Auto merge of #22923 - peterjoel:issue_8539_prefs_refactor, r=jdm

#8539 Config preferences backend restructure

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A procedural macro for generating static structures for use as the backend for config preferences, as well a mapping from string names to accessors.

Preferences can be accessed and updated via a map-like interface with `String` keys, and now also via a convenience macro: `get_pref!(path.to.pref)`. Various `serde`-compatible field attributes are also supported, including renaming the string keys. This could be useful when changing the backend structure without breaking existing usages.

I have added the choice to use `i64` as well as `f64` for storing numbers. As it turns out, none of the existing preferences used non-integer values. Setting a floating point value from a command-line argument requires a decimal point in order to parse correctly.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #8539

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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-----

I have a few outstanding problems or questions:

1. I am unable to get rid of this warning:
    ```
    warning: unnecessary path disambiguator
       --> components/config/prefs.rs:130:51
        |
    130 |         accessor_type = crate::pref_util::Accessor::<Prefs, crate::pref_util::PrefValue>,
        |                                                   ^^ try removing `::`
    ```
    See: https://stackoverflow.com/questions/54710797/how-to-disable-unnecessary-path-disambiguator-warning
2. Several of the preferences in use were not represented in `prefs.json`. Some of these may be in error, but it is hard to tell. For example `js.offthread_compilation.enabled` vs `js.ion.offthread_compilation.enabled` could be different preferences or could be intended to have the same value.
3. Previously, several pieces of code provided default values when accessing a preference that may not be present. For example:
    ```Rust
    let DBL_CLICK_TIMEOUT = Duration::from_millis(
        PREFS
            .get("dom.document.dblclick_timeout")
            .as_u64()
            .unwrap_or(300),
    );
    ```
    Fallback values don't really make sense now and I've added these defaults to `prefs.json`. Does this sound right?
4. I have kept `PrefValue::Missing`, though it doesn't seem very useful any more. An error might be more appropriate now for an incorrect preference key. I've kept this partly because [`webdriver_server` uses it](https://github.com/servo/servo/blob/master/components/webdriver_server/lib.rs#L224).

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components/bluetooth/lib.rs

@@ -21,7 +21,7 @@ use device::bluetooth::{BluetoothGATTDescriptor, BluetoothGATTService};
 use embedder_traits::{EmbedderMsg, EmbedderProxy};
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
 use servo_config::opts;
-use servo_config::prefs::PREFS;
+use servo_config::pref;
 use servo_rand::{self, Rng};
 use std::borrow::ToOwned;
 use std::collections::{HashMap, HashSet};
@@ -65,7 +65,7 @@ pub trait BluetoothThreadFactory {
 impl BluetoothThreadFactory for IpcSender<BluetoothRequest> {
     fn new(embedder_proxy: EmbedderProxy) -> IpcSender<BluetoothRequest> {
         let (sender, receiver) = ipc::channel().unwrap();
-        let adapter = if Some(true) == PREFS.get("dom.bluetooth.enabled").as_boolean() {
+        let adapter = if pref!(dom.bluetooth.enabled) {
             BluetoothAdapter::init()
         } else {
             BluetoothAdapter::init_mock()

components/canvas/webgl_mode/inprocess.rs

@@ -11,7 +11,7 @@ use canvas_traits::webgl::{WebGLSender, WebVRCommand, WebVRRenderHandler};
 use euclid::Size2D;
 use fnv::FnvHashMap;
 use gleam::gl;
-use servo_config::prefs::PREFS;
+use servo_config::pref;
 use std::rc::Rc;
 
 /// WebGL Threading API entry point that lives in the constellation.
@@ -35,7 +35,7 @@ impl WebGLThreads {
             webrender_api_sender,
             webvr_compositor.map(|c| WebVRRenderWrapper(c)),
         );
-        let output_handler = if PREFS.is_dom_to_texture_enabled() {
+        let output_handler = if pref!(dom.webgl.dom_to_texture.enabled) {
             Some(Box::new(OutputHandler::new(
                 webrender_gl.clone(),
                 channel.clone(),

components/config/Cargo.toml

@@ -9,8 +9,6 @@ publish = false
 [lib]
 name = "servo_config"
 path = "lib.rs"
-test = false
-doctest = false
 
 [dependencies]
 euclid = "0.19"
@@ -20,10 +18,12 @@ lazy_static = "1"
 log = "0.4"
 num_cpus = "1.1.0"
 serde = "1.0"
+serde_derive = "1.0"
 serde_json = "1.0"
 servo_geometry = {path = "../geometry"}
 servo_url = {path = "../url"}
 url = "1.2"
+servo_config_plugins = { path = "../config_plugins" }
 
 [dev-dependencies]
 env_logger = "0.6"

components/config/lib.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+#![feature(core_intrinsics)]
 #![deny(unsafe_code)]
 
 #[macro_use]
@@ -11,10 +12,13 @@ extern crate log;
 #[macro_use]
 extern crate serde;
 
+pub mod pref_util;
+#[macro_use]
+pub mod prefs;
+
 pub mod basedir;
 #[allow(unsafe_code)]
 pub mod opts;
-pub mod prefs;
 
 pub fn servo_version() -> String {
     let cargo_version = env!("CARGO_PKG_VERSION");

components/config/opts.rs

@@ -5,13 +5,12 @@
 //! Configuration options for a single run of the servo application. Created
 //! from command line arguments.
 
-use crate::prefs::{self, PrefValue, PREFS};
+use crate::prefs::{self, PrefValue};
 use euclid::TypedSize2D;
 use getopts::Options;
 use servo_geometry::DeviceIndependentPixel;
 use servo_url::ServoUrl;
 use std::borrow::Cow;
-use std::cmp;
 use std::default::Default;
 use std::env;
 use std::fs::{self, File};
@@ -975,17 +974,11 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
         })
         .collect();
 
-    let do_not_use_native_titlebar = opt_match.opt_present("b") ||
-        !PREFS
-            .get("shell.native-titlebar.enabled")
-            .as_boolean()
-            .unwrap();
+    let do_not_use_native_titlebar =
+        opt_match.opt_present("b") || !(pref!(shell.native_titlebar.enabled));
 
-    let enable_subpixel_text_antialiasing = !debug_options.disable_subpixel_aa &&
-        PREFS
-            .get("gfx.subpixel-text-antialiasing.enabled")
-            .as_boolean()
-            .unwrap();
+    let enable_subpixel_text_antialiasing =
+        !debug_options.disable_subpixel_aa && pref!(gfx.subpixel_text_antialiasing.enabled);
 
     let is_printing_version = opt_match.opt_present("v") || opt_match.opt_present("version");
 
@@ -1065,15 +1058,7 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
     }
 
     if let Some(layout_threads) = layout_threads {
-        PREFS.set("layout.threads", PrefValue::Number(layout_threads as f64));
-    } else if let Some(layout_threads) = PREFS.get("layout.threads").as_string() {
-        PREFS.set(
-            "layout.threads",
-            PrefValue::Number(layout_threads.parse::<f64>().unwrap()),
-        );
-    } else if *PREFS.get("layout.threads") == PrefValue::Missing {
-        let layout_threads = cmp::max(num_cpus::get() * 3 / 4, 1);
-        PREFS.set("layout.threads", PrefValue::Number(layout_threads as f64));
+        set_pref!(layout.threads, layout_threads as i64);
     }
 
     ArgumentParsingResult::ChromeProcess
@@ -1104,15 +1089,26 @@ pub fn get() -> RwLockReadGuard<'static, Opts> {
 pub fn parse_pref_from_command_line(pref: &str) {
     let split: Vec<&str> = pref.splitn(2, '=').collect();
     let pref_name = split[0];
-    let value = split.get(1);
-    match value {
-        Some(&"false") => PREFS.set(pref_name, PrefValue::Boolean(false)),
-        Some(&"true") | None => PREFS.set(pref_name, PrefValue::Boolean(true)),
-        Some(value) => match value.parse::<f64>() {
-            Ok(v) => PREFS.set(pref_name, PrefValue::Number(v)),
-            Err(_) => PREFS.set(pref_name, PrefValue::String(value.to_string())),
+    let pref_value = parse_cli_pref_value(split.get(1).cloned());
+    prefs::pref_map()
+        .set(pref_name, pref_value)
+        .expect(format!("Error setting preference: {}", pref).as_str());
+}
+
+fn parse_cli_pref_value(input: Option<&str>) -> PrefValue {
+    match input {
+        Some("true") | None => PrefValue::Bool(true),
+        Some("false") => PrefValue::Bool(false),
+        Some(string) => {
+            if let Some(int) = string.parse::<i64>().ok() {
+                PrefValue::Int(int)
+            } else if let Some(float) = string.parse::<f64>().ok() {
+                PrefValue::Float(float)
+            } else {
+                PrefValue::from(string)
+            }
         },
-    };
+    }
 }
 
 pub fn parse_url_or_filename(cwd: &Path, input: &str) -> Result<ServoUrl, ()> {