The Procedural Macro Ecosystem: proc-macro2, syn, and quote

Procedural macros function by manipulating the proc_macro::TokenStream type. However, the standard proc_macro crate is a compiler built-in and is restricted to crates explicitly defined as procedural macro libraries. To bypass these limitations and streamline development, the Rust ecosystem relies on three foundational crates:

• proc-macro2: This crate offers an API almost identical to proc_macro but can be used in any environment (including tests and binaries). It acts as a bridge, facilitating seamless conversion to and from the compiler-native proc_macro::TokenStream.
• syn: A parsing library that provides a complete syntax tree for any valid Rust source code. It also makes defining custom syntax trees easy.
• quote: As the counterpart to syn, this crate enables you to convert Rust code templates back into a proc_macro2::TokenStream. Its support for “quasi-quoting” allows you to interpolate variables directly into the generated source code.