[small FR] An trivial "has been used" function in pybind11::class_ to avoid unused-raii finding

[tkoeppe]

ClangTidy has a finding https://clang.llvm.org/extra/clang-tidy/checks/bugprone/unused-raii.html which fires on an expression whose type has a non-trivial destructor if the expression is discarded:

struct Foo { Foo() {} ~Foo() {} };

void test() {
  Foo{};  // clang-tidy: warning: object destroyed immediately after creation; did you mean to name the object? [bugprone-unused-raii]
}

This happens for the very common expressions-statements of the form pybind11::class_<...>(...);. The finding does not appear once the value is used (either to initialize a variable or if member functions, such as def, are called on it.

A workaround could be to add an explicit discarding cast-to-void, but that seems quite ugly and hard to justify.

I would like to propose an alternative solution. This one is inspired by absl::Status, which is marked "nodiscard", and it offers a no-op member function IgnoreError so that you can spell f().IgnoreError(); to avoid the warning about not using the return value.

Similarly, we could add some no-op member function to class_ that one could spell to avoid the "immediately destroyed" finding. E.g. if it was hypothetically called end_of_def, we could spell:

pybind11::class_<T>(m, "T").end_of_def();

Would you at all be amenable to such an addition