public final String peekSingleCharForMessage() {
        char peek = peek();
        return peek == EOF ? "[EOF]" : String.valueOf(peek);
    }
Peeks the next character and returns [EOF] if the next character is past
the end of the expression.

@return Returns the peeked next character.
smithy-utils/src/main/java/software/amazon/smithy/utils/SimpleParser.java