langref: improve zero bit types documentation

[perillo]

In the Zero Bit Types section, add the empty enum, empty struct and empty union. Clarify that a non empty zero bit union just needs to have all fields of zero bit type, instead of only one field of zero bit type.

Add a doctest.

[Vexu]

Empty enums and unions are not zero-bit types but noreturn types. Their behavior is a bit buggy currently though #13807.

[perillo]

The following executable:

const std = @import("std");
const debug = std.debug;

pub fn main() void {
    inline for (.{
        enum {},
        union {},
    }) |typ| {
        debug.print("typ: {}\n", .{typ});
        debug.print("@typeInfo(typ): {}\n", .{@typeInfo(typ)});
    }
}

prints:

typ: zero_bit_types.main__enum_3928
@typeInfo(typ): builtin.Type{ .Enum = builtin.Type.Enum{ .tag_type = u0, .fields = {  }, .decls = {  }, .is_exhaustive = true } }
typ: zero_bit_types.main__union_3929
@typeInfo(typ): builtin.Type{ .Union = builtin.Type.Union{ .layout = builtin.Type.ContainerLayout.Auto, .tag_type = null, .fields = {  }, .decls = {  } } }

Thanks.

[perillo]

I also tried the examples in #13807, with zig 0.11.0-dev.1327+b42bd759a.

The empty error set causes a segmentation fault, instead of a panic:
LLVM Emit Object... zsh: segmentation fault (core dumped) zig build-lib lib.zig

What about adding the empty error set to the list, and document the current limitations/issues?

P.S.:
What does it mean:
"Empty enums and unions are not currently recognized as noreturn types so this example compiles just fine but if recognized as such then it hits the same issues."

How can the compiler change the way it recognizes empty enums and unions?

[Vexu]

What about adding the empty error set to the list, and document the current limitations/issues?

An empty error set is also a noreturn type, not a zero-bit type.

P.S.: What does it mean: "Empty enums and unions are not currently recognized as noreturn types so this example compiles just fine but if recognized as such then it hits the same issues."

How can the compiler change the way it recognizes empty enums and unions?

Noreturn types affect the semantic analysis and empty enums and unions currently behave incorrectly because I'm unsure what the proper behavior should be.

[perillo]

Why not just disallow empty struct, empty union and empty enum?

[Vexu]

See #3257

[perillo]

I removed the empty struct, enum and union from the list and updated the commit message.

At the end of the section I added a clarification about problematic empty types.