Feature request: Warning when .c file gets #include'd

[Marek Polacek]

On Fri, Dec 02, 2022 at 03:57:44PM +0100, Jiří Wolker via Gcc wrote:
> 
> Hi,
> 
> I've met a guy that is learning C and got stuck when the linker produced
> a screenful of messages about that he did something define multiple
> times. The cause of the problem was trivial:
> 
>     He did ``#include "something.c"'' in his code.
> 
> In the past, I also did this thing multiple times and I would appreciate
> a warning produced by the gcc when compiling C/C++ file that includes a
> file ending in .c, .cx, .cpp or .cxx.
> 
> What do you think about that?
> 
> I would prefer to make this enabled when -Wextra is used.

The problem is that a lot of beginners don't use -Wextra, so the
warning wouldn't be displayed anyway.
 
> Based on my past experience with using .c files in the include
> directive, I can think of these problems.
> 
>   - Sometimes, I want to include a file that is definitely not a header,
>     but also it is not a stand-alone source file. I personally prefer
>     using .inc suffix, but some people possibly terminate the file names
>     with .c suffix like a source file.
> 
>     Example:  list of enum fields included from a separate file that was
>               generated by a script
> 
>   - In some projects, someone can deliberately want to include another
>     source file. For example, this can happen when doing unit tests and
>     you do not want to specify the source file name on the command line.

Indeed, e.g. elfutils uses this a lot:
#define LIBELFBITS 64
#include "elf32_xlatetom.c"

We use including .c in our testsuite a lot as well.

So I'm afraid this would get -1 from me personally, sorry.

> If you accept that, I can try to implement that. (That would be my first
> contribution to this project and I do not know gcc codebase, but it
> looks like a relatively simple change.) Do you think that it would need
> a copyright assignment?

I think it would.

Thanks,
Marek