Action code and return types

Action code

Action code is normal Rust code with the addition of the following special variables:

$1 ... $n refer to the respective symbol in the production, numbered from 1 (i.e. $1 refers to the first symbol in the production). If the symbol references a rule R then an instance of R's type will be stored in the $i variable; if the symbol references a lexeme then an Option<Lexeme> instance is returned.
$lexer allows access to the lexer and its various functions. The most commonly used of these is the span_str function, which allows us to extract &'input strs from a Span (e.g. to extract the string represented by a Lexeme, we would use $lexer.span_str(lexeme.span())). As this may suggest, actions may also reference the special lifetime 'input (without any $ prefix), which allows strings to be returned / stored by the grammar without copying memory.
$span is a lrpar::Span tuple (with both elements of type usize) which captures how much of the user's input the current production matched.
$$ is equivalent to $ in normal Rust code.

Any other variables beginning with $ are treated as errors.

Productions' return types can be any arbitrary Rust type. You may in addition make use of the following:

The generic parameter StorageT references the type of lexemes and is typically used with the Lexeme type i.e. Lexeme<StorageT>. This allows you to return lexemes from rules.
The lifetime 'input allows you to extract strings whose lifetime is tied to the lexer and return them from rules / store them in structs without copying. Lexer::span_str returns such strings and the typical idiom of use is &'input str.