Hi,

I've been working on integrating my jsonrpc.el library with very new
json.c library in Emacs 27.

After adding plist support to json.c, I got jsonrpc.el mostly working
(and eglot.el working on top of that). There is, however, a rough edge
in json.c that is breaking core functionality in eglot.el: given that
LSP servers may sometimes send :null as a key's value and sometimes omit
the key completely, and given that JSON null is unavoidably parsed as
the keyword :null, I have to explicitly check for non-`nil'ness *and*
non-`:null'ness, whereas before only one check was necessary.  Other
applications may care about the subtle difference, but eglot.el (and, I
think, LSP in general) doesn't.

This patch adds two new keyword arguments to json-parse-string and
json-parse-buffer to control this behaviour.  This is analogous to
json.el's `json-false' and `json-null'.

The defaults I have chosen ensure the change is backward compatible, but
maybe they needn't be, and other defaults compatible to json.el's could
be chosen. This is because I don't know of any json.c users: AFAIK
jsonrpc.el will be the first one.

Patch attached, or find the code in the
scratch/allow-custom-null-and-false-objects-in-jsonc branch

Thanks,
João