Type prepare results with the run query's variables struct

[saga-dasgupta]

Why

A prepare target returns the variables for its run target's input query as untyped JSON. Nothing checks that they match what the run query declares. A renamed, added, or retyped variable deploys fine, and then the run query resolves nothing at checkout.

What

Depends on two bluejay PRs:

• Generate a struct for each operation's variables bluejay#149 generates a struct for the variables of each operation that declares them: <Operation>Variables, or RootVariables for an anonymous operation. A non-null variable with no default is a plain field; any other is an Option whose None omits the key, so the query's default applies.
• Override the types of input object custom scalar fields bluejay#148 adds custom_scalar_overrides to typegen, to type a custom scalar field of an input object as another type.

The first is based on the second. Until both ship in a release, the workspace Cargo.toml patches the bluejay crates in from Shopify/bluejay#149's branch.

typegen implements wasm_api's Serialize and Deserialize for the variables struct through bluejay's new CodeGenerator hooks. The struct reuses the code input objects already use, moved into a shared object_impls, so it serializes directly.

Overriding the prepare result's variables field then types it as the run query's variables struct instead of JsonValue:

#[typegen("schema.graphql", custom_scalar_overrides = {
    "CartValidationsGeneratePrepareResult.variables" => cart_validations_generate_run::InputVariables,
})]
pub mod schema {
    // query Input($ruleSetHandle: String! = "rules-default") { ... }
    #[query("src/cart_validations_generate_run.graphql")]
    pub mod cart_validations_generate_run {}
}

// `CartValidationsGeneratePrepareResult.variables` is now
// `Option<cart_validations_generate_run::InputVariables>`.
Ok(schema::CartValidationsGeneratePrepareResult {
    variables: Some(schema::cart_validations_generate_run::InputVariables {
        rule_set_handle: Some(rule_set_handle),
    }),
})

A prepare target that no longer matches its run query fails to compile:

Change to the run query	Error in the prepare target
Variable renamed or removed	E0560, unknown field
Variable added	E0063, missing field
Variable retyped	E0308, mismatched types
Prepare returns a hand-written JsonValue	E0308, mismatched types

Breaking change

• A variable whose type contains a custom scalar that the schema module aliases to its own type now needs that type to implement wasm_api's Serialize and Deserialize, and Debug, PartialEq and Clone. This applies to any #[query] module, not only run queries. Input objects already need these. The built-in mappings (String, JsonValue, Decimal) all have them.

Limits

• String, ID, and the custom scalars mapped to String (such as Handle) are the same Rust type, so switching between them doesn't fail to compile.
• bluejay reports an error when a variables struct's name clashes with a fragment's or operation's struct. A variable named $self, $Self, $super, $crate or $_ panics the macro, the same way fields with those names already do. Both are covered in Generate a struct for each operation's variables bluejay#149.