add test config from JuliaDiff/ChainRules.jl#441

Author: Miha Zgubic

src/ChainRulesTestUtils.jl

@@ -36,6 +36,7 @@ include("iterator.jl")
 include("output_control.jl")
 include("check_result.jl")
 
+include("rule_config.jl")
 include("finite_difference_calls.jl")
 include("testers.jl")
 

src/rule_config.jl

@@ -0,0 +1,16 @@
+# For testing this config re-dispatches Xrule_via_ad to Xrule without config argument
+struct ADviaRuleConfig <: RuleConfig{Union{HasReverseMode, HasForwardsMode}} end
+
+function ChainRulesCore.frule_via_ad(config::ADviaRuleConfig, ȧrgs, f, args...; kws...)
+    ret = frule(config, ȧrgs, f, args...; kws...)
+    # we don't support actually doing AD: the rule has to exist. lets give helpfulish error
+    ret === nothing && throw(MethodError(frule, (ȧrgs, f, args...)))
+    return ret
+end
+
+function ChainRulesCore.rrule_via_ad(config::ADviaRuleConfig, f, args...; kws...)
+    ret = rrule(config, f, args...; kws...)
+    # we don't support actually doing AD: the rule has to exist. lets give helpfulish error
+    ret === nothing && throw(MethodError(rrule, (f, args...)))
+    return ret
+end