Failure on servo's WebGLContextEvent.webidl

[JMLX42]

Describe the Bug

The parsing of WebGLContextEvent.webidl:

* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15
[Exposed=Window]
interface WebGLContextEvent : Event {
    [Throws] constructor(DOMString type, optional WebGLContextEventInit eventInit = {});
    readonly attribute DOMString statusMessage;
};

// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15
dictionary WebGLContextEventInit : EventInit {
    DOMString statusMessage;
};

fails the following error:

[src/main.rs:29] parsed = Err(
    Failure(
        Code(
            CompleteStr(
                "/* This Source Code Form is subject to the terms of the Mozilla Public\n * License, v. 2.0. If a copy of the MPL was not distributed with this\n * file, You can obtain one at https://mozilla.org/MPL/2.0/. */\n\n// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15\n[Exposed=Window]\ninterface WebGLContextEvent : Event {\n    [Throws] constructor(DOMString type, optional WebGLContextEventInit eventInit = {});\n    readonly attribute DOMString statusMessage;\n};\n\n// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15\ndictionary WebGLContextEventInit : EventInit {\n    DOMString statusMessage;\n};\n",
            ),
            Custom(
                0,
            ),
        ),
    ),
)

Steps to Reproduce

let parsed = weedle::parse("
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/. */

// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15
[Exposed=Window]
interface WebGLContextEvent : Event {
    [Throws] constructor(DOMString type, optional WebGLContextEventInit eventInit = {});
    readonly attribute DOMString statusMessage;
};

// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.15
dictionary WebGLContextEventInit : EventInit {
    DOMString statusMessage;
};
");

The problem appears to be the following content: optional WebGLContextEventInit eventInit = {}. Removing the default value declaration to have optional WebGLContextEventInit eventInit makes the parsing work OK.

Expected Behavior

The WebIDL is parsed properly and the corresponding AST is returned.

[JMLX42]

Replacing:

    [Throws] constructor(DOMString type, optional WebGLContextEventInit eventInit = {});

with:

    [Throws] constructor(DOMString type, optional WebGLContextEventInit eventInit = 0);

make the parsing works. My understanding is that maybe {} is not a handled as a default value for args.

It looks like this part of the parsing is done here:

https://github.com/rustwasm/weedle/blob/master/src/argument.rs#L19

But I don't understand yet why 0 works but {} does not work.

[JMLX42]

Default is defined in common.rs and relies on DefaultValue which is defined in literals.rs and does have a specific case for the empty dict initializer:

https://github.com/rustwasm/weedle/blob/master/src/literal.rs#L63

So it's still unclear why 0 works but not {}.