Tune Stateful iterator

This attempts to address some of the performance regressions observed
with the Stateful iterator #25763. It gets most of the way there,
but unfortunately still ends up allocating the `Stateful` iterator
object rather than propagating through the fields. Getting the rest
of the way there will require some compiler tweaks.

base/iterators.jl

@@ -1016,44 +1016,56 @@ mutable struct Stateful{T, VS}
     # A bit awkward right now, but adapted to the new iteration protocol
     nextvalstate::Union{VS, Nothing}
     taken::Int
-    # Try to find an appropriate type for the (value, state tuple),
-    # by doing a recursive unrolling of the iteration protocol up to
-    # fixpoint.
-    function fixpoint_iter_type(itrT::Type, valT::Type, stateT::Type)
-        nextvalstate = Base._return_type(next, Tuple{itrT, stateT})
-        nextvalstate <: Tuple{Any, Any} || return Any
-        nextvalstate = Tuple{
-            typejoin(valT, fieldtype(nextvalstate, 1)),
-            typejoin(stateT, fieldtype(nextvalstate, 2))}
-        return (Tuple{valT, stateT} == nextvalstate ? nextvalstate :
-            fixpoint_iter_type(itrT,
-                fieldtype(nextvalstate, 1),
-                fieldtype(nextvalstate, 2)))
-    end
-    function Stateful(itr::T) where {T}
+    @inline function Stateful(itr::T) where {T}
         state = start(itr)
         VS = fixpoint_iter_type(T, Union{}, typeof(state))
-        vs = done(itr, state) ? nothing : next(itr, state)::VS
-        new{T, VS}(itr, vs, 0)
+        if done(itr, state)
+            new{T, VS}(itr, nothing, 0)
+        else
+            new{T, VS}(itr, next(itr, state)::VS, 0)
+        end
     end
 end
 
+# Try to find an appropriate type for the (value, state tuple),
+# by doing a recursive unrolling of the iteration protocol up to
+# fixpoint.
+function fixpoint_iter_type(itrT::Type, valT::Type, stateT::Type)
+    nextvalstate = Base._return_type(next, Tuple{itrT, stateT})
+    nextvalstate <: Tuple{Any, Any} || return Any
+    nextvalstate = Tuple{
+        typejoin(valT, fieldtype(nextvalstate, 1)),
+        typejoin(stateT, fieldtype(nextvalstate, 2))}
+    return (Tuple{valT, stateT} == nextvalstate ? nextvalstate :
+        fixpoint_iter_type(itrT,
+            fieldtype(nextvalstate, 1),
+            fieldtype(nextvalstate, 2)))
+end
+
 convert(::Type{Stateful}, itr) = Stateful(itr)
 
-isempty(s::Stateful) = s.nextvalstate === nothing
+@inline isempty(s::Stateful) = s.nextvalstate === nothing
 
-function popfirst!(s::Stateful)
-    isempty(s) && throw(EOFError())
-    val, state = s.nextvalstate
-    s.nextvalstate = done(s.itr, state) ? nothing : next(s.itr, state)
-    s.taken += 1
-    val
+@inline function popfirst!(s::Stateful)
+    vs = s.nextvalstate
+    if vs === nothing
+        throw(EOFError())
+    else
+        val, state = vs
+        if done(s.itr, state)
+            s.nextvalstate = nothing
+        else
+            s.nextvalstate = next(s.itr, state)
+        end
+        s.taken += 1
+        return val
+    end
 end
 
-peek(s::Stateful, sentinel=nothing) = s.nextvalstate !== nothing ? s.nextvalstate[1] : sentinel
-start(s::Stateful) = nothing
-next(s::Stateful, state) = popfirst!(s), nothing
-done(s::Stateful, state) = isempty(s)
+@inline peek(s::Stateful, sentinel=nothing) = s.nextvalstate !== nothing ? s.nextvalstate[1] : sentinel
+@inline start(s::Stateful) = nothing
+@inline next(s::Stateful, state) = popfirst!(s), nothing
+@inline done(s::Stateful, state) = isempty(s)
 IteratorSize(::Type{Stateful{VS,T}} where VS) where {T} =
     isa(IteratorSize(T), SizeUnknown) ? SizeUnknown() : HasLength()
 eltype(::Type{Stateful{VS, T}} where VS) where {T} = eltype(T)

test/iterators.jl

@@ -495,15 +495,15 @@ end
 end
 
 @testset "Iterators.Stateful" begin
-    let a = Iterators.Stateful("abcdef")
+    let a = @inferred(Iterators.Stateful("abcdef"))
         @test !isempty(a)
         @test popfirst!(a) == 'a'
         @test collect(Iterators.take(a, 3)) == ['b','c','d']
         @test collect(a) == ['e', 'f']
     end
-    let a = Iterators.Stateful([1, 1, 1, 2, 3, 4])
+    let a = @inferred(Iterators.Stateful([1, 1, 1, 2, 3, 4]))
         for x in a; x == 1 || break; end
         @test Base.peek(a) == 3
         @test sum(a) == 7
     end
-end
+end