I'm sitting around doing IT shit waiting things to download/backup/install/etc and have nothing better to do, so here's an AI-free explanation with code samples:

It's basically just a code style thing. Standard C allows you to declare unnamed structs/unions within other structs/unions. They must be unnamed, so it'd look like this:

struct test {
        int a;
        struct {
                char b;
                float c;
        };
        double d;
};

Which is fine, but the -fms-extensions flag enables you to do the same thing with named structs. For example:

struct test {
        int a;
        struct test2 {
                char b;
                float c;
        };
        double d;
};

without -fms-extensions, the above will compile, but won't do what you might assume. b and c will be members of struct test2, not test. So something like this won't compile:

struct test my_test;
my_test.b = 1; // error: ‘struct test’ has no member named ‘b’

But with the flag, not only does it work, it also lets you do some convenient things like this:

struct test2 {
        char b;
        float c;
};
struct test {
        int a;
        struct test2;
        double d;
};
//...
struct test my_test;
my_test.b = 1; //OK

That is, you can reuse an existing struct definition, which gives you a nice little tool to organize your code.

Source: https://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html

Once in a while, it turns out that enabling -fms-extensions could allow some slightly prettier code. But every time it has come up, the code that had to be used instead has been deemed "not too awful" and not worth introducing another compiler flag for.

That's probably true for each individual case, but then it's somewhat of a chicken/egg situation.

If we just "bite the bullet" as Linus says and enable it once and for all, it is available whenever a use case turns up, and no individual case has to justify it.

A lore.kernel.org search provides these examples:

• https://lore.kernel.org/lkml/200706301813.58435.agruen@suse.de/
• https://lore.kernel.org/lkml/20180419152817.GD25406@bombadil.infradead.org/
• https://lore.kernel.org/lkml/170622208395.21664.2510213291504081000@noble.neil.brown.name/
• https://lore.kernel.org/lkml/87h6475w9q.fsf@prevas.dk/
• https://lore.kernel.org/lkml/CAHk-=wjeZwww6Zswn6F_iZTpUihTSNKYppLqj36iQDDhfntuEw@mail.gmail.com/

Undoubtedly, there are more places in the code where this could also be used but where -fms-extensions just didn't come up in any discussion.

Basically the extensions are useful sometimes. Note that they have nothing to do with Microsoft other than being invented by them.

Some software require it sadly

Old languages should be able to learn new tricks.

The "extend" phase of EEE.