make last work on any reversible collection (JuliaLang#42991)

NEWS.md

@@ -25,6 +25,8 @@ Language changes
 * `@time` and `@timev` now take an optional description to allow annotating the source of time reports.
   i.e. `@time "Evaluating foo" foo()` ([#42431])
 * New `@showtime` macro to show both the line being evaluated and the `@time` report ([#42431])
+* `last(collection)` will now work on any collection that supports `Iterators.reverse` and `first`, rather than being
+  restricted to indexable collections.
 
 Compiler/Runtime improvements
 -----------------------------

base/abstractarray.jl

@@ -462,11 +462,15 @@ end
     last(coll)
 
 Get the last element of an ordered collection, if it can be computed in O(1) time. This is
-accomplished by calling [`lastindex`](@ref) to get the last index. Return the end
-point of an [`AbstractRange`](@ref) even if it is empty.
+accomplished by calling [`Iterators.reverse`](@ref) and then [`first`](@ref) on that reversed iterator.
+Return the end point of an [`AbstractRange`](@ref) even if it is empty.
 
 See also [`first`](@ref), [`endswith`](@ref).
 
+!!! compat "Julia 1.8"
+    For versions of julia older than 1.8, `last(x)` will only work on collections that support indexing and
+    [`lastindex`](@ref).
+
 # Examples
 ```jldoctest
 julia> last(1:2:10)
@@ -476,7 +480,8 @@ julia> last([1; 2; 3; 4])
 4
 ```
 """
-last(a) = a[end]
+last(a) = first(Iterators.reverse(a))
+last(a::AbstractVector) = a[end]
 
 """
     last(itr, n::Integer)

base/iterators.jl

@@ -103,7 +103,6 @@ size(r::Reverse) = size(r.itr)
 IteratorSize(::Type{Reverse{T}}) where {T} = IteratorSize(T)
 IteratorEltype(::Type{Reverse{T}}) where {T} = IteratorEltype(T)
 last(r::Reverse) = first(r.itr) # the first shall be last
-first(r::Reverse) = last(r.itr) # and the last shall be first
 
 # reverse-order array iterators: assumes more-specialized Reverse for eachindex
 @propagate_inbounds function iterate(A::Reverse{<:AbstractArray}, state=(reverse(eachindex(A.itr)),))

test/iterators.jl

@@ -885,3 +885,8 @@ end
     @test Iterators.peel(x^2 for x in 2:4)[1] == 4
     @test Iterators.peel(x^2 for x in 2:4)[2] |> collect == [9, 16]
 end
+
+@testset "last for iterators" begin
+    @test last(Iterators.map(identity, 1:3)) == 3
+    @test last(Iterators.filter(iseven, (Iterators.map(identity, 1:3)))) == 2
+end