5dd8e020b7
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] https://github.com/llvm/llvm-project/issues/94184
Differential Revision: https://phabricator.services.mozilla.com/D231824
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