Function is_single_expression 

pub fn is_single_expression(out: &ParseOutput) -> bool

Whether the parsed text is exactly one complete R expression, the way R’s own parser judges it — the predicate behind rlang::parse_expr() (and so roxygen2’s can_parse()).

This takes the whole ParseOutput rather than just the CST because a recovered tree alone cannot answer the question: f( yields a well-shaped CALL_EXPR and reports the missing ) only as a diagnostic.

Three things have to hold:

• the parse is diagnostic-free;
• the root holds one expression, optionally followed by a single ;. R’s ; is a statement terminator, not a separator that may stand alone: it must follow an expression on the same line, and a second one opens an empty statement, which is a syntax error (x; parses, ;x, x;;, x\n; and x; y do not);
• no name in the tree is one R’s lexer rejects but arity’s accepts (see has_r_invalid_name).