make ChainRulesCore dependency into an extension (#421)

Author: Kristoffer Carlsson

Project.toml

@@ -9,6 +9,12 @@ LogExpFunctions = "2ab3a3ac-af41-5b50-aa03-7779005ae688"
 OpenLibm_jll = "05823500-19ac-5b8b-9628-191a04bc5112"
 OpenSpecFun_jll = "efe28fd5-8261-553b-a9e1-b2916fc3738e"
 
+[weakdeps]
+ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
+
+[extensions]
+ChainRulesCoreExt = "ChainRulesCore"
+
 [compat]
 ChainRulesCore = "0.9.44, 0.10, 1"
 ChainRulesTestUtils = "0.6.8, 0.7, 1"
@@ -19,9 +25,10 @@ OpenSpecFun_jll = "0.5"
 julia = "1.3"
 
 [extras]
+ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 ChainRulesTestUtils = "cdddcdb0-9152-4a09-a978-84456f9df70a"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["ChainRulesTestUtils", "Random", "Test"]
+test = ["ChainRulesCore", "ChainRulesTestUtils", "Random", "Test"]

src/chainrules.jl renamed to ext/ChainRulesCoreExt.jl

@@ -1,3 +1,9 @@
+module ChainRulesCoreExt
+
+using SpecialFunctions, ChainRulesCore
+
+import SpecialFunctions: sqrtπ, invπ
+
 const BESSEL_ORDER_INFO = """
 derivatives of Bessel functions with respect to the order are not implemented currently:
 https://github.com/JuliaMath/SpecialFunctions.jl/issues/160
@@ -293,3 +299,5 @@ function ChainRulesCore.rrule(::typeof(besselyx), ν::Number, x::Number)
     end
     return Ω, besselyx_pullback
 end
+
+end # module

src/SpecialFunctions.jl

@@ -13,7 +13,6 @@ using IrrationalConstants:
     logπ,
     log2π
 
-import ChainRulesCore
 import LogExpFunctions
 
 using OpenLibm_jll
@@ -91,7 +90,9 @@ include("gamma.jl")
 include("gamma_inc.jl")
 include("betanc.jl")
 include("beta_inc.jl")
-include("chainrules.jl")
+if !isdefined(Base, :get_extension)
+    include("../ext/ChainRulesCoreExt.jl")
+end
 include("deprecated.jl")
 
 for f in (:digamma, :erf, :erfc, :erfcinv, :erfcx, :erfi, :erfinv, :logerfc, :logerfcx,