[Issue 23812] New: ImportC: allow adding function attributes to imported C functions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23812

          Issue ID: 23812
           Summary: ImportC: allow adding function attributes to imported
                    C functions
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: ImportC
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

C functions imported with ImportC currently don't get any function attributes,
so they're currently not callable in `nothrow` `@nogc` `pure` `@safe` contexts.

nothrow and @nogc could be given by default, the only exceptions would be if
the imported function has a callback, or if it is actually implemented in D but
imported through a C file for some reason.

@safe is rarer for C functions since they often receive raw pointers, so we
might just require use of `@trusted` when calling them from a `@safe` context.

pure is also rarer for C, since C libraries often use global variables / stderr
for error reporting, but ImportC could recognize the `__attribute__((pure))`
extension some compilers have.

In any case, new syntax could be created along the lines of:
```C
#pragma nogc nothrow

#include <somelibrary.h>
```

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