Revamp the engine style guide, remove always_specify_types. (#52859)

Based on the (internal) discussion around converging on using the official Dart style guide, with the exception of the code that gets published under `dart:ui`, as that is user-facing, and we'd like to evolve the code style in conjunction with the framework.

I also took the opportunity to specify more about our style guide use in general, mostly to make it easier to understand our conventions, and also call out known problem areas (notably, our over-use of `shared_ptr` and `auto` in some cases). I am happy to split those up, but it was easier to make the markdown changes at once.

I also took @cbracken and folks advice and clarified directly that explicit types in Dart are _not_ bad (with examples).

Author: matanlurey

CONTRIBUTING.md

@@ -14,29 +14,182 @@ contributing guide.
 
 ## Style
 
-The Flutter engine follows Google style for the languages it uses:
-
-- [C++](https://google.github.io/styleguide/cppguide.html)
-  - **Note**: The Linux embedding generally follows idiomatic GObject-based C
-    style. Use of C++ is discouraged in that embedding to avoid creating hybrid
-    code that feels unfamiliar to either developers used to working with
-    `GObject` or C++ developers. For example, do not use STL collections or
-    `std::string`. Exceptions:
-    - C-style casts are forbidden; use C++ casts.
-    - Use `nullptr` rather than `NULL`.
-    - Avoid `#define`; for internal constants use `static constexpr` instead.
-- [Objective-C][objc_style] (including [Objective-C++][objcc_style])
-- [Java][java_style]
-
-C/C++ and Objective-C/C++ files are formatted with `clang-format`, and GN files
-with `gn format`.
+The Flutter engine _generally_ follows Google style for the languages it uses,
+with some exceptions.
 
+### C/C++
+
+Follows the [Google C++ Style Guide][google_cpp_style] and is automatically
+formatted with `clang-format`.
+
+Some additional considerations that are in compliance with the style guide, but
+are worth noting:
+
+#### Judiciously use shared_ptr
+
+The engine currently (as of 2024-05-15) uses `shared_ptr` liberally, which can
+be expensive to copy, and is not always necessary.
+
+The C++ style guide has a
+[section on ownership and smart pointers][cpp_ownership] worth reading:
+
+> Do not design your code to use shared ownership without a very good reason.
+> One such reason is to avoid expensive copy operations, but you should only do
+> this if the performance benefits are significant, and the underlying object is
+> immutable.
+
+Prefer using `std::unique_ptr` when possible.
+
+#### Judiciously use `auto`
+
+The C++ style guide has a [section on type deduction][cpp_auto] that is worth
+reading:
+
+> The fundamental rule is: use type deduction only to make the code clearer or
+> safer, and do not use it merely to avoid the inconvenience of writing an
+> explicit type. When judging whether the code is clearer, keep in mind that
+> your readers are not necessarily on your team, or familiar with your project,
+> so types that you and your reviewer experience as unnecessary clutter will
+> very often provide useful information to others. For example, you can assume
+> that the return type of `make_unique<Foo>()` is obvious, but the return type
+> of `MyWidgetFactory()` probably isn't.
+
+Due to our codebase's extensive use of `shared_ptr`, `auto` can have surprising
+performance implications. See [#49801][pr_49801] for an example.
+
+#### Linux Embedding
+
+> [!NOTE]  
+> The Linux embedding instead follows idiomatic GObject-based C style.
+
+Use of C++ in the [Linux embedding][] is discouraged in that embedding to avoid
+creating hybrid code that feels unfamiliar to either developers used to working
+with `GObject` or C++ developers.
+
+For example, _do not_ use STL collections or `std::string`, but _do_:
+
+- Use C++ casts (C-style casts are forbidden).
+- Use `nullptr` rather than `NULL`.
+- Avoid `#define`; for internal constants use `static constexpr` instead.
+
+### Dart
+
+The Flutter engine _intends_ to follow the [Dart style guide][dart_style] but
+currently follows the [Flutter style guide][flutter_style], with the following
+exceptions:
+
+#### Use of type inference is allowed
+
+The [Dart style guide][dart_inference] only requires explicit types when type
+inference is not possible, but the Flutter style guide always requires explicit
+types. The engine is moving towards the Dart style guide, but this is a gradual
+process. In the meantime, follow these guidelines:
+
+- **Always** annotate when inference is not possible.
+- **Prefer** annotating when inference is possible but the type is not
+  obvious.
+
+Some cases when using `var`/`final`/`const` is appropriate:
+
+- When the type is obvious from the right-hand side of the assignment:
+
+  ```dart
+  // Capitalized constructor name always returns a Foo.
+  var foo = Foo();
+
+  // Similar with factory constructors.
+  var bar = Bar.create();
+
+  // Literals (strings, numbers, lists, maps, etc) always return the same type.
+  var name = 'John Doe';
+  var flag = true;
+  var numbers = [1, 2, 3];
+  var map = {'one': 1, 'two': 2, 'three': 3};
+  ```
+
+- When the type is obvious from the method name:
+
+  ```dart
+  // toString() always returns a String.
+  var string = foo().toString();
+
+  // It's reasonable to assume that length returns an int.
+  var length = string.length;
+  ```
+
+- When the type is obvious from the context:
+
+  ```dart
+  // When variables are in the same scope, reduce() clearly returns an int.
+  var list = [1, 2, 3];
+  var sum = list.reduce((a, b) => a + b);
+  ```
+
+Some cases where an explicit type should be considered:
+
+- When the type is not obvious from the right-hand side of the assignment:
+
+  ```dart
+  // What does 'fetchLatest()' return?
+  ImageBuffer buffer = fetchLatest();
+
+  // What does this large chain of method calls return?
+  Iterable<int> numbers = foo().bar().map((b) => b.baz());
+  ```
+
+- When there are semantic implications to the type:
+
+  ```dart
+  // Without 'num', the map would be inferred as 'Map<String, int>'.
+  const Map<String, num> map = {'one': 1, 'two': 2, 'three': 3};
+  ```
+
+- Or, **when a reviewer requests it**!
+
+  Remember that the goal is to make the code more readable and maintainable, and
+  explicit types _can_ help with that. Code can be changed, so it's always
+  possible to add or remove type annotations later as the code evolves, so avoid
+  bikeshedding over this.
+
+### Java
+
+Follows the [Google Java Style Guide][java_style] and is automatically formatted
+with `google-java-format`.
+
+### Objective-C
+
+Follows the [Google Objective-C Style Guide][objc_style], including for
+Objective-C++ and is automatically formatted with `clang-format`.
+
+### Python
+
+Follows the [Google Python Style Guide][google_python_style] and is
+automatically formatted with `yapf`.
+
+> [!WARNING]
+> Historically, the engine grew a number of one-off Python scripts, often as
+> part of the testing or build infrastructure (i.e. command-line tools). We are
+> instead moving towards using Dart for these tasks, so new Python scripts
+> should be avoided whenever possible.
+
+### GN
+
+Automatically formatted with `gn format`.
+
+[cpp_auto]: https://google.github.io/styleguide/cppguide.html#Type_deduction
+[cpp_ownership]: https://google.github.io/styleguide/cppguide.html#Ownership_and_Smart_Pointers
+[dart_inference]: https://dart.dev/effective-dart/design#types
+[dart_style]: https://dart.dev/effective-dart/style
+[linux embedding]: shell/platform/linux
+[google_cpp_style]: https://google.github.io/styleguide/cppguide.html
+[pr_49801]: https://github.com/flutter/engine/pull/49801
 [code_of_conduct]: https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md
 [contrib_guide]: https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md
 [engine_dev_setup]: https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment
+[flutter_style]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
 [objc_style]: https://google.github.io/styleguide/objcguide.html
-[objcc_style]: https://google.github.io/styleguide/objcguide.html#objective-c
 [java_style]: https://google.github.io/styleguide/javaguide.html
+[google_python_style]: https://google.github.io/styleguide/pyguide.html
 
 ## Testing
 

analysis_options.yaml

@@ -2,7 +2,8 @@
 #
 # This file is a copy of analysis_options.yaml from flutter repo
 # as of 2023-12-18, but with some modifications marked with
-# "DIFFERENT FROM FLUTTER/FLUTTER" below.
+# "DIFFERENT FROM FLUTTER/FLUTTER" below:
+# - Rem
 
 analyzer:
   language:
@@ -29,7 +30,7 @@ linter:
     - always_declare_return_types
     - always_put_control_body_on_new_line
     # - always_put_required_named_parameters_first # we prefer having parameters in the same order as fields https://github.com/flutter/flutter/issues/10219
-    - always_specify_types
+    # - always_specify_types # DIFFERENT FROM FLUTTER/FLUTTER; see https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#dart
     # - always_use_package_imports # we do this commonly
     - annotate_overrides
     # - avoid_annotating_with_dynamic # conflicts with always_specify_types

lib/ui/analysis_options.yaml

@@ -7,4 +7,5 @@ analyzer:
 
 linter:
   rules:
+    always_specify_types: true # dart:ui is shipped as part of flutter/flutter, let's keep them consistent
     unreachable_from_main: false # lint not compatible with how dart:ui is structured