Are there any possible API like field_count() or is it only possible to get that via macros? There is no such built-in API that would allow you to get this information at runtime. Rust does not have runtime reflection (see this question for more information). But it is indeed possible via proc-macros! Note: proc-macros are … Read more
Procedural macros are fundamentally different from normal dependencies in your code. A normal library is just linked into your code, but a procedural macro is actually a compiler plugin. Consider the case of cross-compiling: you are working on a Linux machine, but building a WASM project. A normal crate will be cross-compiled, generate WASM code … Read more
Apart from panicking, there are currently two ways to reports errors from a proc-macro: the unstable Diagnostic API and “the compile_error! trick”. Currently, the latter is mostly used because it works on stable. Let’s see how they both work. The compile_error! trick Since Rust 1.20, the compile_error! macro exists in the standard library. It takes … Read more
That’s a notion introduced to ensure that whatever is in a macro invocation correctly matches (), [] and {} pairs. tt will match any single token or any pair of parenthesis/brackets/braces with their content. For example, for the following program: fn main() { println!(“Hello world!”); } The token trees would be: fn main () ∅ … Read more
It means to import (“use”) the macros from the crate. As of Rust 1.30, this syntax is no longer generally needed and you can use the standard use keyword instead. Review the macros chapter from the first edition of The Rust Programming Language for more detail.
Create a crate for your procedural macros: cargo new my_derive –lib Edit the Cargo.toml to make it a procedural macro crate: [lib] proc-macro = true Implement your procedural macro: extern crate proc_macro; use proc_macro::TokenStream; #[proc_macro_derive(MyMacroHere)] pub fn my_macro_here_derive(input: TokenStream) -> TokenStream { // … } Import the procedural macro and use it: extern crate my_derive; … Read more
cargo rustc –profile=check — -Zunpretty=expanded, but a more concise alternative is the cargo-expand crate. It provides a Cargo subcommand cargo expand which prints the result of macro expansion. It also passes the expanded code through rustfmt which generally results in much more readable code than the default output from rustc. Install by running cargo install … Read more
Macros within the same crate New method (since Rust 1.32, 2019-01-17) foo::bar!(); // works mod foo { macro_rules! bar { () => () } pub(crate) use bar; // <– the trick } foo::bar!(); // works With the pub use, the macro can be used and imported like any other item. And unlike the older method, … Read more