Re-land: Specialize adapt_structure for small tuples (#81)

src/base.jl

@@ -1,7 +1,20 @@
 # predefined adaptors for working with types from the Julia standard library
 
-adapt_structure(to, xs::Union{Tuple,NamedTuple}) = map(adapt(to), xs)
-
+# Use recursion to avoid inference bail-out in `map`
+#adapt_structure(to, xs::Union{Tuple,NamedTuple}) = map(adapt(to), xs)
+adapt_structure(to, xs::NamedTuple) = map(adapt(to), xs)
+# Specialize on small Tuples
+function adapt_structure(to, xs::Tuple)
+  if length(xs) ≤ 20
+    _adapt_tuple_structure(to, xs)
+  else
+    map(adapt(to), xs)
+  end
+end
+_adapt_tuple_structure(to, xs::Tuple) =
+  (adapt(to, first(xs)), _adapt_tuple_structure(to, Base.tail(xs))...)
+_adapt_tuple_structure(to, xs::Tuple{}) = ()
+_adapt_tuple_structure(to, xs::Tuple{<:Any}) = (adapt(to, first(xs)), )
 
 ## Closures