Add `Ast` as top level enum

[youknowone]

One of the possible solution of #48

[youknowone]

cc @MichaReiser

[MichaReiser]

Thanks for working on this and sorry for my late reply. I've been busy with other work and haven't had time to think more about the problem.

I still think that I would favor the flat structure over the nested one because:

• The nested structure requires two marker bits to distinguish the variants. One at the top level, and another for the sub-union. That means, the enum is three bytes larger than a flat structure.
• In my view, the most common access patterns are that we don't care about what the node is OR that we want to test if it is a few very specific nodes. The nested structure requires a nested match except for, in my view, the rare case where you want to match on all statements.
• I think we can accommodate the "matching on all statements" by introducing the helpers is_stmt(), as_stmt() and stmt() on the flat structure.

Would you mind if I change the enum to a flat structure? I will also introduce a AstRef as part of that work because that's actually the enum that I need first.

[youknowone]

Because current Ast enum is the counter part of python side ast.AST(each rust enum matches to inheritance from Python), adding a new enum for flat structure seems better if you need one. Before adding the Ast type, the top level ast.AST was the only missing type from Rust side. On the other hand, adding is_stmt_if() to current Ast seems also reasonable if it fits your requirements. (probably not if you need to use match)

[youknowone]

I wish conversion between Ast and other types like Stmt and Expr doesn't require much cost. If we can achieve it with flat structure, that's also fine.

[MichaReiser]

Because current Ast enum is the counter part of python side ast.AST(each rust enum matches to inheritance from Python), adding a new enum for flat structure seems better if you need one. Before adding the Ast type, the top level ast.AST was the only missing type from Rust side. On the other hand, adding is_stmt_if() to current Ast seems also reasonable if it fits your requirements. (probably not if you need to use match)

I'll add it on the Ruff side if you want to keep using the Ast node for CPython compatibility

[youknowone]

Yes, that will be a good option.