Rust: Use an associated type for TestGetValue to make it clear it's an output parameter

Generic parameters are an "input".
A trait `TestGetValue<T>` can be implemented for the same type multiple
times with different concrete types `T`.
That's never needed for `TestGetValue`, we only have a single type that
should be returned from `test_get_value`.
An associated parameter is the way to do that.

Author: badboy

CHANGELOG.md

@@ -19,6 +19,8 @@
 
 [Full changelog](https://github.com/mozilla/glean/compare/v65.1.1...v65.2.0)
 
+* Rust
+  *  Rust: Use an associated type for `TestGetValue` ([#3259](https://github.com/mozilla/glean/pull/3259))
 * Swift
   * Glean for iOS is now being built with Xcode 16.2 ([#3189](https://github.com/mozilla/glean/pull/3189))
 * General

glean-core/rlb/src/private/event.rs

@@ -72,7 +72,9 @@ impl<K: traits::ExtraKeys> EventMetric<K> {
 }
 
 #[inherent]
-impl<K> TestGetValue<Vec<RecordedEvent>> for EventMetric<K> {
+impl<K> TestGetValue for EventMetric<K> {
+    type Output = Vec<RecordedEvent>;
+
     pub fn test_get_value(&self, ping_name: Option<String>) -> Option<Vec<RecordedEvent>> {
         self.inner.test_get_value(ping_name)
     }

glean-core/rlb/src/private/object.rs

@@ -39,7 +39,9 @@ impl<'a, K> MetricIdentifier<'a> for ObjectMetric<K> {
     }
 }
 
-impl<K> TestGetValue<JsonValue> for ObjectMetric<K> {
+impl<K> TestGetValue for ObjectMetric<K> {
+    type Output = JsonValue;
+
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Gets the currently stored value as JSON-encoded string.

glean-core/src/metrics/boolean.rs

@@ -120,7 +120,8 @@ impl BooleanMetric {
     }
 }
 
-impl TestGetValue<bool> for BooleanMetric {
+impl TestGetValue for BooleanMetric {
+    type Output = bool;
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Gets the currently stored value as a boolean.

glean-core/src/metrics/counter.rs

@@ -157,7 +157,9 @@ impl CounterMetric {
     }
 }
 
-impl TestGetValue<i32> for CounterMetric {
+impl TestGetValue for CounterMetric {
+    type Output = i32;
+
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Gets the currently stored value as an integer.

glean-core/src/metrics/custom_distribution.rs

@@ -303,7 +303,9 @@ impl CustomDistributionMetric {
     }
 }
 
-impl TestGetValue<DistributionData> for CustomDistributionMetric {
+impl TestGetValue for CustomDistributionMetric {
+    type Output = DistributionData;
+
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Gets the currently stored value as an integer.

glean-core/src/metrics/datetime.rs

@@ -277,7 +277,9 @@ impl DatetimeMetric {
     }
 }
 
-impl TestGetValue<Datetime> for DatetimeMetric {
+impl TestGetValue for DatetimeMetric {
+    type Output = Datetime;
+
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Gets the stored datetime value.

glean-core/src/metrics/denominator.rs

@@ -127,7 +127,9 @@ impl DenominatorMetric {
     }
 }
 
-impl TestGetValue<i32> for DenominatorMetric {
+impl TestGetValue for DenominatorMetric {
+    type Output = i32;
+
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Gets the currently stored value as an integer.

glean-core/src/metrics/dual_labeled_counter.rs

@@ -231,7 +231,9 @@ impl DualLabeledCounterMetric {
     }
 }
 
-impl TestGetValue<HashMap<String, HashMap<String, i32>>> for DualLabeledCounterMetric {
+impl TestGetValue for DualLabeledCounterMetric {
+    type Output = HashMap<String, HashMap<String, i32>>;
+
     fn test_get_value(
         &self,
         ping_name: Option<String>,

glean-core/src/metrics/event.rs

@@ -212,7 +212,9 @@ impl EventMetric {
     }
 }
 
-impl TestGetValue<Vec<RecordedEvent>> for EventMetric {
+impl TestGetValue for EventMetric {
+    type Output = Vec<RecordedEvent>;
+
     /// **Test-only API (exported for FFI purposes).**
     ///
     /// Get the vector of currently stored events for this event metric.