allow constant-prop through iterate (#158)

* allow constant-prop through `iterate`

Allows inference to unroll `iterate` for a `FixedSizeArray` with statically known size

* fixup

* Apply suggestions from code review

* Update test/runtests.jl

* fix `iterate` overload

Author: simeonschaub

src/FixedSizeArray.jl

@@ -404,14 +404,9 @@ function Base.reshape(a::FixedSizeArray, size::(NTuple{N,Int} where {N}))
     new_fixed_size_array(parent(a), size)
 end
 
-# `iterate`: the `AbstractArray` fallback doesn't perform well, so add our own methods
+# `iterate`: the `AbstractArray` fallback doesn't perform well, so add our own methods (copied from `Base.Array`)
 
-function Base.iterate(a::FixedSizeArray)
-    iterate(parent(a))
-end
-function Base.iterate(a::FixedSizeArray, state)
-    iterate(parent(a), state)
-end
+Base.iterate(A::FixedSizeArray, i=1) = (@inline; (i - 1)%UInt < length(A)%UInt ? (@inbounds A[i], i + 1) : nothing)
 
 """
     FixedSizeArrayDefault{T,N}(undef, size::NTuple{N,Int})

test/runtests.jl

@@ -698,4 +698,14 @@ end
         @test farr isa Transpose
         @test parent(farr) isa FixedSizeArray
     end
+
+    @testset "Constant-prop through iterate" begin
+        @test Base.return_types() do
+            m = FixedSizeArray{Int}(undef, 3)
+            m[1] = 1
+            m[2] = 2
+            m[3] = 3
+            tuple(m...)
+       end[] == NTuple{3, Int}
+    end
 end