Bug 1571631 has landed. The `MOZ_MUST_USE` macro has been removed and
all uses have been replaced with C++17's `[[nodiscard]]` function
attribute. Use [[nodiscard]] instead of MOZ_MUST_USE.
One caveat: unlike MOZ_MUST_USE, [[nodiscard]] must precede all of a
function declaration's declaration specifiers (like static, extern,
inline, or virtual):
- static inline MOZ_MUST_USE nsresult SomeFunction();
+ [[nodiscard]] static inline nsresult SomeFunction();
Next steps:
1. Replace MOZ_MUST_USE_TYPE type declarations with [[nodiscard]]. Bug
1628542
2. Consider removing `mozilla::Unused`, replacing over 3000 uses of
`Unused << SomeFunction()` with a more idiomatic `(void) SomeFunction()`
cast. mozilla::Unused was a workaround for the fact that gcc didn't
allow MOZ_MUST_USE (`__attribute__((warn_unused_result))`) warnings to
be suppressed with a void cast. Because this change would touch so many
files, it's probably not worth the trouble. Bug 1628542
On 12/21/2020 4:20 PM, Chris Peterson wrote:
Now that Firefox is compiled as C++17 (bug 1560664), you can use C++17's
[[nodiscard]] attribute [1] instead of the MOZ_MUST_USE macro (defined
using clang and gcc's non-standard __attribute__((warn_unused_result))).
I have been slowly replacing MOZ_MUST_USE with [[nodiscard]] in my free
time and hope to eventually remove the MOZ_MUST_USE definition itself.
That is meta bug 1571631.
In the meantime, please:
1. Avoid adding more uses of MOZ_MUST_USE. Use [[nodiscard]].
2. Consider making more functions use [[nodiscard]] when writing or
reviewing new code. Functions that return errors as nsresult or bool are
probably good candidates for [[nodiscard]].
(I looked at adding [[nodiscard]] to the nsresult type definition, but
the results were too noisy.)
One caveat: the [[nodiscard]] attribute must precede all of a function
declaration's declaration specifiers (like static, extern, inline, or
virtual). The __attribute__((warn_unused_result)) attribute (and thus
MOZ_MUST_USE) does not have this order restriction.
- static inline MOZ_MUST_USE nsresult SomeFunction();
+ [[nodiscard]] static inline nsresult SomeFunction();
Once __attribute__((warn_unused_result)) has been replaced with
[[nodiscard]], we can also remove mozilla::Unused, replacing `Unused <<`
with a more idiomatic `(void)` cast (bug 1628542).
[[nodiscard]] can also be applied to types, so we may be able to replace
our custom MOZ_MUST_USE_TYPE clang plugin with [[nodiscard]].
[1] https://en.cppreference.com/w/cpp/language/attributes/nodiscard
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