Bug 1792920, part 3 - Make Rust use a new MozExternalRefCountType instead of XPIDL's nsrefcnt. r=nika

The remaining uses of XPIDL's definition of nsrefcnt in Rust are all
now related to the return type of nsISupport's AddRef and Release
methods. The C++ name of the return types is MozExternalRefCountType,
not nsrefcnt (which is something else entirely), so use a more accurate
name.

I can't use the C++ definition, because it is in C++, and I eventually
want to remove the XPIDL definition, because it isn't accurate for C++,
so in this patch I'll make a specific one for Rust, so the code is a
little easier to read.

Differential Revision: https://phabricator.services.mozilla.com/D159322

xpcom/rust/xpcom/src/refptr.rs

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use crate::interfaces::{nsISupports, nsrefcnt};
+use crate::interfaces::{nsISupports};
 use libc;
 use nserror::{nsresult, NS_OK};
 use std::cell::Cell;
@@ -15,6 +15,10 @@ use std::ptr::{self, NonNull};
 use std::sync::atomic::{self, AtomicUsize, Ordering};
 use threadbound::ThreadBound;
 
+// This should match the definition in mfbt/RefCountType.h, modulo the delicate
+// effort at maintaining binary compatibility with Microsoft COM on Windows.
+pub type MozExternalRefCountType = u32;
+
 /// A trait representing a type which can be reference counted invasively.
 /// The object is responsible for freeing its backing memory when its
 /// reference count reaches 0.
@@ -262,7 +266,7 @@ impl Refcnt {
 
     /// Increment the reference count. Returns the new reference count. This is
     /// unsafe as modifying this value can cause a use-after-free.
-    pub unsafe fn inc(&self) -> nsrefcnt {
+    pub unsafe fn inc(&self) -> MozExternalRefCountType {
         // XXX: Checked add?
         let new = self.0.get() + 1;
         self.0.set(new);
@@ -271,7 +275,7 @@ impl Refcnt {
 
     /// Decrement the reference count. Returns the new reference count. This is
     /// unsafe as modifying this value can cause a use-after-free.
-    pub unsafe fn dec(&self) -> nsrefcnt {
+    pub unsafe fn dec(&self) -> MozExternalRefCountType {
         // XXX: Checked sub?
         let new = self.0.get() - 1;
         self.0.set(new);
@@ -301,14 +305,14 @@ impl AtomicRefcnt {
 
     /// Increment the reference count. Returns the new reference count. This is
     /// unsafe as modifying this value can cause a use-after-free.
-    pub unsafe fn inc(&self) -> nsrefcnt {
+    pub unsafe fn inc(&self) -> MozExternalRefCountType {
         let result = self.0.fetch_add(1, Ordering::Relaxed) + 1;
         result.try_into().unwrap()
     }
 
     /// Decrement the reference count. Returns the new reference count. This is
     /// unsafe as modifying this value can cause a use-after-free.
-    pub unsafe fn dec(&self) -> nsrefcnt {
+    pub unsafe fn dec(&self) -> MozExternalRefCountType {
         let result = self.0.fetch_sub(1, Ordering::Release) - 1;
         if result == 0 {
             // We're going to destroy the object on this thread, so we need

xpcom/rust/xpcom/xpcom_macros/src/lib.rs

@@ -68,8 +68,8 @@
 //!
 //! ```ignore
 //! // Automatic nsISupports implementation
-//! unsafe fn AddRef(&self) -> nsrefcnt;
-//! unsafe fn Release(&self) -> nsrefcnt;
+//! unsafe fn AddRef(&self) -> MozExternalRefCountType;
+//! unsafe fn Release(&self) -> MozExternalRefCountType;
 //! unsafe fn QueryInterface(&self, uuid: &nsIID, result: *mut *mut libc::c_void) -> nsresult;
 //!
 //! // Allocates and initializes a new instance of this type. The values will
@@ -442,7 +442,7 @@ fn gen_casts(
     let qi = quote! {
         #base_qi
         if *uuid == #base_name::IID {
-            // Implement QueryInterface in terms of coersions.
+            // Implement QueryInterface in terms of coercions.
             self.addref();
             *result = self.coerce::<#base_name>()
                 as *const #base_name
@@ -710,7 +710,7 @@ fn xpcom_impl(args: AttributeArgs, template: ItemStruct) -> Result<TokenStream,
             }
 
             /// Automatically generated implementation of AddRef for nsISupports.
-            #vis unsafe fn AddRef(&self) -> ::xpcom::interfaces::nsrefcnt {
+            #vis unsafe fn AddRef(&self) -> ::xpcom::MozExternalRefCountType {
                 let new = self.__refcnt.inc();
                 ::xpcom::trace_refcnt::NS_LogAddRef(
                     self as *const _ as *mut ::xpcom::reexports::libc::c_void,
@@ -722,7 +722,7 @@ fn xpcom_impl(args: AttributeArgs, template: ItemStruct) -> Result<TokenStream,
             }
 
             /// Automatically generated implementation of Release for nsISupports.
-            #vis unsafe fn Release(&self) -> ::xpcom::interfaces::nsrefcnt {
+            #vis unsafe fn Release(&self) -> ::xpcom::MozExternalRefCountType {
                 let new = self.__refcnt.dec();
                 ::xpcom::trace_refcnt::NS_LogRelease(
                     self as *const _ as *mut ::xpcom::reexports::libc::c_void,