Simpler flatten_tuples

Author: Tokazama

lib/ArrayInterfaceCore/src/ArrayInterfaceCore.jl

@@ -66,27 +66,20 @@ julia> ArrayInterfaceCore.flatten_tuples((1, (2, (3,))))
 
 ```
 """
-@inline function flatten_tuples(t::Tuple)
-    if @generated
-        texpr = Expr(:tuple)
-        for i in 1:fieldcount(t)
-            p = fieldtype(t, i)
-            if p <: Tuple
-                for j in 1:fieldcount(p)
-                    push!(texpr.args, :(@inbounds(getfield(getfield(t, $i), $j))))
-                end
-            else
-                push!(texpr.args, :(@inbounds(getfield(t, $i))))
-            end
-        end
-        Expr(:block, Expr(:meta, :inline), texpr)
-    else
-        _flatten(t)
+function flatten_tuples(t::Tuple)
+    fields = _new_field_positions(t)
+    ntuple(Val{nfields(fields)}()) do k
+        i, j = getfield(fields, k)
+        i = length(t) - i
+        @inbounds j === 0 ? getfield(t, i) : getfield(getfield(t, i), j)
     end
 end
-_flatten(::Tuple{}) = ()
-@inline _flatten(t::Tuple{Any,Vararg{Any}}) = (getfield(t, 1), _flatten(Base.tail(t))...)
-@inline _flatten(t::Tuple{Tuple,Vararg{Any}}) = (getfield(t, 1)..., _flatten(Base.tail(t))...)
+_new_field_positions(::Tuple{}) = ()
+@nospecialize
+_new_field_positions(x::Tuple) = (_fl1(x, x[1])..., _new_field_positions(Base.tail(x))...)
+_fl1(x::Tuple, x1::Tuple) = ntuple(Base.Fix1(tuple, length(x) - 1), Val(length(x1)))
+_fl1(x::Tuple, x1) = ((length(x) - 1, 0),)
+@specialize
 
 """
     parent_type(::Type{T}) -> Type