If it were to do complete test for multi-char glyphs it would get way out of the scope of a 7kyu, specially once you consider variant selectors, combining diacritical marks, RTL text and regional indicators. Properly reversing a string is not an easy task.

Return null/nil/None/... if the input is an even number or negative, as it is not possible to print a diamond of even or negative size.

Challenge accepted!
(even-sized diamonds are totally possible)

Thank you

The tests don't check for multi-char glyphs. This means that naive implementations (most, if not all submitted, including mine) would corrupt the strings with multi-char glyphs in them by reversing order of chars in them. Either add test cases for those or add the assumption that input strings are ASCII in instructions.

If there's multiple spaces, your .s method might remove some of them. Spaces should be preserved.

Do we need to reverse graphemes in the words or just the chars are fine?

Grapheme-based reversing would need an external crate to do properly in Rust. Is there a way to plug in dependencies here?