[feature] support quantity/order rule

[HerringtonDarkholme]

❓Problem Now

It is hard to find nodes without children or nodes appearing in specific positions.
For example, user may want to match the first/last argument of an argument list, or to find a node with more than three children (e.g. too many arguments or too many statements).

Though it is possible to use precedes/follows and recursive matches to emulate node position rules, the procedure is definitely excruciating. (I can testify this as the author).

⭐ Suggestion

Support one additional rule:

rule:
  nthChild: 3       
  nthChild: 2n+1
  nthChild:
    position: 3 # or An+B
    ofRule:    # rule object
      pattern: pattern-rule 

💻 Use Cases

1. Find the first/last argument
2. Find functions with too many statements
3. Find functions with too many arguments
4. Find an argument at specific position of an argument list

Related issue #675

[alexzhang1030]

I want to try to implement this feature, but I'm unsure if it will be part of the composite rule or maybe create a new type like the quantity rule. 👀

[HerringtonDarkholme]

Hi @alexzhang1030, thanks for your interest in ast-grep.

I am not 100% sure of the quantity rule's behavior at the moment. If you wanna grab this issue. Would you like to first write a RFC in this issue? We can discuss the behavior of the rule.

Once the behavior is well-defined, we can discuss about how to implement it.

Thanks