The file content is based on Rust source code.
build.rs is not helpful since it runs before compiling Rust source code.
I imagine a developer experience like:
// Rust source code
#[some_macro]
struct A;
Then run cargo build as usual, I can get a text file (perhaps in $OUT_DIR):
// The generated text fle
You defined a struct A.
Is it possible to have such macros?
What do you want to do with this text file?
One technique I've used quite often is to create a test which will make sure a generated file, in this case auto-generated types for an AST, is always up to date:
It's a trick I borrowed from how rust-analyzer handles its AST and various other auto-generated file, which in turn uses matklad's self-modifying code trick.
The above example is for generating Rust code, but I've also used it to make sure a schema.grapql file stays in sync with the upstream GraphQL API.