Bug 220002 – Define _LIBCPP_ENABLE_NODISCARD when building WebKit

• Component: WebKit Misc.
• Status: NEW

[David Kilzer (:ddkilzer)]

Define _LIBCPP_ENABLE_NODISCARD when building WebKit.

This will catch errors where a developer means to declare a variable (like a resource acquisition is initialization (RAII) object), but forgets to give it a variable name:

    SpmeRAIIObject { };

This instantiates the object then destroys it in a single statement because the object's scope is the statement itself.

The developer probably meant to give a variable like this instead:

    SpmeRAIIObject object { };

Note that I tried compiling a Release build of WebKit for macOS with this enabled, but I didn't find any issues.

[Radar WebKit Bug Importer]

<rdar://problem/72453662>

[Joseph Pecoraro]

Nice!

[Joseph Pecoraro]

(In reply to David Kilzer (:ddkilzer) from comment #0)
> Define _LIBCPP_ENABLE_NODISCARD when building WebKit.
> 
> This will catch errors where a developer means to declare a variable (like a
> resource acquisition is initialization (RAII) object), but forgets to give
> it a variable name:
> 
>     SpmeRAIIObject { };

To be clear the `_LIBCPP_ENABLE_NODISCARD` only adds the `[[nodiscard]]` annotation to more of the `std` Standard Library Functions than already have it since C++17.

---

If WebKit has its own classes where this is important (Lock Guards / Mutex Holders, Allocation / Copy / Ownership passing methods) then they should be annotated with `[[nodiscard]]` in order to get such warnings. Currently I see no `[[nodiscard]]` use in WebKit, but there is `WARN_UNUSED_RETURN` which is probably pretty similar.

Things like:

  * RAII constructors
    - WTF::Locker for example
  * Ownership Passing methods that may leak if the result is not used
    - WTF::MallocPtr::leakPtr for example
  * Allocation methods that may leak if the result is not used
    - WTF::fastMalloc for example

[Joseph Pecoraro]

https://en.cppreference.com/w/cpp/language/attributes/nodiscard