AST parent pointer

[SebastienGllmt]

Problem

Right now, there is no good way to go up the tree given a node in the AST. This kind of operation can be done outside the library, but it's hacky so I'd prefer to just add it as an optional feature here.

Proposed solution

Add a ast-parent flag that when set, building the AST will do an additional pass after the AST is completed to fill in the parent pointers.

Related work

Note that unfortunately this change will have a very large conflict with #96. Unfortunately that PR is still a draft so there was no good way to build my work on top of that PR.

Issues

Note that the visitor pattern is for some reason missing a few cases:

visit_cddl
visit_generic_args
visit_generic_params
visit_group_entry
visit_operator
visit_rule

TODO:

• add missing visitor calls
• Finish coding parent visitor
• Run parent visitor to fill the AST if the compiler flag is set
• Write tests
• Check for build errors on different compiler flags (ex: if ast-span is off)

[anweiss]

Thanks @SebastienGllmt. You can disregard #96 for now in the context of this PR. It's been a while since I looked at that. I'd rather try and do something like what is being proposed in #125.

[SebastienGllmt]

Marking this ready to review since I won't have bandwidth to personally work on this for the next while as I mentioned (but happy to review)

The two features we wanted for our use case was:

1. Knowing whether or not a node is part of an embedded structure (ex: if `foo .bytes bar is a top-level node, or if it's a field inside a group). We wanted this because sometimes the code we have to generate is different if a type is embedded inside another or not
2. Finding the rule name a node belongs to. We wanted this to programmatically generate names (ex: any node embedded inside a large structure can be represented by a Rust struct whose name is prefixed by the rule name)

[anweiss]

Thanks a ton for this @SebastienGllmt! This is currently being worked on in https://github.com/anweiss/cddl/tree/dcSpark-ast-parent-pointer, which is based on this PR.

[anweiss]

Closing in favor of #147