servo: Merge #14244 - Use build script to generate binding files (from upsuper:buildtime-bindgen); r=emilio

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This is a WIP patch to make rust-bindgen a build time dependency of style component for stylo. This should make things more automatic. I convert majority of `regen.py` to `build_gecko.rs`.

It currently doesn't work on Windows, because of rust-lang/rust-bindgen#271 (it works when I fix the generated file manually, though). I haven't tested other platforms.

It would break servo's CI for geckolib, because:
1. it needs libclang (which is probably easy to solve)
2. it needs `MOZ_OBJDIR` to be passed in so that it can generate bindings

Manishearth emilio Do you agree with this change? Do you have suggestion for the issues above?

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- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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Source-Repo: https://github.com/servo/servo
Source-Revision: 872ec89a9c546eb05246b5047aabfc032d140eff

UltraBlame original commit: 78635ebabfc9e63971829a549388d1b61a75a503

Author: marco-c

servo/Cargo.lock

@@ -14,6 +14,7 @@ doctest = false
 
 [features]
 gecko = ["nsstring_vendor", "num_cpus", "rayon/unstable"]
+bindgen = ["libbindgen", "regex"]
 servo = ["serde/unstable", "serde", "serde_derive", "heapsize_derive",
          "style_traits/servo", "app_units/plugins", "servo_atoms", "html5ever-atoms",
          "cssparser/heap_size", "cssparser/serde-serialization",
@@ -65,5 +66,8 @@ version = "1.0"
 kernel32-sys = "0.2"
 
 [build-dependencies]
+lazy_static = "0.2"
+libbindgen = {version = "0.1.1", optional = true}
 phf_codegen = "0.7.20"
+regex = {version = "0.1", optional = true}
 walkdir = "0.1"

servo/components/style/Cargo.toml

@@ -5,8 +5,3 @@ This directory contains simple tools for generating the Rust bindings for [stylo
 ## `setup_bindgen.sh`
 
 This clones Servo's version of bindgen, and uses `llvm-3.8` library to build it. It will then be used to generate the Rust bindings.
-
-## `regen.sh`
-
-This will regenerate the bindings for the `ServoBindings.h` file in your gecko
-build. The generated bindings live in `components/style/gecko_bindings/bindings.rs`. For structs, the bindings are in `components/style/gecko_bindings/structs_*`