Bug 1524822 - Fix warnings with rust 1.34+. r=jrmuizel

There's some new limited const fn support in stable, and this is the recommended
way to initialize atomics now.

If this for some reason doesn't compile in all platforms / versions we support
I'll just sprinkle some #[allow(deprecated)] instead.

Also, cargo changes the output of Cargo.lock, see
rust-lang/cargo#6180. So also update those comments.

Differential Revision: https://phabricator.services.mozilla.com/D18495

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/5f006a3c652c3a5def8800b9b0e65c4213b86785

webrender/src/device/gl.rs

@@ -27,7 +27,7 @@ use std::ptr;
 use std::rc::Rc;
 use std::slice;
 use std::sync::Arc;
-use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
+use std::sync::atomic::{AtomicUsize, Ordering};
 use std::thread;
 use webrender_build::shader::ProgramSourceDigest;
 use webrender_build::shader::{parse_shader_source, shader_source_from_file};
@@ -44,7 +44,7 @@ pub struct GpuFrameId(usize);
 /// to be atomic per se, but we make it atomic to satisfy the thread safety
 /// invariants in the type system. We could also put the value in TLS, but that
 /// would be more expensive to access.
-static GPU_BYTES_ALLOCATED: AtomicUsize = ATOMIC_USIZE_INIT;
+static GPU_BYTES_ALLOCATED: AtomicUsize = AtomicUsize::new(0);
 
 /// Returns the number of GPU bytes currently allocated.
 pub fn total_gpu_bytes_allocated() -> usize {

webrender/src/picture.rs

@@ -34,7 +34,7 @@ use scene::{FilterOpHelpers, SceneProperties};
 use scene_builder::Interners;
 use smallvec::SmallVec;
 use std::{mem, u16};
-use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
+use std::sync::atomic::{AtomicUsize, Ordering};
 use texture_cache::{Eviction, TextureCacheHandle};
 use tiling::RenderTargetKind;
 use util::{ComparableVec, TransformedRectKind, MatrixHelpers, MaxRect};
@@ -116,7 +116,7 @@ const MAX_PRIMS_TO_SEARCH: usize = 128;
 
 /// Used to get unique tile IDs, even when the tile cache is
 /// destroyed between display lists / scenes.
-static NEXT_TILE_ID: AtomicUsize = ATOMIC_USIZE_INIT;
+static NEXT_TILE_ID: AtomicUsize = AtomicUsize::new(0);
 
 fn clamp(value: i32, low: i32, high: i32) -> i32 {
     value.max(low).min(high)