I want to collect some notes on differences in StringData that could complicate a migration to std::string_view.

The BIG one: as of its introduction in C++17, std::string_view(cp) is UB for null-valued cp, but this is tolerated for StringData. This will be very difficult to audit for, as this is an implicit constructor.

Renames:

• rawData() should be called data().
• startsWith,endsWith (slated for C++20 as starts_with and ends_with).

Several public members must be brought outside the class:

• The range constructor.
• toString.
• (SERVER-48429) StringData::ComparatorInterface.
• equalCaseInsensitive
• copyTo

Behavior:
Exception behavior should match. std::string_view throws std::out_of_range.
Noexcept and constexpr annotations should be aligned with string_view.

• char operator[](unsigned) should take size_type and return const_reference.

Needs more members:

• Several typedefs need to be added.
• many more find/rfind variants, and these need offset parameters.
• remove_prefix / remove_suffix
• string_view has its own npos
• (SERVER-38248) explicit operator std::string instead of .toString, to approximate the string(string_view) ctor.
• member swap
• at, front, back, max_size, length
• (c?r?)(begin|end) iterator accessors.
• `explicit operator std::string() const`

Apparatus:

• needs std::hash specialization.
• nonmember swap.
• (SERVER-32434) put-to-stream operator<< needs to be made FormattedOutputFunction compliant.
  See https://en.cppreference.com/w/cpp/named_req/FormattedOutputFunction
  It's not as simple as the current stream.write(), unfortunately.