docs/contribute/contributing-to-fidl/design-history.md

# FIDL design history

This page provides a starting point for navigating [RFCs] in the "FIDL" area. It
organizes them in various ways and gives an updated status on each, bringing the
reader up to speed with FIDL's history and current direction.

Previously, these design documents were called [FTPs] or FIDL Tuning Proposals.
The FTP process was later [merged into][rfc-0017] the RFC process. Because of
the way FTPs were renumbered as RFCs, RFC numbers do not imply chronology.
However, each individual list on this page shows RFCs in chronological order:
by submission date for FTP-turned-RFCs, and by review date for all later RFCs.

## By theme

This section organizes FIDL RFCs by theme. Each RFC is listed once, under its
primary theme, even if it touches on multiple themes.

### Governance

* [RFC-0018: FTP process: A modest proposal][rfc-0018]
* [RFC-0049: FIDL tuning process evolution][rfc-0049]
* [RFC-0017: The FTP Process is dead, long live the RFC Process!][rfc-0017]
* [RFC-0131: Design principles of the FIDL wire format][rfc-0131]

### Evolvability

* [RFC-0047: Tables][rfc-0047]
* (Rejected) [RFC-0063: OrdinalRange][rfc-0063]
* [RFC-0058: Introduce a deprecated attribute][rfc-0058]
* [RFC-0061: Extensible unions][rfc-0061]
* [RFC-0020: Interface ordinal hashing][rfc-0020]
* [RFC-0021: Soft transitions for methods add and remove][rfc-0021]
* [RFC-0033: Handling of unknown fields and strictness][rfc-0033]
* [RFC-0029: Increasing method ordinals][rfc-0029]
* [RFC-0037: Transactional message header v3][rfc-0037]
* [RFC-0024: Mandatory source compatibility][rfc-0024]
* [RFC-0040: Identifier uniqueness][rfc-0040]
* [RFC-0048: Explicit union ordinals][rfc-0048]
* [RFC-0083: FIDL versioning][rfc-0083]
* (Rejected) [RFC-0116: Wire format support for sparser FIDL tables][rfc-0116]
* [RFC-0132: FIDL table size limit][rfc-0132]
* [RFC-0138: Handling unknown interactions][rfc-0138]

### Expressivity

* [RFC-0019: Type aliases with using][rfc-0019]
* [RFC-0022: Default values for struct members][rfc-0022]
* [RFC-0066: Programmer advisory explicit defaults][rfc-0066]
* (Rejected) [RFC-0065: No optional strings or vectors][rfc-0065]
* (Rejected) [RFC-0064: Box\<Knox\>][rfc-0064]
* [RFC-0023: Compositional model for protocols][rfc-0023]
* (Rejected) [RFC-0042: Non nullable types][rfc-0042]
* [RFC-0041: Support for unifying services and devices][rfc-0041]
* (Rejected) [RFC-0044: Extensible method arguments][rfc-0044]
* [RFC-0054: Parameter attributes][rfc-0054]
* [RFC-0052: Type aliasing and new types][rfc-0052]
* [RFC-0137: Discard unknown data in FIDL][rfc-0137]
* [RFC-0160: Remove support for FIDL struct defaults][rfc-0160]

### Syntax

* (Rejected) [RFC-0036: Update to struct declarations][rfc-0036]
* (Rejected) [RFC-0038: Separating layout from constraints][rfc-0038]
* (Rejected) [RFC-0039: Types come second][rfc-0039]
* [RFC-0050: Syntax revamp][rfc-0050]
* [RFC-0086: Updates to RFC-0050: FIDL attributes syntax][rfc-0086]
* (Rejected) [RFC-0088: Updates to RFC-0050: FIDL bits, enum, and constraints syntax][rfc-0088]
* [RFC-0087: Updates to RFC-0050: FIDL method parameter syntax][rfc-0087]

### Ergonomics

* [RFC-0053: Epitaphs][rfc-0053]
* [RFC-0056: Empty structs][rfc-0056]
* [RFC-0060: Error handling][rfc-0060]
* [RFC-0025: Bit flags][rfc-0025]
* (Rejected) [RFC-0031: Typed epitaphs][rfc-0031]
* [RFC-0057: Default no handles][rfc-0057]
* [RFC-0196: FIDL large messages][rfc-0196]

### Domain

* [RFC-0120: Standalone use of the FIDL wire format][rfc-0120]
* [RFC-0190: FIDL support for syscalls][rfc-0190]

### Performance

* (Rejected) [RFC-0045: Zero-size empty structs][rfc-0045]
* (Rejected) [RFC-0026: Envelopes everywhere][rfc-0026]
* [RFC-0027: You only pay for what you use][rfc-0027]
* (Rejected) [RFC-0032: Efficient envelopes][rfc-0032]
* (Rejected) [RFC-0035: Automatic flow tracing][rfc-0035]
* [RFC-0113: Efficient envelopes][rfc-0113]
* [RFC-0114: Inlining small values in FIDL envelopes][rfc-0114]
* [RFC-0149: FIDL encode validation not mandatory][rfc-0149]

### Safety & security

* (Rejected) [RFC-0051: Safer structs for C++][rfc-0051]
* (Rejected) [RFC-0062: Method impossible][rfc-0062]
* (Rejected) [RFC-0034: Null terminate strings][rfc-0034]
* [RFC-0028: Handle rights][rfc-0028]

### Tooling

* [RFC-0076: FIDL API summaries][rfc-0076]
* [RFC-0097: FIDL Toolchain][rfc-0097]

### Documentation

* [RFC-0055: Documentation comments][rfc-0055]
* [RFC-0043: Documentation comment format][rfc-0043]

### Simplify design space

* [RFC-0030: FIDL is little endian][rfc-0030]
* [RFC-0059: Reserved bits in vector, string, and array count fields][rfc-0059]

## By language feature

This section lists RFCs that introduced new feature that are easily visible in
the FIDL language. It only includes current features, not obsolete ones.

| Feature                  | RFC        | Note
| ------------------------ | -----------| ------------------------------------------
| `table`                  | [RFC-0047] | Forward and backward compatible data type
| `///`                    | [RFC-0055] | Documentation comments
| `struct Empty {};`       | [RFC-0056] | Empty structs
| `error`                  | [RFC-0060] | Method error result syntax
| `union`                  | [RFC-0061] | Initially called `xunion`, replaced the old static unions
| `@selector`              | [RFC-0020] | Override method hashing with an explicit selector
| `@transitional`          | [RFC-0021] | Attribute for soft transitioning method addition/removal
| `compose`                | [RFC-0023] | Renamed `interface` to `protocol`, added the `compose` feature
| `bits`                   | [RFC-0025] | Bit flags types
| `strict`, `flexible`     | [RFC-0033] | Strict and flexible types
| handle rights            | [RFC-0028] | Annotate required or excluded handle rights
| `service`                | [RFC-0041] | Services are collection of protocols
| `alias`, `type`          | [RFC-0052] | Replaced `using` aliases with `alias` and `type`
| `resource`               | [RFC-0057] | Value and resource types
| anonymous layouts        | [RFC-0050] | Inline type definitions, named contextually or with `@generated_name`
| `@available`             | [RFC-0083] | Version annotations
| `Method(table { ... })`  | [RFC-0087] | Tables and unions as top-level request/response types
| `open`, `closed`, `ajar` | [RFC-0138] | Open and closed interactions

<!-- TODO(https://fxbug.dev/42175244): Uncomment when accepted. -->
<!-- | `terminal`        | [RFC-NNNN] | Terminal events -->

## Current status

This section gives the current status of all FIDL RFCs.

_Legend:_

| Status                     | Meaning
| -------------------------- | ------------------------------------------------------
| Unpublished                | Never made public nor formally reviewed
| Withdrawn                  | Made public, but never formally reviewed
| In review                  | Made public, review pending
| Implemented                | Accepted, implemented, still accurate
| Partially implemented      | Accepted and partially implemented, no plans to finish
| Implementation in progress | Accepted, implementation in progress
| Amended                    | Accepted, still mostly accurate, but amended by a later RFC
| Superseded                 | Accepted but no longer accurate, superseded by a later RFC
| Rejected                   | Formally rejected

| RFC        | Title                          | Status
| ---------- | ------------------------------ | --------------------------------
| [RFC-0018] | FTP process: A modest proposal | Superseded by [RFC-0017]
| [RFC-0019] | Type aliases with using | Superseded by [RFC-0052]
| [RFC-0022] | Default values for struct members | Superseded by [RFC-0160]
| [RFC-0051] | Safer structs for C++ | Rejected
| [RFC-0062] | Method impossible | Rejected
| [RFC-0053] | Epitaphs | Implemented <!-- TODO(https://fxbug.dev/42175244): Change to "Superseded" when we have terminal events. -->
| [RFC-0066] | Programmer advisory explicit defaults | Implemented
| [RFC-0047] | Tables | Amended by [RFC-0116], [RFC-0132]
| [RFC-0055] | Documentation comments | Amended by [RFC-0043]
| [RFC-0063] | OrdinalRange | Rejected; Superseded by [RFC-0020]
| &ndash;    | FIDL in FIDL | Unpublished; Protobuf [can do this][protobuf-self-describe]
| [RFC-0056] | Empty structs | Implemented
| [RFC-0058] | Introduce a deprecated attribute | Superseded by [RFC-0083]
| [RFC-0060] | Error handling | Implemented
| [RFC-0061] | Extensible unions | Implemented
| [RFC-0065] | No optional strings or vectors | Rejected
| [RFC-0064] | Box\<Knox\> | Rejected; see also "FIDL large message support"
| &ndash;    | Sequences | Unpublished; earlier version of "Streams"
| &ndash;    | Streams | Unpublished; see also [measure-tape] (a pragmatic solution)
| [RFC-0020] | Interface ordinal hashing | Implemented
| [RFC-0021] | Soft transitions for methods add and remove | Implemented
| [RFC-0023] | Compositional model for protocols | Implemented
| [RFC-0045] | Zero-size empty structs | Rejected
| [RFC-0025] | Bit flags | Implemented
| [RFC-0026] | Envelopes everywhere | Rejected
| [RFC-0027] | You only pay for what you use | Implemented
| [RFC-0030] | FIDL is little endian | Implemented
| [RFC-0031] | Typed epitaphs | Rejected
| [RFC-0032] | Efficient envelopes | Rejected; Superseded by [RFC-0113]
| [RFC-0033] | Handling of unknown fields and strictness | Amended by [RFC-0137]
| [RFC-0034] | Null terminate strings | Rejected
| [RFC-0029] | Increasing method ordinals | Superseded by [RFC-0037]
| [RFC-0035] | Automatic flow tracing | Rejected
| [RFC-0036] | Update to struct declarations | Rejected
| [RFC-0037] | Transactional message header v3 | Implemented
| [RFC-0038] | Separating layout from constraints | Rejected; Superseded by [RFC-0050]
| [RFC-0039] | Types come second | Rejected; Superseded by [RFC-0050]
| [RFC-0028] | Handle rights | Implemented
| [RFC-0042] | Non nullable types | Rejected (April Fools)
| [RFC-0024] | Mandatory source compatibility | Implemented
| [RFC-0040] | Identifier uniqueness | Implemented (rejected at first, later accepted)
| [RFC-0041] | Support for unifying services and devices | [Partially implemented](https://fxbug.dev/42160684#c36): only in C++, Rust
| [RFC-0044] | Extensible method arguments | Rejected; Superseded by [RFC-0087]
| [RFC-0043] | Documentation comment format | Implemented
| &ndash;    | Versioning | Unpublished; Superseded by [RFC-0083]
| &ndash;    | Required table fields | Unpublished
| [RFC-0048] | Explicit union ordinals | Implemented
| [RFC-0049] | FIDL tuning process evolution | Superseded by [RFC-0017]
| &ndash;    | Unified view of optionality | Unpublished
| &ndash;    | Iterators | Unpublished; see also [measure-tape] (a pragmatic solution)
| [RFC-0054] | Parameter attributes | Superseded by [RFC-0050]
| [RFC-0052] | Type aliasing and new types | [Implementation in progress](https://fxbug.dev/42158155)
| &ndash;    | Restrict non-numeric floating point values | Unpublished
| &ndash;    | Constant expressions | Unpublished
| [RFC-0057] | Default no handles | Implemented
| [RFC-0050] | Syntax revamp | Amended by [RFC-0086], [RFC-0087]
| &ndash;    | FIDL text format | Unpublished
| [RFC-0059] | Reserved bits in vector, string, and array count fields | Implemented; reserved bits no longer used by LLCPP
| [RFC-0017] | The FTP Process is dead, long live the RFC Process! | Implemented
| &ndash;    | FIDL Large message support | [Withdrawn][large-message-cl]; Superseded by [RFC-0196]
| [RFC-0076] | FIDL API summaries | Implemented
| [RFC-0083] | FIDL versioning | [Implementation in progress](https://fxbug.dev/42146818)
| [RFC-0086] | Updates to RFC-0050: FIDL attributes syntax | Implemented
| [RFC-0088] | Updates to RFC-0050: FIDL bits, enum, and constraints syntax | Rejected
| [RFC-0087] | Updates to RFC-0050: FIDL method parameter syntax | [Implementation in progress](https://fxbug.dev/42156243)
| [RFC-0097] | FIDL Toolchain | [Implementation in progress](https://fxbug.dev/42115264)
| [RFC-0113] | Efficient envelopes | Implemented
| [RFC-0114] | Inlining small values in FIDL envelopes | Implemented
| [RFC-0116] | Wire format support for sparser FIDL tables | Rejected
| [RFC-0120] | Standalone use of the FIDL wire format | [Implementation in progress](https://fxbug.dev/42121756)
| [RFC-0131] | Design principles of the FIDL wire format | Implemented
| [RFC-0132] | FIDL table size limit | [Implemented](https://fuchsia-review.googlesource.com/c/fuchsia/+/583988)
| [RFC-0137] | Discard unknown data in FIDL | [Implementation in progress](https://fxbug.dev/42166276)
| [RFC-0138] | Handling unknown interactions | [Implementation in progress](https://fxbug.dev/42169590)
| [RFC-0149] | FIDL encode validation not mandatory | Implemented
| [RFC-0160] | Remove support for FIDL struct defaults | Implemented
| [RFC-0190] | FIDL support for syscalls | [Implementation in progress](https://fxbug.dev/42061412)
| [RFC-0196] | FIDL large messages | [Implementation in progress](https://fxbug.dev/42051196)
| &ndash;    | A JSON representation for FIDL | [In review](https://fuchsia-review.googlesource.com/c/fuchsia/+/688226)
| &ndash;    | FIDL complex constants | [In review](https://fuchsia-review.googlesource.com/c/fuchsia/+/690449)
| &ndash;    | FIDL language support plans | [In review](https://fuchsia-review.googlesource.com/c/fuchsia/+/705622)
| &ndash;    | Terminal events | [In review](https://fuchsia-review.googlesource.com/c/fuchsia/+/632266/)

<!-- link labels -->
[rfcs]: /docs/contribute/governance/rfcs/README.md#proposals
[ftps]: /docs/contribute/governance/deprecated-ftp-process.md
[protobuf-self-describe]: https://developers.google.com/protocol-buffers/docs/techniques#self-description
[measure-tape]: /tools/fidl/measure-tape/README.md
[large-message-cl]: https://fuchsia-review.googlesource.com/c/fuchsia/+/470640
[rfc-0017]: /docs/contribute/governance/rfcs/0017_folding_ftp_into_rfc.md
[rfc-0018]: /docs/contribute/governance/rfcs/0018_ftp_process.md
[rfc-0019]: /docs/contribute/governance/rfcs/0019_using_evolution_uint64.md
[rfc-0020]: /docs/contribute/governance/rfcs/0020_interface_ordinal_hashing.md
[rfc-0021]: /docs/contribute/governance/rfcs/0021_soft_transitions_methods_add_remove.md
[rfc-0022]: /docs/contribute/governance/rfcs/0022_default_values_for_struct.md
[rfc-0023]: /docs/contribute/governance/rfcs/0023_compositional_model_protocols.md
[rfc-0024]: /docs/contribute/governance/rfcs/0024_mandatory_source_compatibility.md
[rfc-0025]: /docs/contribute/governance/rfcs/0025_bit_flags.md
[rfc-0026]: /docs/contribute/governance/rfcs/0026_envelopes_everywhere.md
[rfc-0027]: /docs/contribute/governance/rfcs/0027_you_only_pay_what_you_use.md
[rfc-0028]: /docs/contribute/governance/rfcs/0028_handle_rights.md
[rfc-0029]: /docs/contribute/governance/rfcs/0029_increasing_method_ordinals.md
[rfc-0030]: /docs/contribute/governance/rfcs/0030_fidl_is_little_endian.md
[rfc-0031]: /docs/contribute/governance/rfcs/0031_typed_epitaphs.md
[rfc-0032]: /docs/contribute/governance/rfcs/0032_efficient_envelopes.md
[rfc-0033]: /docs/contribute/governance/rfcs/0033_handling_unknown_fields_strictness.md
[rfc-0034]: /docs/contribute/governance/rfcs/0034_null_terminate_strings.md
[rfc-0035]: /docs/contribute/governance/rfcs/0035_automatic_flow_tracing.md
[rfc-0036]: /docs/contribute/governance/rfcs/0036_update_struct_declarations.md
[rfc-0037]: /docs/contribute/governance/rfcs/0037_transactional_message_header_v3.md
[rfc-0038]: /docs/contribute/governance/rfcs/0038_separating_layout_from_constraints.md
[rfc-0039]: /docs/contribute/governance/rfcs/0039_types_come_second.md
[rfc-0040]: /docs/contribute/governance/rfcs/0040_identifier_uniqueness.md
[rfc-0041]: /docs/contribute/governance/rfcs/0041_unifying_services_devices.md
[rfc-0042]: /docs/contribute/governance/rfcs/0042_non_nullable_types.md
[rfc-0043]: /docs/contribute/governance/rfcs/0043_documentation_comment_format.md
[rfc-0044]: /docs/contribute/governance/rfcs/0044_extensible_method_arguments.md
[rfc-0045]: /docs/contribute/governance/rfcs/0045_zero_size_empty_structs.md
[rfc-0047]: /docs/contribute/governance/rfcs/0047_tables.md
[rfc-0048]: /docs/contribute/governance/rfcs/0048_explicit_union_ordinals.md
[rfc-0049]: /docs/contribute/governance/rfcs/0049_fidl_tuning_process_evolution.md
[rfc-0050]: /docs/contribute/governance/rfcs/0050_syntax_revamp.md
[rfc-0051]: /docs/contribute/governance/rfcs/0051_safer_structs_for_cpp.md
[rfc-0052]: /docs/contribute/governance/rfcs/0052_type_aliasing_named_types.md
[rfc-0053]: /docs/contribute/governance/rfcs/0053_epitaphs.md
[rfc-0054]: /docs/contribute/governance/rfcs/0054_parameter_attributes.md
[rfc-0055]: /docs/contribute/governance/rfcs/0055_documentation_comments.md
[rfc-0056]: /docs/contribute/governance/rfcs/0056_empty_structs.md
[rfc-0057]: /docs/contribute/governance/rfcs/0057_default_no_handles.md
[rfc-0058]: /docs/contribute/governance/rfcs/0058_deprecated_attribute.md
[rfc-0059]: /docs/contribute/governance/rfcs/0059_reserved_bits_count_fields.md
[rfc-0060]: /docs/contribute/governance/rfcs/0060_error_handling.md
[rfc-0061]: /docs/contribute/governance/rfcs/0061_extensible_unions.md
[rfc-0062]: /docs/contribute/governance/rfcs/0062_method_impossible.md
[rfc-0063]: /docs/contribute/governance/rfcs/0063_OrdinalRange.md
[rfc-0064]: /docs/contribute/governance/rfcs/0064_box_knox.md
[rfc-0065]: /docs/contribute/governance/rfcs/0065_optional_strings_or_vectors.md
[rfc-0066]: /docs/contribute/governance/rfcs/0066_programmer_advisory_explicit.md
[rfc-0076]: /docs/contribute/governance/rfcs/0076_fidl_api_summaries.md
[rfc-0083]: /docs/contribute/governance/rfcs/0083_fidl_versioning.md
[rfc-0086]: /docs/contribute/governance/rfcs/0086_rfc_0050_attributes.md
[rfc-0087]: /docs/contribute/governance/rfcs/0087_fidl_method_syntax.md
[rfc-0088]: /docs/contribute/governance/rfcs/0088_rfc_0050_bits_enums_constraints.md
[rfc-0097]: /docs/contribute/governance/rfcs/0097_fidl_toolchain.md
[rfc-0113]: /docs/contribute/governance/rfcs/0113_efficient_envelopes.md
[rfc-0114]: /docs/contribute/governance/rfcs/0114_fidl_envelope_inlining.md
[rfc-0116]: /docs/contribute/governance/rfcs/0116_fidl_sparser_tables.md
[rfc-0120]: /docs/contribute/governance/rfcs/0120_standalone_use_of_fidl_wire_format.md
[rfc-0131]: /docs/contribute/governance/rfcs/0131_fidl_wire_format_principles.md
[rfc-0132]: /docs/contribute/governance/rfcs/0132_fidl_table_size_limit.md
[rfc-0137]: /docs/contribute/governance/rfcs/0137_discard_unknown_data_in_fidl.md
[rfc-0138]: /docs/contribute/governance/rfcs/0138_handling_unknown_interactions.md
[rfc-0149]: /docs/contribute/governance/rfcs/0149_fidl_encode_validation_not_mandatory.md
[rfc-0160]: /docs/contribute/governance/rfcs/0160_fidl_remove_struct_defaults.md
[rfc-0190]: /docs/contribute/governance/rfcs/0190_fidl_support_for_syscalls.md
[rfc-0196]: /docs/contribute/governance/rfcs/0196_fidl_large_messages.md