fix(compiler): identify variables in operation directives

crates/apollo-compiler/src/validation/variable.rs

@@ -177,6 +177,12 @@ pub(crate) fn validate_unused_variables(
             )
         })
         .collect();
+
+    // You're allowed to do `query($var: Int!) @dir(arg: $var) {}`
+    for used in variables_in_directives(&operation.directives) {
+        unused_vars.remove(used);
+    }
+
     let walked = walk_selections(
         document,
         &operation.selection_set,

crates/apollo-compiler/test_data/diagnostics/0009_operation_with_undefined_variables_in_fragment.graphql

@@ -1,11 +1,14 @@
-query ExampleQuery($variable: Int) {
+directive @track(label: String) on FRAGMENT_DEFINITION
+directive @report(label: String) on QUERY | MUTATION | SUBSCRIPTION
+
+query ExampleQuery($variable: Int) @report(label: $queryLabel) {
   topProducts(first: $variable) {
     name
   }
   ... subFrag
 }
 
-fragment subFrag on Query {
+fragment subFrag on Query @track(label: $productsLabel) {
   topProducts(first: $variable) {
     price(setPrice: $value)
   }

crates/apollo-compiler/test_data/diagnostics/0009_operation_with_undefined_variables_in_fragment.txt

@@ -1,7 +1,21 @@
+Error: variable `$queryLabel` is not defined
+   ╭─[0009_operation_with_undefined_variables_in_fragment.graphql:4:51]
+   │
+ 4 │ query ExampleQuery($variable: Int) @report(label: $queryLabel) {
+   │                                                   ─────┬─────  
+   │                                                        ╰─────── not found in this scope
+───╯
+Error: variable `$productsLabel` is not defined
+    ╭─[0009_operation_with_undefined_variables_in_fragment.graphql:11:41]
+    │
+ 11 │ fragment subFrag on Query @track(label: $productsLabel) {
+    │                                         ───────┬──────  
+    │                                                ╰──────── not found in this scope
+────╯
 Error: variable `$value` is not defined
-    ╭─[0009_operation_with_undefined_variables_in_fragment.graphql:10:21]
+    ╭─[0009_operation_with_undefined_variables_in_fragment.graphql:13:21]
     │
- 10 │     price(setPrice: $value)
+ 13 │     price(setPrice: $value)
     │                     ───┬──  
     │                        ╰──── not found in this scope
 ────╯

crates/apollo-compiler/test_data/ok/0042_used_variable_in_operation_directive.graphql

@@ -0,0 +1,12 @@
+directive @x(y: Int!) on QUERY | MUTATION | SUBSCRIPTION
+directive @z(f: Boolean) on FRAGMENT_DEFINITION
+
+type Query { field: Boolean }
+type Mutation { field: Boolean }
+type Subscription { field: Boolean }
+
+query Q ($a: Int!, $f: Boolean) @x(y: $a) { field ...f }
+mutation M ($b: Int!) @x(y: $b) { field }
+subscription S ($c: Int!) @x(y: $c) { field }
+
+fragment f on Query @z(f: $f) { field }