Macro keyword 

macro_rules! keyword {
    ($name:ident = $str:literal; ...) => { ... };
    ($name:ident = $group:path; ...) => { ... };
    ($name:ident = [$($keywords:tt),+]; ...) => { ... };
    ($name:ident != $str:literal; ...) => { ... };
    ($name:ident != $group:path; ...) => { ... };
    ($name:ident != [$($keywords:tt),+]; ...) => { ... };
}

Define types matching keywords.

• Any number of attributes (#[...]), including documentation comments. Keyword documentation is automatically extended by a small auto generated doc comment listing what a keyword definition will match.
• A optional pub declaration.
• Name is the name for the struct to be generated.
• "identifier" is the case sensitive keyword.
• group can be a non empty bracketed list of "identifier" or any an other keyword definition.
• By using = the keyword must match the given definition while != negates the output and matches any identifier that is not in the definition.

Name::parse() will then only match the defined identifier. It will implement Debug and Clone for keywords. Additionally AsRef<str> is implemented for each Keyword to access the identifier string from rust code.

The unsynn! macro supports defining keywords by using keyword Name = "ident";, the pub specification has to come before keyword then. See the unsynn! keyword documentation for details.

In case a invalid keyword is defined (not an identifier) the compilation will panic. But because the actual matching function is optimized and lazy evaluated this will only happen on the first use of the invalid keyword definition.

Keywords implement AsRef<str>, AsRef<Ident> and Keyword::as_str(&self) -> &str. For Keywords that are defined with a single literal string (keyword!{ Foo = "foo"}) the Default trait is implemented. Thus they can be created and inserted statically.

Example

keyword!{
    /// Optional documentation for `If`
    pub If = "if";
    pub Else = "else";
    // keywords can be grouped from existing keywords
    IfElse = [If, Else,];
    // or contain identifiers in double quotes
    IfElseThen = [IfElse, "then"];
    // matching can be negated with `!=`
    NotIfElseThen != [IfElse, "then"];
}

let mut tokens = "if".to_token_iter();
let if_kw = If::parse(&mut tokens).unwrap();
assert_eq!(if_kw.as_str(), "if");