feature-flag-removal

name	feature-flag-removal
description	Use when removing a `base::Feature` and its associated code

Your task is to clean up a base::Feature flag and its associated code.

1. Determine flag's default state:

   • Search for the BASE_FEATURE that defines the flag's default state (either ENABLED_BY_DEFAULT or DISABLED_BY_DEFAULT).
   • If the default is platform-specific (i.e., guarded by #if BUILDFLAG()), ask the user to clarify which should be considered the final state.
   • If no definition exists, check //third_party/blink/renderer/platform/runtime_enabled_features.json5, which is a config that generates base::Features at build time. Treat the "stable" status as ENABLED_BY_DEFAULT and all other statuses as DISABLED_BY_DEFAULT.

2. Remove C++ flag enablement checks:

   • Find C++ callsites where the feature enablement is checked. This usually takes the form of base::FeatureList::IsEnabled(kFeature) calls or a free Is<X>Enabled() helper function that wraps the former.
   • Simplify each enablement check as if the default state were inlined. Usually, you will need to delete one branch of an if-else condition and dedent the other.
   • The final code should almost never contain true or false literals. Any true or false in a larger boolean expression should be simplified as much as possible.

3. Remove obsolete base::FeatureParams:

   • Search for BASE_FEATURE_PARAMs that are linked to the obsolete feature flag.
   • Replace each retrieval of the parameter's value with the default value. Improve readability by using a constant (if the same value needs to be shared in multiple places) or a named call parameter (e.g., FunctionCall(/*param=*/67)).

4. Remove obsolete C++ test coverage:

   • Remove obsolete tests or assertions that exercise the non-default state.
   • Remove base::test::ScopedFeatureLists that explicitly set the feature in the default state.
   • Remove references to the obsolete feature in test names.

5. Clean up Java code:

   • Check if the base::Feature is used in Java code by searching for the constant case version of the base::Feature name (e.g., FOO_BAR for kFooBar).
   • Simplify the Java code in the same way as the C++ case (enablement checks, parameters, tests).

6. Clean up WebUI frontend code:

   • If the base::Feature state was formerly sent to a WebUI frontend (e.g., with AddBoolean("isFooEnabled", base::FeatureList::IsEnabled(kFoo))), clean up isFooEnabled flags from WebUI frontend files (TypeScript/HTML/CSS) in the same way as the C++ case.

7. Simplify APIs and expressions:

   • Try to simplify APIs or expressions. For example:

     • Suppose a method takes a std::optional<T> that's std::nullopt with the feature disabled, and T with the feature enabled. If the feature-disabled case is being removed, then the parameter can just become a value of type T.

     • Inline trivial local variable assignments. For example,

       const auto x = base::FeatureList::IsEnabled(kFoo) ? Enum::A : Enum::B;
       Bar(x);
       

       ... will become:

       const auto x = Enum::A;
       Bar(x);
       

       ... if one naively removes an ENABLED_BY_DEFAULT flag kFoo, but this can be simplified further as:

       Bar(Enum::A);
       

       ... since the assignment to x is adding unnecessary indirection.

8. Remove dead code:

   • In all removed lines, check for functions, classes, and constants that are no longer referenced and delete their declarations and definitions. You may need to delete entire source files.
   • Continue this process for all code that becomes unreferenced.
   • Remove declarations and definitions for the obsolete flag and its parameters.

9. Delete unused imports and build deps:

   • Remove #includes, #imports, and forward declarations that are no longer needed.
   • Remove BUILD.gn deps that are no longer needed.
   • Remove deleted source files from BUILD.gn.

10. Delete flag-metadata.json entry:

    • If you modified //chrome/browser/about_flags.cc or //ios/chrome/browser/flags/about_flags.mm, remove the corresponding entry from //chrome/browser/flag-metadata.json. By convention, a base::Feature declared as kFooBar has foo-bar as its about:flags name.

11. Clean up //testing/variations/fieldtrial_testing_config.json:

    • Remove references to the obsolete feature flag and its parameters.
    • Delete any experiments or studies that become empty.

12. Delete unused strings or resources:

    • At build time, .grd and .grdp files generate identifiers starting with IDR_ and IDS_ for resources and localized strings, respectively. In Java, these are static members on an R class (e.g., R.string.foo corresponds to IDS_FOO).
    • If any string or resource has no more users in either C++ or Java:
      • Delete its definition in a .grd(p) file.
      • Delete orphaned resource files.
      • For an orphaned string IDS_FOO, delete its corresponding IDS_FOO.png.sha1 file, which references a screenshot to help translators.

13. Update metrics:

    • If an enumerated histogram no longer emits certain values, find the obsolete values in a enums.xml file under //tools/metrics/histograms/ and prepend (Obsolete) to the <int label=...>.
    • If an entire histogram is no longer emitted, remove it from its histograms.xml file under //tools/metrics/histograms/.
    • Never rename histograms or renumber enum <int> values.
    • Do not touch the flag enums in //tools/metrics/histograms/enums.xml. The entries are still needed for decoding data in future Chrome versions.

14. Update comments:

    • Check immediately above each modified code block for obsolete comments.
    • If a comment still partially applies, reword it. Otherwise, delete it.

15. Verify the build is not broken:

    • Determine what build directory the user wants to use (either from system context, or by asking).
    • Build everything with autoninja -C <dir>.
    • Fix any breakages.

16. Verify all tests still pass:

    • Determine what build directory the user wants to use.
    • Use git diff to determine modified test files.
    • Run each test file with tools/autotest.py -C <dir> <test-file> to confirm the tests still pass.
    • Fix any failures.

17. Format the code:

    • Run git cl format.
    • Remove extraneous whitespace (more than one consecutive blank lines, or whitespace starting or ending a class or method).

• Adhere strictly to existing naming conventions. Do not rename variables or constants for stylistic preference. The goal is removal and simplification, not renaming.
• Do not modify any logic for unrelated features. You'll often see families of related features that share a prefix, but you should only focus on the stale feature.