script: Make the resource task communication use IPC channels.
This change makes Servo use serialized messages over IPC channels for resource loading. The goal is to make it easier to make Servo multiprocess in the future. This patch does not make Servo multiprocess now; there are many other channels that need to be changed to IPC before that can happen. It does introduce a dependency on https://github.com/serde-rs/serde and https://github.com/pcwalton/ipc-channel for the first time.
At the moment, `ipc-channel` uses JSON for serialization. This is because serde does not yet have official support for bincode. When serde gains support for bincode, I'll switch to that. For now, however, the JSON encoding and decoding will constitute a significant performance regression in resource loading.
To avoid having to send boxed `AsyncResponseTarget` trait objects across process boundaries, this series of commits changes `AsyncResponseTarget` to wrap a sender only. It is then the client's responsibility to spawn a thread to proxy calls from that sender to the consumer of the resource data. This only had to be done in a few places. In the future, we may want to collapse those threads into one per process to reduce overhead. (It is impossible to continue to use `AsyncResponseTarget` as a boxed trait object across processes, regardless of how much work is done on `ipc-channel`. Vtables are fundamentally incompatible with IPC across mutually untrusting processes.)
In general, I was pretty pleased with how this turned out. The main changes are adding serialization functionality to various objects that `serde` does not know how to serialize natively—the most complicated being Hyper objects—and reworking `AsyncResponseTarget`. The overall structure of the code is unchanged, and other than `AsyncResponseTarget` no functionality was lost in moving to serialization and IPC.
r? @jdm
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6586)
<!-- Reviewable:end -->
Use local slice_chars
StrExt::slice_chars is deprecated and will be removed in Rust. This
lifts the implementation from Rust libstd and puts it in util::str.
This fixes a bunch of deprecation warnings in Servo.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6876)
<!-- Reviewable:end -->
StrExt::slice_chars is deprecated and will be removed in Rust. This
lifts the implementation from Rust libstd and puts it in util::str.
This fixes a bunch of deprecation warnings in Servo.
Send display lists over IPC in multiprocess mode.
This patch set introduces the `--multiprocess` (`-M`) switch. Right now, all it does it cause display lists to be serialized, but eventually it will cause actual processes to be spawned.
r? @metajack
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6795)
<!-- Reviewable:end -->
`OptionalIpcSender<T>`.
`OptionalIpcSender<T>`dynamically switches between in-process and
out-of-process communication depending on whether multiprocess mode is
enabled.
The multiprocess command-line switch doesn't actually turn on
multiprocess mode yet, but it does control the behavior of
`OptionalIpcSender<T>`.
compositing: Don't count the root layer when checking to see when to take the screenshot.
… as it isn't painted after first layout comes in.
r? @glennw
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6863)
<!-- Reviewable:end -->
