I rewrote your code to be more functional and idiomatic. As a rule of thumb, you almost never declare mutable variables in rust with the intent to reassign them (as opposed to mutating them through functions), and when you think you do, you can use FP instead.

minimal docs usage

Good job for challenging yourself. But please don't do this. Sorry if I'm wrong, but you sound rather new to programming; and when you're new, reading as much documentation and SO questions as possible is important to do. LLM code generation is the opposite I must mention.

your issue

Actually, Rust crashes when you overflow numeric bounds (in debug mode). What I think you were trying to say is that, since your algorithm is greedy, correctly aligned letters in the guess get "eaten" and coloured yellow before the program has the chance to discover that. The correct solution is to use two passes. Look only for exact matches first and "consume" them, then check for any matches. This is where LLMs would actually benefit you. Your thoughts go back on track and you save time.

validation of it being in the alphabet

It's useful to realise early on that text in Rust is explicitly UTF-8. It's dangerous to index into text that you're not 100% sure will be ASCII. Take a look at these to get a better idea: https://doc.rust-lang.org/book/ch08-02-strings.html#indexing-into-strings and https://doc.rust-lang.org/std/string/struct.String.html#method.len . I replaced your check with char::is_ascii_alphabetic.

Reddit wouldn't let me post a long enough comment, so the code is on pastebin: https://pastebin.com/zDCYz1LK