mozilla · freshstrangemusic · Aug 17, 2026
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # v156.0 (In progress)
 
+## ✨ What's Changed ✨
+
+### Nimbus
+
+- The `pref-key` annotation for FML feature variables has been removed. ([#TODO](TODO))
+
 [Full Changelog](In progress)
 
 # v155.0 (_2026-08-13_)
@@ -36,7 +42,7 @@ In the cases where the failed before, they're now no-ops.
 
 ### Remote Settings
 - Replacing v1 routes with v2 routes, removing added v2 routes ([#7492](https://github.com/mozilla/application-services/pull/7339))
-- Verify signature of imported data when `.get()` is called with `sync_if_empty: true` ([#7518](https://github.com/mozilla/application-services/pull/7518)) 
+- Verify signature of imported data when `.get()` is called with `sync_if_empty: true` ([#7518](https://github.com/mozilla/application-services/pull/7518))
 - Do not quote `_since` values with the v2 API ([#7523](https://github.com/mozilla/application-services/pull/7523))
 
 ### Sync Manager

diff --git a/...nts/nimbus/android/src/main/java/org/mozilla/experiments/nimbus/internal/FeatureHolder.kt b/...nts/nimbus/android/src/main/java/org/mozilla/experiments/nimbus/internal/FeatureHolder.kt
@@ -182,18 +182,9 @@ interface FMLObjectInterface {
 }
 
 /**
- * A bare-bones interface for the FML generated features.
+ * A marker interface for FML generated feature classes.
  *
  * App developers should use the generated concrete classes, which
  * implement this interface.
  */
-interface FMLFeatureInterface : FMLObjectInterface {
-    /**
-     * A test if the feature configuration has been modified somehow, invalidating any experiment
-     * that uses it.
-     *
-     * This may be `true` if a `pref-key` has been set in the feature manifest and the user has
-     * set that preference.
-     */
-    fun isModified(): Boolean = false
-}
+interface FMLFeatureInterface : FMLObjectInterface {}
diff --git a/components/support/nimbus-fml/fixtures/fe/misc-features.yaml b/components/support/nimbus-fml/fixtures/fe/misc-features.yaml
@@ -10,14 +10,6 @@ about:
 channels:
   - debug
 features:
-  onboarding:
-    description: A feature containing a field with pref-key
-    variables:
-      enabled:
-        description: If true, enable new style onboarding.
-        type: Boolean
-        pref-key: enrollment_enabled
-        default: false
   messaging:
     description: A feature allowing coenrollment
     allow-coenrollment: true

diff --git a/components/support/nimbus-fml/fixtures/fe/pref_overrides.fml.yaml b/components/support/nimbus-fml/fixtures/fe/pref_overrides.fml.yaml
diff --git a/components/support/nimbus-fml/src/backends/frontend_manifest.rs b/components/support/nimbus-fml/src/backends/frontend_manifest.rs
@@ -142,7 +142,6 @@ impl From<PropDef> for FieldBody {
 impl From<PropDef> for FeatureFieldBody {
     fn from(value: PropDef) -> Self {
         Self {
-            pref_key: value.pref_key.clone(),
             gecko_pref: value.gecko_pref.clone(),
             string_alias: value.string_alias.as_ref().map(TypeRef::to_string),
             field: value.into(),

diff --git a/components/support/nimbus-fml/src/backends/kotlin/gen_structs/bundled.rs b/components/support/nimbus-fml/src/backends/kotlin/gen_structs/bundled.rs
@@ -86,16 +86,6 @@ impl CodeType for TextCodeType {
         }
     }
 
-    fn preference_getter(
-        &self,
-        oracle: &dyn CodeOracle,
-        prefs: &dyn Display,
-        pref_key: &dyn Display,
-    ) -> Option<String> {
-        let ct = oracle.find(&TypeRef::String);
-        ct.preference_getter(oracle, prefs, pref_key)
-    }
-
     fn is_resource_id(&self, literal: &Literal) -> bool {
         match literal {
             serde_json::Value::String(v) => is_resource_id(v),

diff --git a/components/support/nimbus-fml/src/backends/kotlin/gen_structs/filters.rs b/components/support/nimbus-fml/src/backends/kotlin/gen_structs/filters.rs
@@ -49,21 +49,6 @@ pub fn property(
     Ok(ct.property_getter(oracle, &vars, &prop, &default))
 }
 
-pub fn preference_getter(
-    type_: impl Borrow<TypeIdentifier>,
-    _: &dyn askama::Values,
-    prefs: impl fmt::Display,
-    pref_key: impl fmt::Display,
-) -> Result<String, askama::Error> {
-    let oracle = &ConcreteCodeOracle;
-    let ct = oracle.find(type_.borrow());
-    if let Some(getter) = ct.preference_getter(oracle, &prefs, &pref_key) {
-        Ok(getter)
-    } else {
-        unreachable!("The preference for type {} isn't available. This is a bug in Nimbus FML Kotlin generator", type_.borrow());
-    }
-}
-
 pub fn to_json(
     prop: impl fmt::Display,
     _: &dyn askama::Values,

diff --git a/components/support/nimbus-fml/src/backends/kotlin/gen_structs/primitives.rs b/components/support/nimbus-fml/src/backends/kotlin/gen_structs/primitives.rs
@@ -66,15 +66,6 @@ impl CodeType for BooleanCodeType {
             _ => unreachable!("Expecting a boolean"),
         }
     }
-
-    fn preference_getter(
-        &self,
-        _oracle: &dyn CodeOracle,
-        prefs: &dyn Display,
-        pref_key: &dyn Display,
-    ) -> Option<String> {
-        Some(format!("{prefs}.getBoolean({}, false)", quoted(pref_key)))
-    }
 }
 
 pub(crate) struct IntCodeType;
@@ -131,15 +122,6 @@ impl CodeType for IntCodeType {
             _ => unreachable!("Expecting a number"),
         }
     }
-
-    fn preference_getter(
-        &self,
-        _oracle: &dyn CodeOracle,
-        prefs: &dyn Display,
-        pref_key: &dyn Display,
-    ) -> Option<String> {
-        Some(format!("{prefs}.getInt({}, 0)", quoted(pref_key)))
-    }
 }
 
 pub(crate) struct StringCodeType;
@@ -199,15 +181,6 @@ impl CodeType for StringCodeType {
             _ => unreachable!("Expecting a string"),
         }
     }
-
-    fn preference_getter(
-        &self,
-        _oracle: &dyn CodeOracle,
-        prefs: &dyn Display,
-        pref_key: &dyn Display,
-    ) -> Option<String> {
-        Some(format!("{prefs}.getString({}, \"\")", quoted(pref_key)))
-    }
 }
 
 #[cfg(test)]

diff --git a/components/support/nimbus-fml/src/backends/kotlin/templates/FeatureTemplate.kt b/components/support/nimbus-fml/src/backends/kotlin/templates/FeatureTemplate.kt
@@ -4,20 +4,4 @@
 {{ inner.doc()|comment("") }}
 public class {{ inner.name()|class_name }}  {% call kt::render_constructor() %} : FMLFeatureInterface {
     {% call kt::render_class_body(inner) %}
-
-    {%- if inner.has_prefs() %}
-    override fun isModified(): Boolean =
-        {% call kt::prefs() %}?.let { prefs ->
-            listOf(
-            {%- for p in inner.props() %}
-            {%- if p.has_prefs() %}
-                {{ p.pref_key().unwrap()|quoted }},
-            {%- endif %}
-            {%- if p.has_gecko_prefs() %}
-                {{ p.gecko_pref().unwrap().pref()|quoted }},
-            {%- endif %}
-            {%- endfor %}
-            ).any { prefs.contains(it) }
-        } ?: false
-    {%- endif %}
 }
diff --git a/components/support/nimbus-fml/src/backends/kotlin/templates/macros.kt b/components/support/nimbus-fml/src/backends/kotlin/templates/macros.kt
@@ -47,29 +47,6 @@ private constructor(
    {%- let defaults = format!("_defaults.{}", prop_kt) %}
    {%- let getter = p.typ()|property(p.name(), "_variables", defaults) %}
   {{ p.doc()|comment("  ") }}
-   {%- if !p.has_prefs() %}
-   val {{ prop_kt }}: {{ type_kt }} by lazy {
-      {{ getter }}
-   }
-   {%- else %}
-   val {{ prop_kt }}: {{ type_kt }}
-      get() =
-         {%- let prefs = "it" %}
-         {%- let key = p.pref_key().unwrap() %}
-         {% call prefs() %}?.let {
-            if ({{ prefs }}.contains({{ key|quoted }})) {
-               try {
-                  {{ p.typ()|preference_getter(prefs, key) }}
-               } catch (e:  ClassCastException) {
-                  // This only gets to here if the app has written the
-                  // wrong type to this preference.
-                  null
-               }
-            } else {
-               null
-            }
-         } ?: {{ getter }}
-   {%- endif %}
 {% endfor %}
 
    {#- toJSON #}

diff --git a/components/support/nimbus-fml/src/backends/mod.rs b/components/support/nimbus-fml/src/backends/mod.rs
@@ -145,15 +145,6 @@ pub trait CodeType {
         None
     }
 
-    fn preference_getter(
-        &self,
-        _oracle: &dyn CodeOracle,
-        _prefs: &dyn Display,
-        _pref_key: &dyn Display,
-    ) -> Option<String> {
-        None
-    }
-
     /// Call from the template
     fn as_json(&self, oracle: &dyn CodeOracle, prop: &dyn Display) -> String {
         self.as_json_transform(oracle, prop)

diff --git a/components/support/nimbus-fml/src/backends/swift/templates/FeatureTemplate.swift b/components/support/nimbus-fml/src/backends/swift/templates/FeatureTemplate.swift
@@ -2,27 +2,4 @@
 {%- let inner = self.inner() %}
 {%- let class_name = inner.name()|class_name -%}
 {% call swift::render_class(inner) %}
-
-{%- if inner.has_prefs() %}
-
-extension {{ class_name }}: FMLFeatureInterface {
-    public func isModified() -> Bool {
-        guard let prefs = {% call swift::prefs() %} else {
-            return false
-        }
-        let keys = [
-            {%- for p in inner.props() %}
-            {%- if p.has_prefs() %}
-            {{ p.pref_key().unwrap()|quoted }},
-            {%- endif %}
-            {%- endfor %}
-        ]
-        if let _ = keys.first(where: { prefs.object(forKey: $0) != nil }) {
-            return true
-        }
-        return false
-    }
-}
-{%- else %}
 extension {{ class_name }}: FMLFeatureInterface {}
-{%- endif %}
diff --git a/components/support/nimbus-fml/src/backends/swift/templates/macros.swift b/components/support/nimbus-fml/src/backends/swift/templates/macros.swift
@@ -13,15 +13,13 @@
 {% let class_name = inner.name()|class_name -%}
 
 {{ inner.doc()|comment("") }}
-public class {{class_name}}: FMLObjectInterface {
+public class {{class_name}}: FMLFeatureInterface {
     private let _variables: Variables
     private let _defaults: Defaults
-    private let _prefs: UserDefaults?
 
-    private init(variables: Variables = NilVariables.instance, prefs: UserDefaults? = nil, defaults: Defaults) {
+    private init(variables: Variables = NilVariables.instance, defaults: Defaults) {
         self._variables = variables
         self._defaults = defaults
-        self._prefs = prefs
     }
     {# The struct holds the default values that come from the manifest. They should completely
     specify all values needed for the  feature #}
@@ -70,33 +68,10 @@ public class {{class_name}}: FMLObjectInterface {
     {%- let defaults = format!("_defaults.{}", prop_swift) %}
     {%- let getter = p.typ()|property(p.name(), "self._variables", defaults) %}
     {{ p.doc()|comment("    ") }}
-    {%- if !p.has_prefs() %}
     public lazy var {{ prop_swift }}: {{ type_swift }} = {
         {{ getter }}
     }()
-    {%- else %}
-    public var {{ prop_swift }}: {{ type_swift }} {
-        {%- let prefs = "prefs" %}
-        {%- let key = p.pref_key().unwrap() %}
-        {#- Using `object(forKey:)` here as it returns an optional that just needs to be cast.
-            `integer(forKey:)` returns zero, `bool(forKey:)` returns `false` if the key is not
-            present.
-
-            Now we only need to use the type label as part of the cast, we don't need to implement
-            separate type specific preference getters.
-
-            `has_prefs()` checks if the type can be got from UserDefaults.
-            #}
-        if let {{ prefs }} = {% call prefs() %},
-            let {{ prop_swift }} = {{ prefs }}.object(forKey: {{ key|quoted }}) as? {{ type_swift }} {
-            return {{ prop_swift }}
-        }
-        return {{ getter }}
-    }
-    {%- endif %}
     {% endfor %}
 }
 
 {%- endmacro %}}
-
-{% macro prefs() %}self._prefs{% endmacro %}
diff --git a/components/support/nimbus-fml/src/fixtures/mod.rs b/components/support/nimbus-fml/src/fixtures/mod.rs
@@ -54,7 +54,6 @@ impl PropDef {
             typ: typ.clone(),
             default: value.clone(),
             doc: format!("{nm} property of type {typ}"),
-            pref_key: None,
             gecko_pref: None,
             string_alias: None,
         }
@@ -64,18 +63,17 @@ impl PropDef {
         nm: &str,
         typ: &TypeRef,
         value: &Value,
-        pref_key: &str,
-        pref_branch: PrefBranch,
+        pref: &str,
+        branch: PrefBranch
     ) -> Self {
         Self {
             name: nm.to_string(),
             typ: typ.clone(),
             default: value.clone(),
             doc: format!("{nm} property of type {typ}"),
-            pref_key: None,
             gecko_pref: Some(GeckoPrefDef {
-                pref: pref_key.into(),
-                branch: pref_branch,
+                pref: pref.into(),
+                branch: branch,
             }),
             string_alias: None,
         }
@@ -87,7 +85,6 @@ impl PropDef {
             typ: typ.clone(),
             default: value.clone(),
             doc: nm.to_string(),
-            pref_key: None,
             gecko_pref: None,
             string_alias: Some(sa.clone()),
         }
@@ -99,7 +96,6 @@ impl PropDef {
             doc: doc.to_string(),
             typ: typ.clone(),
             default: default.clone(),
-            pref_key: None,
             gecko_pref: None,
             string_alias: None,
         }