Set up a place for body validations & add the first new check

[dylan-apollo]

A bunch of refactoring to set us up for body validations—and adds a single check, if you use empty $args with no path but have no arguments defined on the field.

This doesn't use the visitor stuff because we're interested in the opposite info—where's the data coming from, not where is it going to. Left a bunch of TODOs where we should add more validations.

[svc-apollo-docs]

✅ Docs Preview Ready

No new or changed pages found.

[benjamn]

I think the JSONSelection::external_var_paths method (provided by pub(crate) trait ExternalVarPaths) will help this validation code be a little less coupled to the internal PathList structure, and also catch more $args paths that are currently skipped.

[benjamn]

Probably need to process trailing in more than just the $args case, since other $args paths could appear elsewhere along the path, within -> method arguments for example.

[benjamn]

We're now recommending/enforcing the $.data syntax for single-key paths, so $ can definitely be followed by a .key as well as a -> method.

[benjamn]

I wish the PathList enum could remain pub(super), because its internal structure needs to change fairly often, and adding more pattern matching against those internal details makes changing them harder.

Have you considered using the ExternalVarPaths trait?

router/apollo-federation/src/sources/connect/json_selection/parser.rs

Lines 30 to 32 in 619b794

	pub(crate) trait ExternalVarPaths {
	fn external_var_paths(&self) -> Vec<&PathSelection>;
	}

This should give you all the variable paths referring to external inputs (like $this, $args, and $config), fully flattened, so you can easily loop over them and validate the $args paths.

[dylan-apollo]

We're working on a more general interface for iterating over mapping inputs/outputs. We'll use that for validating bodies instead of the AST directly