split Base.lastindex semantics, avoid sentinel value

[goretkin]

It seems like lastindex serves two roles that are tempting to conflate, but could possibly benefit from being split up.

"what is the index of the last element"
This is what lastindex currently does, except for when the collection is empty (so there is no last element)

julia> lastindex([])
0

Possibly it should return nothing or throw a BoundsError, but 0 is really the only reasonable choice because of the second use:

"if I push!ed another element, what would the index to it be, minus 1"
This is the use case I find myself wanting, and it's simple enough to do lastindex(...) + 1 as can be found in many uses of lastindex e.g.

julia/stdlib/Base64/src/buffer.jl

Line 21 in 4b6ab68

capacity(buffer::Buffer) = Int(pointer(buffer.data, lastindex(buffer.data) + 1) - buffer.ptr)

The behavior of lastindex can't be changed until 2.0, clearly. But if the distinction I am making above is useful, then we should have a separate function, possibly newindex. Having two closely related functions might be confusing, and users might not understand the big deal, since it's easy enough to do + 1 or - 1 depending on what you need, but I would argue that splitting up functions like this issue suggests makes it easier to handle boundaries / edge cases.

To be clear,

julia> my_newindex([])
1

[goretkin]

I forgot to mention, if a collection can't be push!'d, then newindex should return an error. i.e. my_newindex(42) is not defined, even though lastindex(42) is, which I think further motivates the split.

[goretkin]

Related:
#26511
#22354
#25385

Further motivation for this semantic split: if e.g. last(1:0) is defined to throw BoundsError, then lastindex([]) would necessarily as well.

Perhaps it would be not a pun to use Base.nextind(a::AbstractVector) for what I called my_newindex(a).

[EDIT: added more links 2020-aug-5]

[mbauman]

There's a third very prominent role for lastindex:

julia> Meta.@lower A[end]
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─ %1 = Base.lastindex(A)
│   %2 = Base.getindex(A, %1)
└──      return %2
))))

julia> Meta.@lower A[end, end]
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope'
1 ─ %1 = Base.lastindex(A, 1)
│   %2 = Base.lastindex(A, 2)
│   %3 = Base.getindex(A, %1, %2)
└──      return %3
))))

Returning 0 for an empty collection is very helpful for syntaxes like A[begin:end]. Note that the empty behavior of firstindex is analogous for the same reason.