Go slices cannot change the structure (len or cap) of their "parent" slice. (Unlike Java, the semantics of every operation are even well-defined.) In some sense this is the root of the problem - the "child" slice can start using spare capacity it 'inherited' with no way to tell the parent it has done so, and the parent may inadvertantly pass some of its length as capacity down to its child.

You can't insert or remove elements, but you can overrun the buffer??

I have a hard time thinking of a situation when that would be useful at all, let alone useful enough to be the default and widely-used behaviour! If I want to let the callee add stuff to the end of my list, can't I just pass them the whole list and let them modify that?

You cannot "overrun the buffer" nor can a callee add stuff to the end of your list; you can give them a memory buffer with unused space and they may use it, and then you can also mistakenly use it later.

Honestly, you seem to be too detached to understand this. If you really want to know how it works go through some official Go documentation.

I coincidentally was just looking at some docs, and I think I have a handle on it (to it?:) now.

The specific strange design decision in Go is that you can create a child slice that has its own starting offset and length, but may or may not inherit its parent’s capacity. Just because it’s well-defined doesn’t mean it isn’t tricky.

Even if you kept the semantics the same but made cap=len for subslices by default, surely that would be an improvement. Rather than a “rare slice trick”, it ought to be normal. Or is there an advantage to the current default that I’m overlooking?

cap==len is effectively the default - most slicing is done with [start:end], not [start:end:additional-cap]

No it’s not -- I just checked the language specification and it says:

After slicing the array a

a := [5]int{1, 2, 3, 4, 5}

s := a[1:4]

the slice s has type []int, length 3, capacity 4

Many of us here are arguing that the default should be cap==len==3, but it’s not, it’s 4.

Welp. TIL.

Yeah, that's much more error prone I think. It's the sort of thing that only kinda makes sense on zero-valued arrays... and even then it's dubious at best.

Bleh. I'm gonna go review some old code now D:

I mean, if you want Java, it's right there.

The advantage is that you get the fast path by default. Since Go programs are generally not awash in buffer reuse bugs despite its semantic trickiness, this seems like something different languages can reasonably prioritize differently.