Add parsing + validation support for FieldSet scalars

[SimonSapin]

Apollo Federation has multiple directives that take an argument of type FieldSet to designate one or more fields of an object (or interface?) type. FieldSet is a custom scalar serialized as a string. Its value is expected to be parsed as a SelectionSet with optional outer brackets. (This is GraphQL syntax inside of a string in GraphQL syntax.)

Currently apollo-rs can only parse GraphQL documents, but we’d rather Rust federation not need string concatenation hacks to create a parser input that looks like a document.

API sketch

In apollo-parser:

• Give apollo_parser::SyntaxTree a Cst type parameter
• apollo_parser::Parser::new(string).parse() now returns SyntaxTree<apollo_parser::cst::Document> instead of just SyntaxTree
• Add apollo_parser::Parser::new(string).parse_fieldset() which returns SyntaxTree<cst::SelectionSet>
• The SyntaxTree::document(self) -> cst::Document method now only exists on SyntaxTree<cst::Document>
• Add SyntaxTree::selection_set(self) -> cst::SelectionSet for SyntaxTree<cst::SelectionSet>

In apollo-compiler:

• Add a top-level free function:

  fn parse_fieldset(
      schema: &Schema,
      type_name: impl Into<ast::NamedType>,
      source_text: impl Into<String>,
      path: impl AsRef<Path>,
  ) -> executable::SelectionSet

• Add a Parser method with the same signature

[SimonSapin]

Validation is also needed:

could parse query { __typename } fragment F on SomeObjectType { <insert FieldSet string here> } […]

in the case above with FieldSet, we’d like to skip the operation validation https://spec.graphql.org/draft/#sec-Fragments-Must-Be-Used

I propose that:

• We add a pub struct FieldSet(pub SelectionSet); wrapper type in the executable module,
• parse_fieldset returns it instead of SelectionSet
• It has a validate method with the same signature as ExecutableDocument::validate
  • For compatibility with Salsa-based validation, this method may create a temporary ast::Document that looks like above
  • Eventually when they are migrated to not require Salsa, only call validation functions relevant to selection sets and skip creating a temporary document entirely

[lrlna]

I was thinking to go about slightly differently, which may simplify this design. I was hoping that Parser::new(string).selection_set() would return a SyntaxNode which could then be added to an existing ExecutableDocument that's currently getting modified.

This would allow us to not have a special validation method for just the selection set - the FieldSet would be validated along with the executable document it will inserted into.

[lrlna]

From some internal conversations, it seems like FieldSet validation is explicitly required without it being part of an executable document, which means we'll add a specific path for this validation in the compiler.

[SimonSapin]

Fixed in #685