[TEST] PSA: FIDL now supports named method payloads

[Alex Zaslavsky]

If you don't read/write FIDL, or interact with generated FIDL bindings, you may stop reading now.

TL;DR:

• Named method payloads are now supported in FIDL:

// Old, repetitive way protocol OldProtocol {     InvertNumber(struct {num int16;}) -> (struct {num int16;});     HalveNumber(struct {num int16;}) -> (struct {num int16;}); }; // New, DRY way type NumberPayload = struct {     num int16; }; protocol NewProtocol {     InvertNumber(NumberPayload) -> (NumberPayload);     HalveNumber(NumberPayload) -> (NumberPayload); };

• This style is encouraged for new method definitions where appropriate, but already existing definitions will not be proactively converted to support this new syntax.

Example

Named payload conversions are 100% API preserving for struct payloads, which as of this writing are the only payloads in fuchsia.git, though support for table and union payloads is just around the corner.   The OldProtocol/NewProtocol example above will result in the following, functionally identical HLCPP output:

class OldProtocol {   public:     /* … */     using InvertNumberCallback = fit::function<void(int16_t)>;     virtual void InvertNumber(int16_t num, InvertNumberCallback callback) = 0;     using HalveNumberCallback = fit::function<void(int16_t)>;     virtual void HalveNumber(int16_t num, HalveNumberCallback callback) = 0; }; class NewProtocol {   public:     /* … */     using InvertNumberCallback = fit::function<void(int16_t)>;     virtual void InvertNumber(int16_t num, InvertNumberCallback callback) = 0;     using HalveNumberCallback = fit::function<void(int16_t)>;     virtual void HalveNumber(int16_t num, HalveNumberCallback callback) = 0; };

And this (also identical) Rust output:

pub trait OldProtocolProxyInterface: Send + Sync {    type InvertNumberResponseFut: std::future::Future<Output = Result<(i16), fidl::Error>> + Send;    fn r#invert_number(&self, num: i16) -> Self::InvertNumberResponseFut;    type HalveNumberResponseFut: std::future::Future<Output = Result<(i16), fidl::Error>> + Send;    fn r#halve_number(&self, num: i16) -> Self::HalveNumberResponseFut; } pub trait NewProtocolProxyInterface: Send + Sync {    type InvertNumberResponseFut: std::future::Future<Output = Result<(i16), fidl::Error>> + Send;    fn r#invert_number(&self, num: i16) -> Self::InvertNumberResponseFut;    type HalveNumberResponseFut: std::future::Future<Output = Result<(i16), fidl::Error>> + Send;    fn r#halve_number(&self, num: i16) -> Self::HalveNumberResponseFut; }