Hello,

Unlike project-shell-command, project-compile first prompts for a
command, then binds default-directory and calls compile.  Binding
default-directory first makes completion work from the project root,
which is useful for completing on filenames relative to the root, on
targets from the toplevel Makefile, etc.

I see three ways to achieve this:

(1) Rewrite project-compile with call-interactively, the way
project-shell-command is written (see patch #1).

(2) Set COMMAND to nil in the interactive spec, then prompt for it after
binding default-directory (see patch #2).

(3) Let-binding default-directory once in the interactive spec, and
again before calling compile.


I'm assuming (1) is out of the question, given 2020-06-02
"* lisp/progmodes/project.el (project-vc-dir, project-shell): New
commands." (2c1e5b9e77).  I'm CC'ing Juri to get his opinion though;
project-compile is new in Emacs 28.1, so its argument list is not yet
set in stone.

I've taken a stab at (2), but my patch changes the semantics of COMMAND
for an edge case: for now calling (project-compile nil) from Lisp causes
(compile nil) to be called (which errors out); with my patch,
(project-compile nil) yields a prompt.

This can be solved using called-interactively-p (or using an optional
INTERACTIVE argument); I just don't know if it's worth the hassle?

I haven't given much thought to (3), so I haven't yet figured out how to
avoid prompting twice for the project (on the rare occasions where
prompting is needed).


Thanks for your time.