From Ruby 2.7, we require C compiler to support C99 features that work on environments supported by platform maintainers.

• C99 features that cannot be built on Travis/AppVeyor/RubyCI are prohibited.
  • Ruby 2.7 requires Visual Studio 2013+, which is tested on AppVeyor and RubyCI.
  • Ruby 2.7 requires Oracle Developer Studio 12+ or GCC on Solaris 10+, which are tested on RubyCI. (adding 12.4 and/or earlier is pending, but planned)

Here is a list of features that are known to work on both Visual Studio 2013 and Oracle Developer Studio 12.5, confirmed by this c99.c.

This is just for a quick reference to bypass testing on CI, and you do NOT need to update this list to use your favorite C99 feature as long as it works on Travis/AppVeyor/RubyCI.

• // comments
• mixed declarations and code
• designated initializers
• compound literals
• relaxed constraints on aggregate and union initialization
• trailing comma allowed in enum declaration

See also:

• ISO/IEC 9899:1999
• C99 library support in Visual Studio 2013

• stdbool.h seems to have been added in Oracle Solaris Studio 12.3, so we need to use missing/stdbool.h header for Oracle Solaris Studio 12.2 or earlier, which is included in our internal.h. See r66739.
• restrict keyword does not exist on Visual Studio 2013, but __restrict exists and is used instead.
  • See also: r66598 and r66715.
• Switching va_copy definition like r62220 was problematic for cross compiling and removed in r62463.
  • r62191 was initially introduced for old Visual Studio. We may be able to just use va_copy with Ruby 2.7's Visual Studio 2013 requirement.

• Visual Studio 2013
  • variable length arrays
  • _Complex
  • snprintf
  • some format specifier behaviors:
    • size_t length modifier: %z
    • infinite/indefinite/NaN conversion of floating point number

According to the header files committed in https://github.com/ruby/ruby/pull/2991,

We assume C99 for ruby itself but we don't assume languages of extension libraries. They could be written in C++98.