std.ArrayList(Aligned): copy default fields from Unmanaged

[BratishkaErik]

Synchronizes default fields and doc comments between Unmanaged and Managed versions:

• Make default values for the items and capacity fields same in all ArrayList versions. This allows to initialize ArrayList "directly" (= via struct initialization) and removes inconsistency with Unmanaged:

var unmanaged: ArrayListUnmanaged(u8) = .{};
errdefer unmanaged.deinit();

// ...

var managed: ArrayList(u8) = .{ .allocator = allocator };
errdefer managed.deinit();

// ...

• Mention "direct" initialization and initCapacity in doc comments, also removes inconsistency.

[BratishkaErik]

I don't understand how this method is dangerous, but OK.

[ifreund]

I don't understand how this method is dangerous, but OK.

As explained in the documentation andrew added, this allows initializing the Arraylist with an inconsistent state, for example:

var list: std.ArrayList(u32) = .{
    .capacity = 42,
    .allocator = allocator,
};

This code would look a lot more instantly wrong with the .items = &.{}, initial value in the initializer.

Note that the existing Unmanaged data structures tend to violate this principle as well.

[BratishkaErik]

Note that the existing Unmanaged data structures tend to violate this principle as well.

This is what I had in mind too, and I got a little bit cofused by referenced doc:

Default field values are only appropriate when the data invariants of a struct cannot
be violated by omitting that field from an initialization.

IIUC it doesn't apply to the ArrayList* at all, since it requires only allocator field — same as in init function — and default fields can be safely omitted.

Thanks for explanation.