Check that rrule x_ad DNE

Author: mattBrzezinski

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesTestUtils"
 uuid = "cdddcdb0-9152-4a09-a978-84456f9df70a"
-version = "0.1.2"
+version = "0.1.3"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/ChainRulesTestUtils.jl

@@ -175,8 +175,13 @@ function rrule_test(f, ȳ, xx̄s::Tuple{Any, Any}...; rtol=1e-9, atol=1e-9, fdm
 
     # Correctness testing via finite differencing.
     x̄s_fd = _make_fdm_call(fdm, f, ȳ, xs, x̄s .== nothing)
-    map(x̄s_ad, x̄s_fd) do x̄_ad, x̄_fd
-        @test isapprox(x̄_ad, x̄_fd; rtol=rtol, atol=atol, kwargs...)
+    for (x̄_ad, x̄_fd) in zip(x̄s_ad, x̄s_fd)
+        if x̄_fd === nothing
+            # The way we've structured the above, this tests that the rule is a DoesNotExistRule
+            @test x̄_ad isa DoesNotExist
+        else
+            @test isapprox(x̄_ad, x̄_fd; rtol=rtol, atol=atol, kwargs...)
+        end
     end
 end