Move non-gfx things out of gfx_traits and create a base crate (#32296)

For a long time, `gfx_traits` has held a lot of things unrelated to graphics
and also unrelated to the `gfx` crate (which is mostly about fonts).
This is a cleanup which does a few things:

1. Move non `gfx` crate things out of `gfx_traits`. This is important in
   order to prevent dependency cycles with a different integration between
   layout, script, and fonts.
2. Rename the `msg` crate to `base`. It didn't really contain anything
   to do with messages and instead mostly holds ids, which are used
   across many different crates in Servo. This new crate will hold the
   *rare* data types that are widely used.

Details:

 - All BackgroundHangMonitor-related things from base to a new
   `background_hang_monitor_api` crate.
 - Moved `TraversalDirection` to `script_traits`
 - Moved `Epoch`-related things from `gfx_traits` to `base`.
 - Moved `PrintTree` to base. This should be widely useful in Servo.
 - Moved `WebrenderApi` from `base` to `webrender_traits` and renamed it
   to `WebRenderFontApi`.

components/script/dom/audiocontext.rs

@@ -4,9 +4,9 @@
 
 use std::rc::Rc;
 
+use base::id::PipelineId;
 use dom_struct::dom_struct;
 use js::rust::HandleObject;
-use msg::constellation_msg::PipelineId;
 use servo_media::audio::context::{LatencyCategory, ProcessingState, RealTimeAudioContextOptions};
 
 use crate::dom::baseaudiocontext::{BaseAudioContext, BaseAudioContextOptions};

components/script/dom/baseaudiocontext.rs

@@ -8,10 +8,10 @@ use std::collections::{HashMap, VecDeque};
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};
 
+use base::id::PipelineId;
 use dom_struct::dom_struct;
 use js::rust::CustomAutoRooterGuard;
 use js::typedarray::ArrayBuffer;
-use msg::constellation_msg::PipelineId;
 use servo_media::audio::context::{
     AudioContext, AudioContextOptions, OfflineAudioContextOptions, ProcessingState,
     RealTimeAudioContextOptions,

components/script/dom/bindings/structuredclone.rs

@@ -8,6 +8,7 @@ use std::collections::HashMap;
 use std::os::raw;
 use std::ptr;
 
+use base::id::{BlobId, MessagePortId};
 use js::glue::{
     CopyJSStructuredCloneData, DeleteJSAutoStructuredCloneBuffer, GetLengthOfJSStructuredCloneData,
     NewJSAutoStructuredCloneBuffer, WriteBytesToJSStructuredCloneData,
@@ -22,7 +23,6 @@ use js::jsapi::{
 use js::jsval::UndefinedValue;
 use js::rust::wrappers::{JS_ReadStructuredClone, JS_WriteStructuredClone};
 use js::rust::{CustomAutoRooterGuard, HandleValue, MutableHandleValue};
-use msg::constellation_msg::{BlobId, MessagePortId};
 use script_traits::serializable::BlobImpl;
 use script_traits::transferable::MessagePortImpl;
 use script_traits::StructuredSerializedData;

components/script/dom/blob.rs

@@ -7,11 +7,11 @@ use std::num::NonZeroU32;
 use std::ptr::NonNull;
 use std::rc::Rc;
 
+use base::id::{BlobId, BlobIndex, PipelineNamespaceId};
 use dom_struct::dom_struct;
 use encoding_rs::UTF_8;
 use js::jsapi::JSObject;
 use js::rust::HandleObject;
-use msg::constellation_msg::{BlobId, BlobIndex, PipelineNamespaceId};
 use net_traits::filemanager_thread::RelativePos;
 use script_traits::serializable::BlobImpl;
 use uuid::Uuid;

components/script/dom/dedicatedworkerglobalscope.rs

@@ -7,6 +7,7 @@ use std::sync::atomic::AtomicBool;
 use std::sync::Arc;
 use std::thread::{self, JoinHandle};
 
+use base::id::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
 use crossbeam_channel::{unbounded, Receiver, Sender};
 use devtools_traits::DevtoolScriptControlMsg;
 use dom_struct::dom_struct;
@@ -15,7 +16,6 @@ use ipc_channel::router::ROUTER;
 use js::jsapi::{Heap, JSContext, JSObject, JS_AddInterruptCallback};
 use js::jsval::UndefinedValue;
 use js::rust::{CustomAutoRooter, CustomAutoRooterGuard, HandleValue};
-use msg::constellation_msg::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
 use net_traits::image_cache::ImageCache;
 use net_traits::request::{
     CredentialsMode, Destination, ParserMetadata, Referrer, RequestBuilder, RequestMode,

components/script/dom/dissimilaroriginwindow.rs

@@ -2,11 +2,11 @@
  * 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/. */
 
+use base::id::PipelineId;
 use dom_struct::dom_struct;
 use js::jsapi::{Heap, JSObject};
 use js::jsval::{JSVal, UndefinedValue};
 use js::rust::{CustomAutoRooter, CustomAutoRooterGuard, HandleValue};
-use msg::constellation_msg::PipelineId;
 use script_traits::{ScriptMsg, StructuredSerializedData};
 use servo_url::ServoUrl;
 

components/script/dom/document.rs

@@ -13,6 +13,7 @@ use std::rc::Rc;
 use std::slice::from_ref;
 use std::time::{Duration, Instant};
 
+use base::id::BrowsingContextId;
 use canvas_traits::webgl::{self, WebGLContextId, WebGLMsg};
 use content_security_policy::{self as csp, CspList};
 use cookie::Cookie;
@@ -33,7 +34,6 @@ use metrics::{
     ProgressiveWebMetric,
 };
 use mime::{self, Mime};
-use msg::constellation_msg::BrowsingContextId;
 use net_traits::pub_domains::is_pub_domain;
 use net_traits::request::RequestBuilder;
 use net_traits::response::HttpsState;

components/script/dom/element.rs

@@ -15,6 +15,7 @@ use std::{fmt, mem};
 use cssparser::match_ignore_ascii_case;
 use devtools_traits::AttrInfo;
 use dom_struct::dom_struct;
+use embedder_traits::InputMethodType;
 use euclid::default::{Rect, Size2D};
 use html5ever::serialize::TraversalScope::{ChildrenOnly, IncludeNode};
 use html5ever::serialize::{SerializeOpts, TraversalScope};
@@ -25,7 +26,6 @@ use html5ever::{
 use js::jsapi::Heap;
 use js::jsval::JSVal;
 use js::rust::HandleObject;
-use msg::constellation_msg::InputMethodType;
 use net_traits::request::CorsSettings;
 use net_traits::ReferrerPolicy;
 use script_layout_interface::ReflowGoal;

components/script/dom/globalscope.rs

@@ -14,6 +14,10 @@ use std::thread::JoinHandle;
 use std::time::Instant;
 use std::{mem, ptr};
 
+use base::id::{
+    BlobId, BroadcastChannelRouterId, MessagePortId, MessagePortRouterId, PipelineId,
+    ServiceWorkerId, ServiceWorkerRegistrationId,
+};
 use content_security_policy::CspList;
 use crossbeam_channel::Sender;
 use devtools_traits::{PageError, ScriptToDevtoolsControlMsg};
@@ -34,10 +38,6 @@ use js::rust::{
     MutableHandleValue, ParentRuntime, Runtime,
 };
 use js::{JSCLASS_IS_DOMJSCLASS, JSCLASS_IS_GLOBAL};
-use msg::constellation_msg::{
-    BlobId, BroadcastChannelRouterId, MessagePortId, MessagePortRouterId, PipelineId,
-    ServiceWorkerId, ServiceWorkerRegistrationId,
-};
 use net_traits::blob_url_store::{get_blob_origin, BlobBuf};
 use net_traits::filemanager_thread::{
     FileManagerResult, FileManagerThreadMsg, ReadFileProgress, RelativePos,

components/script/dom/history.rs

@@ -5,15 +5,15 @@
 use std::cell::Cell;
 use std::cmp::Ordering;
 
+use base::id::HistoryStateId;
 use dom_struct::dom_struct;
 use js::jsapi::Heap;
 use js::jsval::{JSVal, NullValue, UndefinedValue};
 use js::rust::HandleValue;
-use msg::constellation_msg::{HistoryStateId, TraversalDirection};
 use net_traits::{CoreResourceMsg, IpcSend};
 use profile_traits::ipc;
 use profile_traits::ipc::channel;
-use script_traits::{ScriptMsg, StructuredSerializedData};
+use script_traits::{ScriptMsg, StructuredSerializedData, TraversalDirection};
 use servo_url::ServoUrl;
 
 use crate::dom::bindings::codegen::Bindings::HistoryBinding::HistoryMethods;

components/script/dom/htmliframeelement.rs

@@ -4,11 +4,11 @@
 
 use std::cell::Cell;
 
+use base::id::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
 use bitflags::bitflags;
 use dom_struct::dom_struct;
 use html5ever::{local_name, namespace_url, ns, LocalName, Prefix};
 use js::rust::HandleObject;
-use msg::constellation_msg::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
 use profile_traits::ipc as ProfiledIpc;
 use script_layout_interface::ReflowGoal;
 use script_traits::IFrameSandboxState::{IFrameSandboxed, IFrameUnsandboxed};

components/script/dom/htmlimageelement.rs

@@ -9,6 +9,7 @@ use std::sync::{Arc, Mutex};
 use std::{char, i32, mem};
 
 use app_units::{Au, AU_PER_PX};
+use base::id::PipelineId;
 use cssparser::{Parser, ParserInput};
 use dom_struct::dom_struct;
 use euclid::Point2D;
@@ -19,7 +20,6 @@ use ipc_channel::router::ROUTER;
 use js::jsapi::JSAutoRealm;
 use js::rust::HandleObject;
 use mime::{self, Mime};
-use msg::constellation_msg::PipelineId;
 use net_traits::image::base::{Image, ImageMetadata};
 use net_traits::image_cache::{
     CorsStatus, ImageCache, ImageCacheResult, ImageOrMetadataAvailable, ImageResponse,

components/script/dom/htmlinputelement.rs

@@ -11,7 +11,7 @@ use std::{f64, ptr};
 use chrono::naive::{NaiveDate, NaiveDateTime};
 use chrono::{DateTime, Datelike, Weekday};
 use dom_struct::dom_struct;
-use embedder_traits::FilterPattern;
+use embedder_traits::{FilterPattern, InputMethodType};
 use encoding_rs::Encoding;
 use html5ever::{local_name, namespace_url, ns, LocalName, Prefix};
 use js::jsapi::{
@@ -21,7 +21,6 @@ use js::jsapi::{
 use js::jsval::UndefinedValue;
 use js::rust::jsapi_wrapped::{ExecuteRegExpNoStatics, ObjectIsRegExp};
 use js::rust::{HandleObject, MutableHandleObject};
-use msg::constellation_msg::InputMethodType;
 use net_traits::blob_url_store::get_blob_origin;
 use net_traits::filemanager_thread::FileManagerThreadMsg;
 use net_traits::{CoreResourceMsg, IpcSend};

components/script/dom/htmlscriptelement.rs

@@ -12,6 +12,7 @@ use std::ptr;
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};
 
+use base::id::PipelineId;
 use content_security_policy as csp;
 use dom_struct::dom_struct;
 use encoding_rs::Encoding;
@@ -24,7 +25,6 @@ use js::rust::{
     transform_str_to_source_text, CompileOptionsWrapper, FinishOffThreadStencil, HandleObject,
     Stencil,
 };
-use msg::constellation_msg::PipelineId;
 use net_traits::request::{
     CorsSettings, CredentialsMode, Destination, ParserMetadata, RequestBuilder,
 };

components/script/dom/messageport.rs

@@ -8,10 +8,10 @@ use std::convert::TryInto;
 use std::num::NonZeroU32;
 use std::rc::Rc;
 
+use base::id::{MessagePortId, MessagePortIndex, PipelineNamespaceId};
 use dom_struct::dom_struct;
 use js::jsapi::{Heap, JSObject, MutableHandleObject};
 use js::rust::{CustomAutoRooter, CustomAutoRooterGuard, HandleValue};
-use msg::constellation_msg::{MessagePortId, MessagePortIndex, PipelineNamespaceId};
 use script_traits::PortMessageTask;
 
 use crate::dom::bindings::codegen::Bindings::EventHandlerBinding::EventHandlerNonNull;

components/script/dom/node.rs

@@ -13,6 +13,7 @@ use std::sync::Arc as StdArc;
 use std::{cmp, iter};
 
 use app_units::Au;
+use base::id::{BrowsingContextId, PipelineId};
 use bitflags::bitflags;
 use devtools_traits::NodeInfo;
 use dom_struct::dom_struct;
@@ -22,7 +23,6 @@ use js::jsapi::JSObject;
 use js::rust::HandleObject;
 use libc::{self, c_void, uintptr_t};
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image::base::{Image, ImageMetadata};
 use script_layout_interface::{
     GenericLayoutData, HTMLCanvasData, HTMLMediaData, LayoutElementType, LayoutNodeType, QueryMsg,

components/script/dom/offlineaudiocontext.rs

@@ -7,9 +7,9 @@ use std::rc::Rc;
 use std::sync::{mpsc, Arc, Mutex};
 use std::thread::Builder;
 
+use base::id::PipelineId;
 use dom_struct::dom_struct;
 use js::rust::HandleObject;
-use msg::constellation_msg::PipelineId;
 use servo_media::audio::context::OfflineAudioContextOptions as ServoMediaOfflineAudioContextOptions;
 
 use crate::dom::audiobuffer::{AudioBuffer, MAX_SAMPLE_RATE, MIN_SAMPLE_RATE};

components/script/dom/paintworkletglobalscope.rs

@@ -10,6 +10,7 @@ use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
+use base::id::PipelineId;
 use crossbeam_channel::{unbounded, Sender};
 use dom_struct::dom_struct;
 use euclid::{Scale, Size2D};
@@ -20,7 +21,6 @@ use js::jsapi::{
 use js::jsval::{JSVal, ObjectValue, UndefinedValue};
 use js::rust::wrappers::{Call, Construct1};
 use js::rust::{HandleValue, Runtime};
-use msg::constellation_msg::PipelineId;
 use net_traits::image_cache::ImageCache;
 use pixels::PixelFormat;
 use profile_traits::ipc;

components/script/dom/serviceworker.rs

@@ -4,10 +4,10 @@
 
 use std::cell::Cell;
 
+use base::id::ServiceWorkerId;
 use dom_struct::dom_struct;
 use js::jsapi::{Heap, JSObject};
 use js::rust::{CustomAutoRooter, CustomAutoRooterGuard, HandleValue};
-use msg::constellation_msg::ServiceWorkerId;
 use script_traits::{DOMMessage, ScriptMsg};
 use servo_url::ServoUrl;
 

components/script/dom/serviceworkerglobalscope.rs

@@ -7,6 +7,7 @@ use std::sync::Arc;
 use std::thread::{self, JoinHandle};
 use std::time::{Duration, Instant};
 
+use base::id::PipelineId;
 use crossbeam_channel::{after, unbounded, Receiver, Sender};
 use devtools_traits::DevtoolScriptControlMsg;
 use dom_struct::dom_struct;
@@ -14,7 +15,6 @@ use ipc_channel::ipc::{IpcReceiver, IpcSender};
 use ipc_channel::router::ROUTER;
 use js::jsapi::{JSContext, JS_AddInterruptCallback};
 use js::jsval::UndefinedValue;
-use msg::constellation_msg::PipelineId;
 use net_traits::request::{CredentialsMode, Destination, ParserMetadata, Referrer, RequestBuilder};
 use net_traits::{CustomResponseMediator, IpcSend};
 use parking_lot::Mutex;

components/script/dom/serviceworkerregistration.rs

@@ -4,9 +4,9 @@
 
 use std::cell::Cell;
 
+use base::id::ServiceWorkerRegistrationId;
 use devtools_traits::WorkerId;
 use dom_struct::dom_struct;
-use msg::constellation_msg::ServiceWorkerRegistrationId;
 use script_traits::{ScopeThings, WorkerScriptLoadOrigin};
 use servo_url::ServoUrl;
 use uuid::Uuid;

components/script/dom/servoparser/mod.rs

@@ -6,6 +6,7 @@ use std::borrow::Cow;
 use std::cell::Cell;
 use std::mem;
 
+use base::id::PipelineId;
 use base64::engine::general_purpose;
 use base64::Engine as _;
 use content_security_policy::{self as csp, CspList};
@@ -20,7 +21,6 @@ use html5ever::tree_builder::{ElementFlags, NextParserState, NodeOrText, QuirksM
 use html5ever::{local_name, namespace_url, ns, Attribute, ExpandedName, LocalName, QualName};
 use hyper_serde::Serde;
 use mime::{self, Mime};
-use msg::constellation_msg::PipelineId;
 use net_traits::{
     FetchMetadata, FetchResponseListener, Metadata, NetworkError, ResourceFetchTiming,
     ResourceTimingType,

components/script/dom/servoparser/prefetch.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/. */
 
+use base::id::PipelineId;
 use html5ever::buffer_queue::BufferQueue;
 use html5ever::tokenizer::states::RawKind;
 use html5ever::tokenizer::{
@@ -9,7 +10,6 @@ use html5ever::tokenizer::{
 };
 use html5ever::{local_name, Attribute, LocalName};
 use js::jsapi::JSTracer;
-use msg::constellation_msg::PipelineId;
 use net_traits::request::{CorsSettings, CredentialsMode, ParserMetadata, Referrer};
 use net_traits::{CoreResourceMsg, FetchChannels, IpcSend, ReferrerPolicy, ResourceThreads};
 use servo_url::{ImmutableOrigin, ServoUrl};

components/script/dom/testworkletglobalscope.rs

@@ -4,10 +4,10 @@
 
 use std::collections::HashMap;
 
+use base::id::PipelineId;
 use crossbeam_channel::Sender;
 use dom_struct::dom_struct;
 use js::rust::Runtime;
-use msg::constellation_msg::PipelineId;
 use servo_url::ServoUrl;
 
 use crate::dom::bindings::cell::DomRefCell;

components/script/dom/window.rs

@@ -16,6 +16,7 @@ use std::{cmp, env, mem};
 
 use app_units::Au;
 use backtrace::Backtrace;
+use base::id::{BrowsingContextId, PipelineId};
 use base64::Engine;
 use bluetooth_traits::BluetoothRequest;
 use canvas_traits::webgl::WebGLChan;
@@ -26,7 +27,6 @@ use dom_struct::dom_struct;
 use embedder_traits::{EmbedderMsg, PromptDefinition, PromptOrigin, PromptResult};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect};
 use euclid::{Point2D, Rect, Scale, Size2D, Vector2D};
-use gfx_traits::combine_id_with_fragment_type;
 use ipc_channel::ipc::{self, IpcSender};
 use ipc_channel::router::ROUTER;
 use js::conversions::ToJSValConvertible;
@@ -38,7 +38,6 @@ use js::rust::{
 };
 use malloc_size_of::MallocSizeOf;
 use media::WindowGLContext;
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use net_traits::image_cache::{
     ImageCache, ImageResponder, ImageResponse, PendingImageId, PendingImageResponse,
 };
@@ -50,7 +49,8 @@ use profile_traits::ipc as ProfiledIpc;
 use profile_traits::mem::ProfilerChan as MemProfilerChan;
 use profile_traits::time::ProfilerChan as TimeProfilerChan;
 use script_layout_interface::{
-    Layout, PendingImageState, QueryMsg, Reflow, ReflowGoal, ScriptReflow, TrustedNodeAddress,
+    combine_id_with_fragment_type, FragmentType, Layout, PendingImageState, QueryMsg, Reflow,
+    ReflowGoal, ScriptReflow, TrustedNodeAddress,
 };
 use script_traits::webdriver_msg::{WebDriverJSError, WebDriverJSResult};
 use script_traits::{
@@ -2110,10 +2110,7 @@ impl Window {
             .borrow_mut()
             .insert(node.to_opaque(), Vector2D::new(x_ as f32, y_ as f32));
         let scroll_id = ExternalScrollId(
-            combine_id_with_fragment_type(
-                node.to_opaque().id(),
-                gfx_traits::FragmentType::FragmentBody,
-            ),
+            combine_id_with_fragment_type(node.to_opaque().id(), FragmentType::FragmentBody),
             self.pipeline_id().into(),
         );
 

components/script/dom/windowproxy.rs

@@ -5,6 +5,7 @@
 use std::cell::Cell;
 use std::ptr;
 
+use base::id::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
 use dom_struct::dom_struct;
 use embedder_traits::EmbedderMsg;
 use html5ever::local_name;
@@ -27,7 +28,6 @@ use js::jsval::{JSVal, NullValue, PrivateValue, UndefinedValue};
 use js::rust::wrappers::{JS_TransplantObject, NewWindowProxy, SetWindowProxy};
 use js::rust::{get_object_class, Handle, MutableHandle};
 use js::JSCLASS_IS_GLOBAL;
-use msg::constellation_msg::{BrowsingContextId, PipelineId, TopLevelBrowsingContextId};
 use net_traits::request::Referrer;
 use script_traits::{
     AuxiliaryBrowsingContextLoadInfo, HistoryEntryReplacement, LoadData, LoadOrigin, NewLayoutInfo,

components/script/dom/workerglobalscope.rs

@@ -7,6 +7,7 @@ use std::rc::Rc;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 
+use base::id::{PipelineId, PipelineNamespace};
 use crossbeam_channel::Receiver;
 use devtools_traits::{DevtoolScriptControlMsg, WorkerId};
 use dom_struct::dom_struct;
@@ -14,7 +15,6 @@ use ipc_channel::ipc::IpcSender;
 use js::jsval::UndefinedValue;
 use js::panic::maybe_resume_unwind;
 use js::rust::{HandleValue, ParentRuntime};
-use msg::constellation_msg::{PipelineId, PipelineNamespace};
 use net_traits::request::{
     CredentialsMode, Destination, ParserMetadata, RequestBuilder as NetRequestInit,
 };

components/script/dom/worklet.rs

@@ -18,11 +18,11 @@ use std::sync::atomic::{AtomicIsize, Ordering};
 use std::sync::Arc;
 use std::thread;
 
+use base::id::PipelineId;
 use crossbeam_channel::{unbounded, Receiver, Sender};
 use dom_struct::dom_struct;
 use js::jsapi::{GCReason, JSGCParamKey, JSTracer, JS_GetGCParameter, JS_GC};
 use malloc_size_of::malloc_size_of_is_0;
-use msg::constellation_msg::PipelineId;
 use net_traits::request::{Destination, RequestBuilder, RequestMode};
 use net_traits::IpcSend;
 use servo_url::{ImmutableOrigin, ServoUrl};

components/script/dom/workletglobalscope.rs

@@ -5,13 +5,13 @@
 use std::borrow::Cow;
 use std::sync::Arc;
 
+use base::id::PipelineId;
 use crossbeam_channel::Sender;
 use devtools_traits::ScriptToDevtoolsControlMsg;
 use dom_struct::dom_struct;
 use ipc_channel::ipc::IpcSender;
 use js::jsval::UndefinedValue;
 use js::rust::Runtime;
-use msg::constellation_msg::PipelineId;
 use net_traits::image_cache::ImageCache;
 use net_traits::ResourceThreads;
 use parking_lot::Mutex;

components/script/dom/xrsystem.rs

@@ -5,10 +5,10 @@
 use std::cell::Cell;
 use std::rc::Rc;
 
+use base::id::PipelineId;
 use dom_struct::dom_struct;
 use ipc_channel::ipc::{self as ipc_crate, IpcReceiver};
 use ipc_channel::router::ROUTER;
-use msg::constellation_msg::PipelineId;
 use profile_traits::ipc;
 use servo_config::pref;
 use webxr_api::{Error as XRError, Frame, Session, SessionInit, SessionMode};