Remove Struct{}/Union{} (and maybe Enum.value?)

[tadeokondrak]

This was mentioned in #3652 but I don't think it got much discussion after that.

Structs/unions:

make zig fmt convert Foo{.a = b} to @as(Foo, .{.a = b}), wait 1 release cycle, and remove the deprecated old syntax. If this looks bad to you, consider that most struct literals will look like this: const foo: Foo = .{.a = b};.

Enums:

That's a good point - you could make an analogy with Enum.name being redundant with @as(Enum, .name).

[daurnimator]

Note #3652 (comment)

let's leave it be for a bit, and see how it feels. If it makes it worse in practice to use @as(Foo, .{.a = b}) rather than Foo{.a = b}, then it's worth considering to keep the redundant syntax.

[tauoverpi]

If .{} worked with error unions this wouldn't probably be too much and the same with [_]T{} in the case of &[_]T{}. then it would possibly be better at the cost of being slightly more noisy.

[Vexu]

Duplicate of #5038