[cfe-dev] [RFC] A new attribute for enums

[Akira Hatanaka via cfe-dev]

I’d like to propose a new attribute for enums.

### The proposed attribute ###
The attribute is tentatively named “enum_style” and takes two arguments. The first argument determines the style of the enum, either “option” or “choice". “option” implies that the enum can be used as a one-bit flag, just like “flag_enum", and it’s OK to OR the values to create a new value. “choice” implies the enum cannot be used like “option” enums. The second argument is used to indicate whether or not the enum can be extended. If an enum is marked “closed”, clang can assume a variable of the enum type always has a value that is in the range determined by the enumerators listed in the enum definition. If it’s marked “open”, it doesn’t have the restriction. 

There are four possible combinations:

1. "choice, closed"
2. "choice, open"
3. "option, closed"
4. "option, open"

Attribute “flag_enum” we have today is equivalent to "option,closed".

In addition, I’m considering adding a command line option that specifies the default enum-style for unannotated enums.

### Motivation for the new attribute ###
There are several areas that can be improved using the new attribute and command line option.

1. Warnings
The new attribute can improve the accuracy of enum-related warnings such as -Wassign-enum and give better control over when the warnings are issued.

-Wassign-enum currently warns whenever a value that is out of the range determined by the enumerators is assigned to an enum variable. For a flag-enum, a value is in range if it can be created by ORing the enumerators listed in the enum definition or is a complement of one of the in-range values. For enums that are not flag-enums, only the values of the enumerators listed in the enum definition are considered to be in range. The warning is helpful in catching out-of-range values that are unintentionally assigned, but would be too strict if a project intentionally extended an enum by defining out-of-range “private” values (this does happen often). If the compiler knows an enum is “open”, it can choose not to issue warnings.

Another problem with the current approach is that all the enums the compiler sees have to be classified into flag-enums or non-flag-enums and the flag-enums have to be annotated. This requires a lot of work up front to determine whether or not an enum is a flag-enum, and sometimes it’s not even possible to annotate the enums if they are defined in a third party library that cannot be modified. With the command line option for specifying the default enum-style, users can instruct the compiler not to issue warnings if the enum is unannotated (the default can be either "choice,open” or “option,open”) and add the attributes to the enum definitions in an incremental fashion.

2. Code-completion and debugging
Code-completion tools can offer better suggestions based on whether the enum is a choice or an option. For example, if we had an "option" enum like this:

enum __attribute((enum_style(option, closed)))__ MyEnum {
  E1 = 1, E2 = 2,
};

and the user requests a code completion for this:

MyEnum = E1 | <esc>

code completion would recognize MyEnum is an “option" enum and could offer E2 as a suggestion. If MyEnum were a “choice" enum, it would offer no suggestions.

Debugging experience can be improved too. For example, when a MyEnum variable is set to 3, the debugger could show “E1 | E2” instead of showing the raw value 3.

3. clang importer
In order to determine whether a C or ObjC enum maps to an enum or option set in swift, swift’s clang importer looks at whether the enum was declared using macros such as CF_ENUM or CF_OPTIONS (the macros are explained in the link below). With the proposed attribute, this hack can be removed.

https://developer.apple.com/library/content/releasenotes/ObjectiveC/ModernizationObjC/AdoptingModernObjective-C/AdoptingModernObjective-C.html <https://developer.apple.com/library/content/releasenotes/ObjectiveC/ModernizationObjC/AdoptingModernObjective-C/AdoptingModernObjective-C.html>

4. Optimization
There is a command line option named -fstrict-enums, which allows the compiler to optimize code using the assumption that the value of an enum variable is in range. This option can safely be used only if it is known that the values the variables of enum types can take are always in range. With the new attribute, the compiler can focus on variables of enum types that are marked “closed” and optimize them and leave other variables unoptimized.

### What will change ###
I plan to add support for the new attribute in Sema and change the code that issues warning. I’m not planning to work on the IRGen optimization that takes advantage of the information the new attribute provides. It will be left as future work.
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