Bug 1500362 - Make GkAtoms opaque to avoid lld-link.exe errors r=emilio

Depends on D15078

Differential Revision: https://phabricator.services.mozilla.com/D15800

--HG--
extra : moz-landing-system : lando

layout/style/ServoBindings.toml

@@ -167,6 +167,7 @@ whitelist-vars = [
     "nsContentUtils_.*",
     "GECKO_IS_NIGHTLY",
     "mozilla::detail::gGkAtoms",
+    "mozilla::detail::kGkAtomsArrayOffset",
 ]
 whitelist-types = [
     "RawGecko.*",
@@ -377,6 +378,7 @@ opaque-types = [
     "mozilla::dom::.*Callback", # Pulls in ErrorResult and other things that
                                 # don't align properly on Linux 32-bit
     "mozilla::SchedulerGroup", # Non-standard-layout packing of field into superclass
+    "mozilla::detail::GkAtoms", # https://bugzilla.mozilla.org/show_bug.cgi?id=1517685
 ]
 
 # All cbindgen-types are in mod "structs::root::mozilla".

servo/components/style/gecko_string_cache/mod.rs

@@ -15,7 +15,9 @@ use crate::gecko_bindings::bindings::Gecko_Atomize;
 use crate::gecko_bindings::bindings::Gecko_Atomize16;
 use crate::gecko_bindings::bindings::Gecko_ReleaseAtom;
 use crate::gecko_bindings::structs::{nsAtom, nsDynamicAtom, nsStaticAtom};
+use crate::gecko_bindings::structs::root::mozilla::detail::GkAtoms_Atoms_AtomsCount;
 use crate::gecko_bindings::structs::root::mozilla::detail::gGkAtoms;
+use crate::gecko_bindings::structs::root::mozilla::detail::kGkAtomsArrayOffset;
 use nsstring::{nsAString, nsStr};
 use precomputed_hash::PrecomputedHash;
 use std::borrow::{Borrow, Cow};
@@ -57,11 +59,32 @@ pub struct Atom(usize);
 /// where `'a` is the lifetime of something that holds a strong reference to that atom.
 pub struct WeakAtom(nsAtom);
 
+/// The number of static atoms we have.
+const STATIC_ATOM_COUNT: usize = GkAtoms_Atoms_AtomsCount as usize;
+
+/// Returns the Gecko static atom array.
+///
+/// We have this rather than use rust-bindgen to generate
+/// mozilla::detail::gGkAtoms and then just reference gGkAtoms.mAtoms, so we
+/// avoid a problem with lld-link.exe on Windows.
+///
+/// https://bugzilla.mozilla.org/show_bug.cgi?id=1517685
+#[inline]
+fn static_atoms() -> &'static [nsStaticAtom; STATIC_ATOM_COUNT] {
+    unsafe {
+        let addr = &gGkAtoms as *const _ as usize + kGkAtomsArrayOffset as usize;
+        &*(addr as *const _)
+    }
+}
+
+/// Returns whether the specified address points to one of the nsStaticAtom
+/// objects in the Gecko static atom array.
 #[inline]
 fn valid_static_atom_addr(addr: usize) -> bool {
     unsafe {
-        let start = gGkAtoms.mAtoms.get_unchecked(0) as *const _;
-        let end = gGkAtoms.mAtoms.get_unchecked(gGkAtoms.mAtoms.len()) as *const _;
+        let atoms = static_atoms();
+        let start = atoms.get_unchecked(0) as *const _;
+        let end = atoms.get_unchecked(STATIC_ATOM_COUNT) as *const _;
         let in_range = addr >= start as usize && addr < end as usize;
         let aligned = addr % mem::align_of::<nsStaticAtom>() == 0;
         in_range && aligned
@@ -341,7 +364,7 @@ impl Atom {
     /// checking in release builds.
     #[inline]
     pub unsafe fn from_index(index: u16) -> Self {
-        let ptr = gGkAtoms.mAtoms.get_unchecked(index as usize) as *const _;
+        let ptr = static_atoms().get_unchecked(index as usize) as *const _;
         let handle = make_static_handle(ptr);
         let atom = Atom(handle);
         debug_assert!(valid_static_atom_addr(ptr as usize));

xpcom/ds/nsGkAtoms.h

@@ -109,6 +109,14 @@ struct GkAtoms {
   const nsStaticAtom mAtoms[static_cast<size_t>(Atoms::AtomsCount)];
 };
 
+// The offset from the start of the GkAtoms object to the start of the
+// nsStaticAtom array inside it.  This is used in Rust to avoid problems
+// with lld-link.exe on Windows when rust-bindgen generates a non-opaque
+// version of GkAtoms.
+//
+// https://bugzilla.mozilla.org/show_bug.cgi?id=1517685
+const ptrdiff_t kGkAtomsArrayOffset = offsetof(GkAtoms, mAtoms);
+
 // The GkAtoms instance is `extern const` so it can be defined in a .cpp file.
 //
 // XXX: The NS_EXTERNAL_VIS is necessary to work around an apparent GCC bug: