Check that if only one adjoint exists, it is not thunked (#53)

* Test no thunking when unnecessary

* Remove test that now fails by design

* Add note that InplaceableThunk is allowed

* Increment version number

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesTestUtils"
 uuid = "cdddcdb0-9152-4a09-a978-84456f9df70a"
-version = "0.4.3"
+version = "0.5.0"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/testers.jl

@@ -192,8 +192,9 @@ function rrule_test(f, ȳ, xx̄s::Tuple{Any, Any}...; rtol=1e-9, atol=1e-9, fdm
     x̄s_ad = ∂s[2:end]
     @test ∂self === NO_FIELDS  # No internal fields
 
+    x̄s_is_dne = x̄s .== nothing
     # Correctness testing via finite differencing.
-    x̄s_fd = _make_fdm_call(fdm, (xs...) -> f(xs...; fkwargs...), ȳ, xs, x̄s .== nothing)
+    x̄s_fd = _make_fdm_call(fdm, (xs...) -> f(xs...; fkwargs...), ȳ, xs, x̄s_is_dne)
     for (x̄_ad, x̄_fd) in zip(x̄s_ad, x̄s_fd)
         if x̄_fd === nothing
             # The way we've structured the above, this tests the propagator is returning a DoesNotExist
@@ -202,4 +203,11 @@ function rrule_test(f, ȳ, xx̄s::Tuple{Any, Any}...; rtol=1e-9, atol=1e-9, fdm
             @test isapprox(x̄_ad, x̄_fd; rtol=rtol, atol=atol, kwargs...)
         end
     end
+
+    if count(!, x̄s_is_dne) == 1
+        # for functions with pullbacks that only produce a single non-DNE adjoint, that
+        # single adjoint should not be `Thunk`ed. InplaceableThunk is fine.
+        i = findfirst(!, x̄s_is_dne)
+        @test !(isa(x̄s_ad[i], Thunk))
+    end
 end

test/testers.jl

@@ -40,10 +40,7 @@ primalapprox(x) = x
         # define rrule using ChainRulesCore's v0.9.0 convention, conjugating the derivative
         # in the rrule
         function ChainRulesCore.rrule(::typeof(sinconj), x)
-            # usually we would not thunk for a single output, because it will of course be
-            # used, but we do here to ensure that test_scalar works even if a scalar rrule
-            # thunks
-            sinconj_pullback(ΔΩ) = (NO_FIELDS, @thunk(conj(cos(x)) * ΔΩ))
+            sinconj_pullback(ΔΩ) = (NO_FIELDS, conj(cos(x)) * ΔΩ)
             return sin(x), sinconj_pullback
         end