[WIP] Make DOM bindings methods not take raw JSContext pointers

[csmoe]

address #20377

• create a struct JSContext(*mut jsapi::JSContext) type
• implement Deref for this new type so it's easy to obtain the inner context pointer
• add an unsafe from_ptr static method that returns a new instance of the type
• make the code generator declare trait methods that accept the new type instead of *mut JSContext
• adjust generated code to pass JSContext::from_ptr(cx) when necessary
• update all of the methods defined in components/script/dom that need to be changed
• remove any unnecessary uses of #[allow(unsafe_code)] on methods that no longer accept raw pointers

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes fix #__ (github issue number if applicable).

• There are tests for these changes OR
• These changes do not require tests because _____

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This change is 

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @asajeffrey (or someone else) soon.

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/script_runtime.rs
• @fitzgen: components/script/script_runtime.rs
• @KiChjang: components/script/script_runtime.rs

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!
• These commits modify script code, but no tests are modified. Please consider adding a test!

[KiChjang]

This looks like it would not compile, since there's a naming collision.

[KiChjang]

I believe this is a misnomer -- the struct that wraps the *mut JSContext is actually the safe JSContext, since we make sure that we don't have a null pointer reference when creating the struct. I would instead rename the JSContext import to RawJSContext.

[csmoe]

thanks, I'll address your advice in next commit.

[csmoe]

there are lots of raw JSContext waiting to be wrapped in dom, introduction hasn't been done, more commits will be pushed one day later.

[jdm]

Yes, that is definitely something we can do incrementally over time.

[bors-servo]

☔ The latest upstream changes (presumably #20246) made this pull request unmergeable. Please resolve the merge conflicts.

[csmoe]

There are some methods defined locally but declared from outside crates, for example:

servo/components/script/dom/bindings/conversions.rs

Lines 75 to 81 in a69ecee

	impl<T: Float + ToJSValConvertible> ToJSValConvertible for Finite<T> {
	#[inline]
	unsafe fn to_jsval(&self, cx: *mut JSContext, rval: MutableHandleValue) {
	let value = **self;
	value.to_jsval(cx, rval);
	}
	}

the to_jsval method is declared in mozjs crate:

pub trait ToJSValConvertible {
    /// Convert `self` to a `JSVal`. JSAPI failure causes a panic.
    #[inline]
    unsafe fn to_jsval(&self, cx: *mut JSContext, rval: MutableHandleValue);
}

this prevents me to introduce safe JSContext here. So can I leave that kinds of methods unchanged?

[jdm]

Yes. We can migrate this type into the upstream rust-mozjs crate at a later time if we decide it's worthwhile.

[KiChjang]

I'm not sure if the migration is possible, since I believe that these FromJSValConvertible implementations are implemented on types that only exist in servo and not rust-mozjs.

[jdm]

I meant migrating the new wrapper struct.

[csmoe]

I suspect that it'll be better if the safe JSContext were introduced into rust-mozjs before this commit, it's really trival to wrap RawJSContext in dom, there are many methods called directly from rust-mozjs, I have been working on this task, but the mixture makes codegen and other dom::components so hard to maintain.

If it's ok, I'll volunteer to create a PR for rust-mozjs and the related introductions for servo.

[jdm]

@csmoe Are you still planning to continue working on this?

[csmoe]

@jdm I'll restart this

[bors-servo]

☔ The latest upstream changes (presumably #21131) made this pull request unmergeable. Please resolve the merge conflicts.