Make it easier to define high-level ASTs?

[oxalica]

Currently we provides traits and auxiliary functions in rowan::ast, but it's still difficult to use.

• In the example, we define ASTs using a custom macro.
  

  rowan/examples/s_expressions.rs

  Lines 268 to 270 in 6a94212

  	ast_node!(Root, ROOT);
  	ast_node!(Atom, ATOM);
  	ast_node!(List, LIST);

• In rust-analyzer, we even use code generation to produce a 4.8k lines file crates/syntax/src/ast/generated/nodes.rs.

Could we make it easier to define ASTs and child extractors?
Maybe using some DSL with proc-macros or build.rs for generation?

[azdavis]

I use a mix of

• rust-analyzer/ungrammar
  
• my own language-util crates, specifically syntax-gen and token

Example usage site in my project rjsonnet is here

Note that my language-util crates are not on crates.io at this time. I don't really want to go to the effort of publishing them, and they may have breaking changes or disappear without notice at any time. So if really want to use them, probably pin to a specific git commit or fork them to yourself.