add Returns for 1.6

Author: mcabbott

src/utils.jl

@@ -91,3 +91,22 @@ end
 function rrule(cfg::RuleConfig{>:HasReverseMode}, ::typeof(_fast_broadcast!), f::F, x::AbstractArray, ys...)  where {F<:Function}
     rrule_via_ad(cfg, broadcast, f, x, ys...)
 end
+
+# Could get this from Compat.jl instead
+# https://github.com/JuliaLang/julia/pull/39794
+if VERSION < v"1.7.0-DEV.793"
+    struct Returns{V} <: Function
+        value::V
+        Returns{V}(value) where {V} = new{V}(value)
+        Returns(value) = new{Core.Typeof(value)}(value)
+    end
+
+    (obj::Returns)(args...; kw...) = obj.value
+    function Base.show(io::IO, obj::Returns)
+        show(io, typeof(obj))
+        print(io, "(")
+        show(io, obj.value)
+        print(io, ")")
+    end
+end
+