RFC-0086：RFC-0050 更新：FIDL 屬性語法

@discoverable
@no_doc
@transport("Channel")
protocol P {};

const DEFAULT_TRANSPORT string = "Channel";

@transport(DEFAULT_TRANSPORT)
protocol P {};

const C bool = true;
@native(req_a="Foo",req_b=3)                  // OK: neither opt arg is set
@native(req_a="Foo",req_b=3,opt_c=C)          // OK: only opt_a is set
@native(req_a="Foo",req_b=3,opt_d=-4)         // OK: only opt_b is set
@native(req_a="Foo",req_b=3,opt_c=C,opt_d=-4) // OK: both opt args are set
@native(req_a="Foo",req_b=3,opt_d=-4,opt_c=C) // OK: arg order is irrelevant
@native(opt_d=-4,req_a="Foo",req_b=3)         // OK: arg order is irrelevant
@native(req_b=3)                              // Error: missing req_a
@native(req_a="Foo")                          // Error: missing req_b
type S = struct {};

@custom(a="Bar",b=true) // OK: grammatically correct
@custom("Bar",true)     // Error: bad grammar - multiple args require keywords
@custom("Bar")          // OK: correct grammar (though signature now unclear)
@custom(true)           // OK: correct grammar (signature even more unclear)
@custom()               // Error: bad grammar - cannot have empty arguments list
@custom                 // OK: grammatically correct
type S = struct {};

attribute = "@", IDENTIFIER , ( "(" , constant | attribute-args, ")" ) ;
attribute-args = attribute-arg | attribute-arg, "," attribute-args;
attribute-arg = IDENTIFIER , "=" , constant;

@foo_bar
@FooBar   // Error: re-used attribute name "foo_bar"
@fooBar   // Error: re-used attribute name "foo_bar"
@Foo_Bar  // Error: re-used attribute name "foo_bar"
@foo__bar // Error: re-used attribute name "foo_bar"
@FOOBar   // Error: re-used attribute name "foo_bar"
type S = struct {};

"attribute-arg": {
  {
    "description": "Definition of an attribute argument",
    "type": "object",
    "required": [
        "name",
        "value",
    ],
    "properties": {
      "name": {
        "description": "Name of the attribute argument",
        "type": "string",
      },
      "value": {
        "description": "Value of the attribute argument",
        "$ref": "#/definitions/constant",
      },
      "location": {
        "description": "Source location of the attribute argument",
        "$ref": "#/definitions/location"
      },
    },
  },
},
"attribute-args": {
  {
    "description": "Definition of an attributes argument list",
    "type": "array",
    "items": {
      "description": "List of arguments",
      "$ref": "#/definitions/attribute-arg",
    },
  },
},
"attribute": {
  {
    "description": "Definition of an attribute",
    "type": "object",
    "required": [
        "name",
        "arguments",
        "location",
    ],
    "properties": {
      "name": {
        "description": "Attribute name",
        "type": "string",
      },
      "arguments": {
        "description": "Attribute arguments",
        "$ref": "#/definitions/attribute-args",
      },
      "location": {
        "description": "Source location of the declaration",
        "$ref": "#/definitions/location",
      },
    },
  },
},
"compose": {
  {
    "description": "Compose member of an interface declaration",
    "type": "object",
    "required": [
        "name",
        "location",
    ],
    "properties": {
      "name": {
        "$ref": "#/definitions/compound-identifier",
        "description": "Name of the composed interface"
      },
      "maybe_attributes": {
        "description": "Optional list of attributes of the compose declaration",
        "$ref": "#/definitions/attributes-list",
      },
      "location": {
        "description": "Source location of the compose declaration",
        "$ref": "#/definitions/location",
      },
    },
  },
},

"maybe_attributes": [
  {
    "name": "native",
    "arguments": [
      // Note: the omitted opt_d is not included in the IR
      {
        "name": "req_a",
        "value": {
          "kind": "literal",
          "value": "Foo",
          "expression": "\"Foo\"",
          "value": {
            "kind": "string",
            "value": "Foo",
            "expression": "\"Foo\"",
          },
        },
      },
      {
        "name": "req_b",
        "value": {
          "kind": "literal",
          "value": "3",
          "expression": "3",
          "literal": {
            "kind": "numeric",
            "value": "3",
            "expression": "3",
          },
        },
      },
      {
        "name": "opt_c",
        "value": {
          "kind": "identifier",
          "value": "true",
          "expression": "C",
          "identifier": "example/C",
        },
      },
    ],
    "location": {
        "filename": "/path/to/src/fidl/file/example.fidl",
        "line": 4,
        "column": 0,
        "length": 36,
    },
  },
],

@available(since=2)
type User = table {
  // Was created with struct at version 2, so no @available attribute is needed.
  1: is_admin bool;
  // Deprecated in favor of, and eventually replaced by, the name field.
  @available(deprecated=3,removed=4,note="use UTF-8 `name` instead")
  2: uid vector<uint8>;
  // The current (unreleased) version constrains this definition.
  // Placing "removed" before "since" is discouraged, but won't fail compilation.
  @available(removed="HEAD",since=3)
  3: name string;
  @available(since="HEAD")
  3: name string:60;
};

type MyAttrArgs = table {
  1: foo string;
  2: bar int32;
  3: baz bool;
};
@myAttr(MyAttrArgs{foo: "abc", bar: 1, baz: false})
type MyStruct = struct {...};

const MAX_STR string = "123";
const MAX_NUM uint64 = 123;

@max_handles(123)     // Error: cannot use non-string types
@max_handles(MAX_STR) // Error: cannot use const identifiers
@max_handles(MAX_NUM) // Error: cannot use non-string types or const identifiers
@max_handles("123")   // OK: only string literals are allowed
type S = struct {};

@this_attr("Foo")
@test_for_this_attr(false)
protocol P {};