Add convenience methods {Field,Directive}Definition::argument_by_name (#801)

* Add convenience methods `{Field,Directive}Definition::argument_by_name`

* chglg

Author: goto-bus-stop

crates/apollo-compiler/CHANGELOG.md

@@ -21,11 +21,13 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## BREAKING
 - **`InputValueDefinition::is_required()` returns false if it has a default value - [goto-bus-stop], [pull/798]**
-  Now `argument.is_required() == true` only if the type is non-null and there is no
-  default value, meaning a value must be provided when it's used.
+  - Now `argument.is_required() == true` only if the type is non-null and there is no
+    default value, meaning a value must be provided when it's used.
 
 ## Features
 - **Implement `fmt::Display` for `ComponentName` - [goto-bus-stop], [pull/795]**
+- **Add `FieldDefinition::argument_by_name` and `DirectiveDefinition::argument_by_name` - [goto-bus-stop], [pull/FIXME]**
+  - These methods return an argument definition by name, or `None`.
 
 [goto-bus-stop]: https://github.com/goto-bus-stop]
 [pull/795]: https://github.com/apollographql/apollo-rs/pull/795

crates/apollo-compiler/src/ast/impls.rs

@@ -459,6 +459,11 @@ impl FragmentDefinition {
 }
 
 impl DirectiveDefinition {
+    /// Returns the definition of an argument by a given name.
+    pub fn argument_by_name(&self, name: &str) -> Option<&Node<InputValueDefinition>> {
+        self.arguments.iter().find(|argument| argument.name == name)
+    }
+
     serialize_method!();
 }
 
@@ -624,6 +629,7 @@ impl FromIterator<Directive> for DirectiveList {
 }
 
 impl Directive {
+    /// Returns the value provided to the named argument.
     pub fn argument_by_name(&self, name: &str) -> Option<&Node<Value>> {
         self.arguments
             .iter()
@@ -836,6 +842,11 @@ impl Type {
 }
 
 impl FieldDefinition {
+    /// Returns the definition of an argument by a given name.
+    pub fn argument_by_name(&self, name: &str) -> Option<&Node<InputValueDefinition>> {
+        self.arguments.iter().find(|argument| argument.name == name)
+    }
+
     serialize_method!();
 }