[ccan] [PATCH] list: fix -Wcast-qual warnings

[Rusty Russell]

Eric Wong <normalperson at yhbt.net> writes:
> These warnings were encountered when adding ccan/list to cmogstored
> (which uses -Wcast-qual).  This changes the public API for checking,
> but I doubt anybody is affected.

I hate -Wcast-qual; it's a terrible warning.

There are genuine reasons to cast away const in C, and if -Wcast-qual
only warned about casts which changed *more* than the constness it might
be useful.  But it doesn't.

Unfortunately the list_check* functions cast away const so you can use
them to pass through both const and non-const lists (just like
"strchr()").

The correct fix is:
(1) Add a test to show why this has to cast away const.
(2) Use ccan/cast which supports this.
(3) Run ccanlint -v --cflags="-g3 -ggdb -Wall -Wstrict-prototypes
-Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wwrite-strings -Wundef -DCCAN_STR_DEBUG=1 -Wcast-qual"
    to see what breaks.

There's one wrinkle: ccan/cast is LGPL.  I'm the main author, so I've
just pinged the only other contributor to check it's OK to relicense.

Expect commits soon...

Thanks!
Rusty.