Optimize context field redaction application

src/app_error/context.rs

@@ -89,9 +89,16 @@ impl Context {
 
     /// Attach a metadata [`Field`].
     #[must_use]
-    pub fn with(mut self, field: Field) -> Self {
+    pub fn with(mut self, mut field: Field) -> Self {
+        if let Some((_, policy)) = self
+            .field_policies
+            .iter()
+            .rev()
+            .find(|(name, _)| *name == field.name())
+        {
+            field.set_redaction(*policy);
+        }
         self.fields.push(field);
-        Self::apply_field_redactions(&mut self.fields, &self.field_policies);
         self
     }
 

src/app_error/tests.rs

@@ -295,6 +295,21 @@ fn with_details_text_attaches_payload() {
     assert_eq!(err.details.as_deref(), Some("retry later"));
 }
 
+#[test]
+fn context_with_preserves_default_redaction() {
+    let err = super::Context::new(AppErrorKind::Service)
+        .with(field::str("request_id", "abc-123"))
+        .into_error(DummyError);
+
+    let metadata = err.metadata();
+    assert_eq!(metadata.len(), 1);
+    assert_eq!(
+        metadata.get("request_id"),
+        Some(&FieldValue::Str(Cow::Borrowed("abc-123")))
+    );
+    assert_eq!(metadata.redaction("request_id"), Some(FieldRedaction::None));
+}
+
 #[test]
 fn context_redact_field_overrides_policy() {
     let err = super::Context::new(AppErrorKind::Service)
@@ -325,6 +340,22 @@ fn context_redact_field_mut_applies_policies() {
     assert_eq!(metadata.redaction("token"), Some(FieldRedaction::Hash));
 }
 
+#[test]
+fn context_with_uses_latest_matching_policy() {
+    let err = super::Context::new(AppErrorKind::Service)
+        .redact_field("token", FieldRedaction::Hash)
+        .redact_field("token", FieldRedaction::Redact)
+        .with(field::str("token", "super-secret"))
+        .into_error(DummyError);
+
+    let metadata = err.metadata();
+    assert_eq!(
+        metadata.get("token"),
+        Some(&FieldValue::Str(Cow::Borrowed("super-secret")))
+    );
+    assert_eq!(metadata.redaction("token"), Some(FieldRedaction::Redact));
+}
+
 #[test]
 fn app_error_redact_field_updates_metadata() {
     let err = AppError::internal("boom")