Problem:
- Many constructors are defined as `{}` rather than using the ` =
default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
instead of requiring the caller to default construct. This violates
the C++ Core Guidelines suggestion to declare single-argument
constructors explicit
(https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).
Solution:
- Change default constructors to use the compiler-provided default
constructor.
- Remove implicit conversion operators from `nullptr_t` and change
usage to enforce type consistency without conversion.
I was confused by the trim() API, thinking it would mutate the span it
was called on. Mark all const functions that return a new span with
[[nodiscard]] so we can catch such mistakes.
Problem:
- `Span` is not `constexpr` aware.
Solution:
- Add `constexpr` support for all parts that do not require
`reinterpret_cast`.
- Modify tests which use the `constexpr` functions.
Two changes were made
1. copy_to() and copy_trimmed_to() now return how many bytes were
copied.
2. The argument was changed to Span<typename RemoveConst<T>::Type>
because the following would not work:
ReadonlyBytes bytes0;
Bytes bytes1;
// Won't work because this calls Span<const u8>::copy_to(Span<u8>)
// but the method was defined as Span<const u8>::copy_to(Span<const u8>)
bytes0.copy_to(bytes1);
I accidently wrote `Span<RemoveConst<T>>` when I meant
`Span<RemoveConst<T>::Type>`.
Changing that wouldn't be enough though, this constructor can only be
defined if T is not const, otherwise it would redefine the copy
constructor. This can be avoided by overloading the cast operator.
