missing: add length(::SkipMissing)

Define `length(::SkipMissing)` with `O(N)` complexity.

Author: thazhemadam

base/missing.jl

@@ -246,6 +246,8 @@ IteratorSize(::Type{<:SkipMissing}) = SizeUnknown()
 IteratorEltype(::Type{SkipMissing{T}}) where {T} = IteratorEltype(T)
 eltype(::Type{SkipMissing{T}}) where {T} = nonmissingtype(eltype(T))
 
+length(s::SkipMissing) = count(!ismissing, s.x)
+
 function iterate(itr::SkipMissing, state...)
     y = iterate(itr.x, state...)
     y === nothing && return nothing

test/missing.jl

@@ -441,6 +441,17 @@ end
     @test collect(x) == [1, 2, 4]
     @test collect(x) isa Vector{Int}
 
+    @testset "length" begin
+        allmiss = Vector{Union{Int,Missing}}(missing, 10)
+        @test (length∘skipmissing)(allmiss) == 0
+
+        somemiss = [1, missing, 2, 3, 4, missing, 5, 6, 7, 8, 9, 10]
+        @test (length∘skipmissing)(somemiss) == 10
+
+        nomiss = rand(1:10, 10)
+        @test (length∘skipmissing)(nomiss) == 10
+    end
+
     @testset "indexing" begin
         x = skipmissing([1, missing, 2, missing, missing])
         @test collect(eachindex(x)) == collect(keys(x)) == [1, 3]