improve performance of generic dotu similar to dot, cf. JuliaLang#27678

stdlib/LinearAlgebra/src/generic.jl

@@ -763,22 +763,27 @@ julia> dotu(σ, σ)
 function dotu(x, y) # arbitrary iterables
     ix = iterate(x)
     iy = iterate(y)
-    if ix == nothing
-        if iy != nothing
+    if ix === nothing
+        if iy !== nothing
             throw(DimensionMismatch("x and y are of different lengths!"))
         end
         return zero(eltype(x)) * zero(eltype(y))
     end
-    if iy == nothing
+    if iy === nothing
         throw(DimensionMismatch("x and y are of different lengths!"))
     end
-    s = ix[1] * iy[1]
-    ix, iy = iterate(x, ix[2]), iterate(y, iy[2])
-    while ix != nothing && iy != nothing
-        s += ix[1] * iy[1]
-        ix, iy = iterate(x, ix[2]), iterate(y, iy[2])
+    (vx, xs) = ix
+    (vy, ys) = iy
+    s = vx * vy
+    while true
+        ix = iterate(x, xs)
+        iy = iterate(y, ys)
+        ix === nothing && break
+        iy === nothing && break
+        (vx, xs), (vy, ys) = ix, iy
+        s += vx * vy
     end
-    if !(iy == nothing && ix == nothing)
+    if !(iy === nothing && ix === nothing)
         throw(DimensionMismatch("x and y are of different lengths!"))
     end
     return s