Add a UI test for correct parsing

Author: achan1989

src/test/ui/parser/stmt_expr_attrs_placement.rs

@@ -0,0 +1,22 @@
+#![feature(stmt_expr_attributes)]
+
+// Test that various placements of the inner attribute are parsed correctly,
+// or not.
+
+fn main() {
+    let a = #![allow(warnings)] (1, 2);
+    //~^ ERROR an inner attribute is not permitted in this context
+
+    let b = (#![allow(warnings)] 1, 2);
+
+    let c = {
+        #![allow(warnings)]
+        (#![allow(warnings)] 1, 2)
+    };
+
+    let d = {
+        #![allow(warnings)]
+        let e = (#![allow(warnings)] 1, 2);
+        e
+    };
+}

src/test/ui/parser/stmt_expr_attrs_placement.stderr

@@ -0,0 +1,10 @@
+error: an inner attribute is not permitted in this context
+  --> $DIR/stmt_expr_attrs_placement.rs:7:13
+   |
+LL |     let a = #![allow(warnings)] (1, 2);
+   |             ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files. Outer attributes, like `#[test]`, annotate the item following them.
+
+error: aborting due to previous error
+