Deal with exceptions from custom element reaction steps

[jdm]

whatwg/html#3235

[tigercosmos]

@jdm Could you point out where is the related code for https://html.spec.whatwg.org/multipage/custom-elements.html#cereactions ?
I try to find it in components/script/dom/customelementregistry.rs, but with no result. 😞

[cbrewster]

@tigercosmos the relevant code is in codegen:

servo/components/script/dom/bindings/codegen/CodegenRust.py

Lines 3258 to 3270 in 36f5b69

	if hasCEReactions:
	self.cgRoot.append(CGGeneric("push_new_element_queue();\n"))
	self.cgRoot.append(CGList([
	CGGeneric("let result: "),
	result,
	CGGeneric(" = "),
	call,
	CGGeneric(";"),
	]))
	if hasCEReactions:
	self.cgRoot.append(CGGeneric("pop_current_element_queue();\n"))

[tigercosmos]

I am not familiar with CodeGen. What's the purpose of CodeGen? Why we use this way?
I am confused about code like this:

        self.cgRoot.append(CGList([
            CGGeneric("let result: "),
            result,
            CGGeneric(" = "),
            call,
            CGGeneric(";"),
        ]))

[jdm]

The code that is generated by CodeGenRust.py is responsible for:

• creating JS objects that have properties and methods that match WebIDL interfaces
• providing glue code between the JS properties and methods that are implemented in native Rust code
• converting JS value types to native Rust types in well-defined ways

Since each of these is very repetitive and easy to mess up, we generate lots of very similar code during the build instead of handwriting it. Python code like the one you quote are building up Rust code piece by piece - a CGList is a list of generated code snippets; CGGeneric is just a raw string that will be included in the generated code.

[jdm]

You can find the generated code output for a given Something.webidl in target/debug/build/script-[hash]/out/Bindings/InterfaceBinding.rs.