Using variables in scalars

I would like some clarification on using variables in scalars.

Given the following schema:

```graphql
scalar Custom

type Query {
hello(custom: Custom!): String!
}
```

We know that the following query _could_ be valid (depending on `Custom`'s wire format):

```graphql
query Example {
hello(custom: { name: "Tim" })
}
```

Would the following example also be valid?

```graphql
query Example($name: String!) {
hello(custom: { name: $name })
}
```

If so, does the executor first resolve `$name` and then call the scalar's deserializer with `{ name: "Tim" }`?

Can someone shed some light on this?