in
  <rust-lang#28777>.
- Document the intention of the test.

tests/ui/parser/assoc/assoc-oddities-3.rs

@@ -1,3 +1,10 @@
+//! Check that braces has the expected precedence in relation to index op and some arithmetic
+//! bin-ops involving nested braces.
+//!
+//! This is a regression test for [Wrapping expr in curly braces changes the operator precedence
+//! #28777](https://github.com/rust-lang/rust/issues/28777), which was fixed by
+//! <https://github.com/rust-lang/rust/pull/30375>.
+
 //@ run-pass
 
 fn that_odd_parse(c: bool, n: usize) -> u32 {
@@ -7,7 +14,28 @@ fn that_odd_parse(c: bool, n: usize) -> u32 {
     x + if c { a } else { b }[n]
 }
 
+/// See [Wrapping expr in curly braces changes the operator precedence
+/// #28777](https://github.com/rust-lang/rust/issues/28777). This was fixed by
+/// <https://github.com/rust-lang/rust/pull/30375>. #30375 added the `that_odd_parse` example above,
+/// but that is not *quite* the same original example as reported in #28777, so we also include the
+/// original example here.
+fn check_issue_28777() {
+    // Before #30375 fixed the precedence...
+
+    // ... `v1` evaluated to 9, indicating a parse of `(1 + 2) * 3`, while
+    let v1 = { 1 + { 2 } * { 3 } };
+
+    // `v2` evaluated to 7, indicating a parse of `1 + (2 * 3)`.
+    let v2 = 1 + { 2 } * { 3 };
+
+    // Check that both now evaluate to 7, as was fixed by #30375.
+    assert_eq!(v1, 7);
+    assert_eq!(v2, 7);
+}
+
 fn main() {
     assert_eq!(4, that_odd_parse(true, 1));
     assert_eq!(8, that_odd_parse(false, 1));
+
+    check_issue_28777();
 }