Bug 1935345 - [1/1] add AttributeMacros to .clang-format r=sylvestre

C++ technically allows the construct `struct foo bar{}` as an
initialization: "struct foo" (a C-ism) is the type name, while "bar" is
the variable name and "{}" is the brace-initialization.

This construct should be formatted in a completely different way than
the type-declaration `struct FOO bar {}`: here "FOO" is a macro that
expands to an `__attribute__((...)))` definition or a comment, "bar" is
the type name, and "{}" is the class body.

These constructs are formally ambiguous at the level of a code formatter
without access to macro-expansion and the like. Unfortunately, after
years of assuming that the intended construct -- common in C++ codebases
like, say, Gecko -- is the latter, clang-format has suddenly decided to
assume instead that it's the former. [1]

To preserve the previous formatter behavior, add the `AttributeMacros`
option to our `.clang-format`, starting it off with a list of all the
attribute-macro names from mfbt/Attributes.h which are marked as
applying to classes.

[1] llvm/llvm-project#94184

Differential Revision: https://phabricator.services.mozilla.com/D231824

Author: selenography

.clang-format

@@ -24,6 +24,31 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END_INHERITED|\
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END_INHERITED|\
 NS_QUERYFRAME_TAIL.*$"
 
+# Attribute macros used in classes. Workaround for behavior noted at
+# https://github.com/llvm/llvm-project/issues/94184.
+AttributeMacros: [
+    MOZ_STATIC_CLASS,
+    MOZ_STATIC_LOCAL_CLASS,
+    MOZ_STACK_CLASS,
+    MOZ_NONHEAP_CLASS,
+    MOZ_HEAP_CLASS,
+    MOZ_NON_TEMPORARY_CLASS,
+    MOZ_TEMPORARY_CLASS,
+    MOZ_RAII,
+    MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS,
+    MOZ_TRIVIAL_CTOR_DTOR,
+    MOZ_IS_REFPTR,
+    MOZ_IS_SMARTPTR_TO_REFCOUNTED,
+    MOZ_NEEDS_NO_VTABLE_TYPE,
+    MOZ_NON_MEMMOVABLE,
+    MOZ_NEEDS_MEMMOVABLE_TYPE,
+    MOZ_NEEDS_MEMMOVABLE_MEMBERS,
+    MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS,
+    MOZ_NON_PARAM,
+    MOZ_NON_AUTOABLE
+]
+
+
 SortIncludes: false
 IndentPPDirectives: AfterHash
 StatementMacros: [MARKUPMAP, ASSERT_TRUE, ASSERT_FALSE, TEST, CHECK]