Metaprogramming in Rust has two forms—macros and procedural macros. Both of these utilities accept an abstract syntax tree as new input and output symbols to be compiled. Procedural macros are very similar to normal macros but with fewer restrictions on how they work and how they are defined.
Macros defined with the macro_rules! syntax are defined recursively by matching the input syntax to produce output. It is crucial to understand that macro matching happens after parsing. This means the following:
- Macros must follow certain rules when creating new syntax forms
- The AST is decorated with information regarding each node's grammar category
Macros can match individual tokens, or a macro can match (and capture) an entire grammar category. The Rust grammar categories are as follows:
- tt: This is a token tree (which is a token output from the lexer before...