allow multiple kinds of return types from main

[andrewrk]

Currently, this is the main entry point function:

pub fn main() -> %void {

}

The bootstrap code runs it, checks the return value, and makes the exit/ExitProcess syscall.

If a zig user wants to do their own process exiting, however, they should be able to make the return type noreturn, and then the bootstrap code does not need to handle cleanup.

This shaves off a couple of bytes of bloat for some of our more hardcore users such as demoscene programmers.

[thejoshwolfe]

Another usecase for returning noreturn is if a program can't stop, won't stop. This is common for daemons such as groovebasin.

[skyfex]

Microcontroller firmware is one more usecase.

[andrewrk]

more proposal:

allow multiple kinds of return types from main

• noreturn - main never returns
• void - main does not fail
• %void - possibly print an error return trace

switch (@typeId(@returnType(root.main))) {
    builtin.TypeId.NoReturn => {
        root.main();
    },
    builtin.TypeId.Void => {
        root.main();
        std.os.exit(0);
    },
    builtin.TypeId.ErrorUnion => {
        if (builtin.mode == builtin.Mode.ReleaseFast) {
            root.main() catch std.os.exit(1);
            std.os.exit(0);
        } else {
            root.main() catch |err, trace| {
                std.debug.warn("error: {}\n", @errorName(err));
                std.debug.dumpStackTrace(trace);
                std.os.exit(1);
            };
            std.os.exit(0);
        }
    },
    else => @compileError("expected return type of main to be 'noreturn', 'void', or '%void'"),
}