RFC-0019: Using evolution = uint64;

RFC-0019: Using evolution = uint64;
Status	Accepted
Areas	FIDL
Description	Add a mechanism to give more descriptive names to primitive types. Remove status as a built-in feature of the language. Introduce a zx library to contain the primitive types from .
Authors	abarth@google.com
Date submitted (year-month-day)	2018-07-17
Date reviewed (year-month-day)	2018-08-03

Summary

Add a mechanism to give more descriptive names to primitive types. Remove status as a built-in feature of the language. Introduce a zx library to contain the primitive types from <zircon/types.h>.

Relation to other RFCs

This RFC was superseded by:

• RFC-0052: Type aliasing and new types

Motivation

Often developers want to assign more descriptive names to primitive types. For example, status is a more descriptive name for int32, but status is built into the language and other types cannot be used in the same way.

Design

This proposal affects only the FIDL source language.

1. Extend the using keyword to be able to assign descriptive names to primitive types. Specifically, add the following production to the FIDL grammar:

using-list = ( using | using-declaration )* ;
using-declaration = "using" , IDENTIFIER ,  "=" , primitive-type , ";" ;

1. Remove the status primitive type from the language. The status primitive type can now be defined using the language itself instead of being a built-in feature.

2. Remove the status reserved word. We no longer need to reserve the word status now that we can define the name in the language. Add a zx library that contains declarations for the primitive types defined by the Zircon system interface. Ideally, this library would be generated from a future syscalls.abigen that contained this information. For example:

library zx;

using status = int32;
using time = uint64;
using duration = uint64;
[...]

Notice that these declarations must appear in the using-list and must refer directly to primitive types. This approach avoids complexity in the compiler because their use can be immediately translated into the underlying primitive type.

Further, notice that there are no proposed changes to the generated code in any target language. These types are represented in the target languages using the underlying primitive types.

Documentation and examples

Example usage

    1: AdvanceTime(zx.duration duration) -> (zx.status status);

Backwards compatibility

This change is a non-backwards compatible change to the FIDL source language because it removes the status primitive type. However, migrating existing clients of status is trivial because they can simply use zx.status instead.

Performance

The proposed change has no impact on performance.

Security

The proposed change has no impact on security.

Testing

The feature will be tested in the fidl-test suite in Zircon by adding a FIDL file that uses each of the types from the zx library.

Drawbacks, alternatives, and unknowns

This proposal is straightforward to implement in the FIDL frontend and requires no changes to the language-specific backends.

An alternative is to not solve the problem and continue to use primitive types directly.

Prior art and references

This feature is very common in programming languages. The syntax is borrowed loosely from C++.