[Xlang] [Compiler] Flatbuffer Schema IDL support

[chaokunyang]

Feature Request

Add FlatBuffers IDL support to Fory compiler, enabling users to define schemas using FlatBuffers .fbs files and generate serialization code through Fory's compilation pipeline.

Is your feature request related to a problem? Please describe

Currently, Fory compiler only supports its native IDL format. Users who have existing FlatBuffers schemas or prefer FlatBuffers IDL syntax cannot leverage Fory's high-performance serialization without manually converting their schemas. This creates friction for teams migrating from FlatBuffers to Fory or those who want to use FlatBuffers IDL as their schema definition language.

Describe the solution you'd like

Implement FlatBuffers IDL parser and translator in Fory compiler with the following specifications:

1. FlatBuffers IDL Parser

• Parse .fbs files and translate to Fory IR (Intermediate Representation)

2. Scalar Type Mapping

FlatBuffers Type	Fory Type
byte	int8
ubyte	uint8
short / int16	int16
ushort / uint16	uint16
int / int32	varint32
uint / uint32	var_uint32
long / int64	varint64
ulong / uint64	var_uint64
float / float32	float32
double / float64	float64

3. Complex Type Mapping

FlatBuffers Construct	Fory Mapping
struct	Fory struct with evolving=false
table	Fory struct with evolving=true
enum	Fory enum
[T] (array/vector)	Fory ListType
attribute	Fory option
field attributes (e.g., deprecated, priority: 1)	Fory field options
union	Fory union

4. Auto growing field id

When translate flatbuffer table/struct schema to fory, we should allocate an autogrowing field id to each field, since flatbuffer use this order for schema compatibility.

flatbuffer:

table Foo {
  x: float;
  y: float;
}

fory:

message {
  float32 x = 0;
  float32 y = 1;
}

5. Implementation Tasks

• Implement FlatBuffers IDL lexer and parser
• Implement IR translator from FlatBuffers AST to Fory IR
• Add error handling for unsupported features (union types)
• Rename integration_tests/idl_tests/proto to integration_tests/idl_tests/idl and fix all tests
• Create comprehensive .fbs test file at integration_tests/idl_tests/idl covering all supported FlatBuffers IDL features except union
• Add documentation docs/compiler/flatbuffers-idl.md detailing:
  • Differences between FlatBuffers IDL and Fory IDL
  • Usage guide for FlatBuffers IDL with Fory compiler
  • Generated code differences and examples

Describe alternatives you've considered

1. Manual schema conversion: Users could manually convert FlatBuffers schemas to Fory IDL, but this is error-prone and time-consuming for large schemas.

2. External conversion tool: A standalone tool to convert .fbs to .fory files, but this adds extra steps and doesn't integrate well with the compilation pipeline.

3. Protocol Buffers only: Continue supporting only Protobuf IDL, but this limits adoption from the FlatBuffers community.

Additional context

Example FlatBuffers IDL input:

namespace MyGame.Sample;

enum Color : byte { Red = 0, Green, Blue }

struct Vec3 {
  x: float;
  y: float;
  z: float;
}

table Monster {
  pos: Vec3;
  mana: short = 150;
  hp: short = 100;
  name: string;
  friendly: bool = false (deprecated, priority: 1);
  inventory: [ubyte];
  color: Color = Blue;
}

root_type Monster;

Expected behavior:

• struct Vec3 maps to Fory struct with evolving=false (fixed layout)
• table Monster maps to Fory struct with evolving=true (schema evolution supported)
• Field attributes like deprecated and priority: 1 map to Fory field options
• Default values are preserved in generated code

References:

• FlatBuffers Schema Documentation
• Fory Compiler Documentation