Duncan Coutts authored Nov 13, 2022 and Marge Bot committed Apr 03, 2024

Now instead of it being just used on Windows to select between the WinIO
vs the MIO or Win32-legacy I/O managers, it is now used on all platforms
for selecting the I/O manager to use.

Right now it remains the case that there is only an actual choice on
Windows, but that will change later.

Document the --io-manager flag in the user guide.

This change is also reflected in the RTS flags types in the base
library. Deprecate the export of IoSubSystem from GHC.RTS.Flags with a
message to import it from GHC.IO.Subsystem.

The way the 'IoSubSystem' is detected also changes. Instead of looking
at the RTS flag, there is now a C bool global var in the RTS which gets
set on startup when the I/O manager is selected. This bool var says
whether the selected I/O manager classifies as "native" on Windows,
which in practice means the WinIO I/O manager has been selected.

Similarly, the is_io_mng_native_p RTS helper function is re-implemented
in terms of the selected I/O manager, rather than based on the RTS
flags.

We do however remove the ./configure --native-io-manager flag because
we're bringing the WinIO/MIO/Win32-legacy choice under the new general
scheme for selecting I/O managers, and that new scheme involves no
./configure time user choices, just runtime RTS flag choices.