(Do not merge) Introduce pins

[nox]

Just pushing for people to take a look at it.

Things I want to do before we consider merging it:

• document the stuff;
• bikeshed the macro syntax;
• use said macro in more places;
• support non-'static types;
• protect code against dropping Pin values out of order;
• introduce an UnpinnedDefault trait;
• introduce a TracedVec type that doesn't implement drain, moving iterators, etc.

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/microtask.rs, components/script/script_runtime.rs, components/script/dom/bindings/mod.rs, components/script/dom/bindings/pin.rs, components/script/dom/macros.rs
• @fitzgen: components/script/microtask.rs, components/script/script_runtime.rs, components/script/dom/bindings/mod.rs, components/script/dom/bindings/pin.rs, components/script/dom/macros.rs
• @KiChjang: components/script/microtask.rs, components/script/script_runtime.rs, components/script/dom/bindings/mod.rs, components/script/dom/bindings/pin.rs, components/script/dom/macros.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!

[nox]

Cc @jdm @asajeffrey

[nox]

Note that this encoding allows Pin to be used trivially with Dom<T>: we can just implement Unpinned<&T> for Dom<T>.

[asajeffrey]

Ooooh, nice! On first glance, it looks good. I was wondering why we have Unpinned? It looks like the only place it's used is in the macro, Can we do something like:

macro_rules! pinned {
   (let ref $name:ident: $ty:ty := $expr:expr) => {
        let mut $name = $crate::dom::bindings::pin::Pin::<$ty>::new();
        let $name = unsafe { $name.pin($expr) };
    };
}

and ditto for let mut ref by adding a pin_mut() method to Pin?

[nox]

I was wondering why we have Unpinned?

Because owning a Dom<T> is unsafe (in the absence of the lint), so you can't just feed it directly to the Pin.

and ditto for let mut ref by adding a pin_mut() method to Pin?

Providing any mutable access is UB because it introduces aliasing when JS traces the pins.

[asajeffrey]

Ah that makes sense.

The IRC conversation about this: https://mozilla.logbot.info/servo/20180216#c14315715

[nox]

Note too that WebIDL unions can be trivially pinned with that encoding too:

#[derive(JSTraceable)]
pub enum DocumentOrBlob {
    Document(Dom<Document>),
    Blob(Dom<Blob>),
}

unsafe impl<'a> Unrooted<&'a Document> for DocumentOrBlob {
    unsafe fn unrooted(value: &'a Document) -> Self {
        DocumentOrBlob::Document(Unrooted::unrooted(value))
    }
}

unsafe impl<'a> Unrooted<&'a Blob> for DocumentOrBlob {
    unsafe fn unrooted(value: &'a Blob) -> Self {
        DocumentOrBlob::Blob(Unrooted::unrooted(value))
    }
}

[asajeffrey]

Some mucking around on the Playground: https://play.rust-lang.org/?gist=0b254dd788285573b19bf3abd8194ac8&version=stable, the gist being https://gist.github.com/anonymous/0b254dd788285573b19bf3abd8194ac8

IRC chat: https://mozilla.logbot.info/servo/20180220#c14335616 and https://mozilla.logbot.info/servo/20180221#c14337818 and https://mozilla.logbot.info/servo/20180221#c14339978

[bors-servo]

☔ The latest upstream changes (presumably #20101) made this pull request unmergeable. Please resolve the merge conflicts.

[nox]

@asajeffrey I replaced some rooted_vec! uses by pinned!.

[bors-servo]

☔ The latest upstream changes (presumably #20430) made this pull request unmergeable. Please resolve the merge conflicts.

[jdm]

Closing due to inactivity.