RFC-0056: Empty structs

RFC-0056: Empty structs
Status	Accepted
Areas	FIDL
Description	Allow empty structs in FIDL library declarations and define the encoding of that to be a struct with a single zero-valued uint8.
Authors	ianloic@google.com
Date submitted (year-month-day)	2018-08-30
Date reviewed (year-month-day)	2018-09-11

Summary

Allow empty structs in FIDL library declarations and define the encoding of that to be a struct with a single zero-valued uint8.

Motivation

Today, we see the need for empty structs come up in various areas. For instance, when using the command pattern where one of the commands does not require any arguments. Say you have a "ping" command, where simply selecting this union option is enough to convey the intent. Generalizing, empty structs are useful to represent 'unit types' when simulating Algebraic Data Types.

However, empty structs are disallowed in some target languages: you can't have empty C/C++ structs, and we want FIDL wire encoding to be mappable to C data types.

As a result of the need, and the current constraint, we've seen a proliferation of structs that are semantically empty, but in practice have a single small placeholder field so that they compile.

Here are some examples:

• fuchsia.modular.GlobalScope
• fuchsia.modular.FocusStory

This proposal would decouple the constraints of a particular set of languages from the FIDL language while maintaining wire format compatibility with C structs.

Design

The FIDL frontend compiler should accept empty struct definitions. The TypeShape of an empty struct should be the same as a struct with a single uint8 field.

Bindings generators for languages that can handle empty structs should generate actual empty structs but care must be taken to ensure that empty structs are correctly encoded as a struct that takes a byte. For C and C++ a single uint8_t __reserved field should be generated.

Implementation strategy

1. Allow fidlc to accept empty structs and synthesize a uint8 field in the IR.
2. Update the fidl_compatibility_test to include an empty struct.
3. Update the documentation.
4. Update fidlc to not emit a bonus bogus field for empty structs, update the bindings generators and validate that this works using fidl_compatibility_test.
5. Remove the placeholder fields from the structs identified above.

Documentation and examples

The FIDL documentation already includes (invalid) examples of empty structs. The language documentation could simply remove the constraint on zero-length structs. The wire format definition would gain a description of how to encode empty structs.

Backwards compatibility

This change is backwards compatible as it loosens rather than constrains the FIDL language. If interface authors replace a semantically empty struct that contains just a byte sized placeholder field (boolean, uint8, int8) with an empty struct then that will be even be represented the same on the wire.

Performance

This should have no impact on build or IPC performance but for languages that allow empty structs they will save on the work of moving around meaningless values.

Security

No impact.

Testing

The compilation of empty structs will be added as a fidlc test. The wire format of empty structs will be tested as part of the fidl_compatibility_test.

Drawbacks, alternatives, and unknowns

We could leave things the way they are but this seems like a simple improvement.

Prior art and references

Most programming languages and IDLs allow empty structs.