tests: add whitespace tests for vertical tab behavior

Add two small tests to highlight how vertical tab is handled differently.

- vertical_tab_lexer.rs checks that the lexer treats vertical tab as whitespace
- ascii_whitespace_excludes_vertical_tab.rs shows that split_ascii_whitespace does not split on it

This helps document the difference between the Rust parser (which accepts vertical tab)
and the standard library’s ASCII whitespace handling.

See: rust-lang/rust-project-goals#53

Author: Brace1000

tests/ui/whitespace/ascii_whitespace_excludes_vertical_tab.rs

@@ -0,0 +1,23 @@
+// This test checks that split_ascii_whitespace does not split on a
+// vertical tab (\x0B). The standard library follows the WhatWG ASCII
+// whitespace definition, which does not include vertical tab.
+//
+// So in this case, "a\x0Bb" should stay as a single piece.
+//
+// See: https://github.com/rust-lang/rust-project-goals/issues/53
+
+fn main() {
+    let s = "a\x0Bb";
+
+    let parts: Vec<&str> = s.split_ascii_whitespace().collect();
+
+    assert_eq!(parts.len(), 1,
+        "vertical tab should not be treated as ASCII whitespace");
+
+    let s2 = "a b";
+    let parts2: Vec<&str> = s2.split_ascii_whitespace().collect();
+    assert_eq!(parts2.len(), 2,
+        "regular space should split correctly");
+
+    println!("All assertions passed.");
+}

tests/ui/whitespace/vertical_tab_lexer.rs

@@ -0,0 +1,12 @@
+// This test checks that the Rust lexer treats a vertical tab (\x0B)
+// as whitespace. That matches how the language parses code, even though
+// the standard library's is_ascii_whitespace does not include it.
+//
+// See: https://github.com/rust-lang/rust-project-goals/issues/53
+
+fn main() {
+    let x = 5;
+    let y = 10;
+    let z = x + y;
+    println!("{}", z);
+}