make @non_differentiable use identical pullbacks when possible

Fixes #678

src/rule_definition_tools.jl

@@ -418,27 +418,32 @@ function _nondiff_frule_expr(__source__, primal_sig_parts, primal_invoke)
     end
 end
 
-function tuple_expression(primal_sig_parts)
+function _make_pullback_for_non_differentiable(::Val{N}) where {N}
+    Vararg{Any,N}  # throw early for invalid `N`, must be nonnegative `Int`
+    function pullback_for_non_differentiable(::Any)
+        ntuple(Returns(NoTangent()), Val(N))
+    end
+end
+
+function tuple_length_expression(primal_sig_parts)
     has_vararg = _isvararg(primal_sig_parts[end])
     return if !has_vararg
         num_primal_inputs = length(primal_sig_parts)
-        Expr(:tuple, ntuple(_ -> NoTangent(), num_primal_inputs)...)
+        :($num_primal_inputs)
     else
         num_primal_inputs = length(primal_sig_parts) - 1 # - vararg
         length_expr =
             :($num_primal_inputs + length($(esc(_unconstrain(primal_sig_parts[end])))))
-        @strip_linenos :(ntuple(i -> NoTangent(), $length_expr))
+        @strip_linenos :($length_expr)
     end
 end
 
 function _nondiff_rrule_expr(__source__, primal_sig_parts, primal_invoke)
     esc_primal_sig_parts = map(esc, primal_sig_parts)
-    tup_expr = tuple_expression(primal_sig_parts)
+    tup_len_expr = tuple_length_expression(primal_sig_parts)
     primal_name = first(primal_invoke.args)
     pullback_expr = @strip_linenos quote
-        function $(esc(propagator_name(primal_name, :pullback)))(@nospecialize(_))
-            return $(tup_expr)
-        end
+        _make_pullback_for_non_differentiable(Val{$(tup_len_expr)}())
     end
 
     @gensym kwargs

test/rule_definition_tools.jl

@@ -42,6 +42,47 @@ end
 
 @testset "rule_definition_tools.jl" begin
     @testset "@non_differentiable" begin
+        @testset "`_make_pullback_for_non_differentiable`" begin
+            f = ChainRulesCore._make_pullback_for_non_differentiable
+            @testset "throws on invalid input" begin
+                @test_throws Exception f(Val(0.0))
+                @test_throws Exception f(Val(-1))
+            end
+            @testset "identical objects" begin
+                for i ∈ 0:5
+                    v = Val(i)
+                    @test f(v) === f(v)
+                end
+            end
+            @testset "correctness" begin
+                for i ∈ 0:5
+                    expected = ntuple((_ -> NoTangent()), i)
+                    @test f(Val(i))(:arbitrary) === expected
+                end
+            end
+            @testset "dispatch" begin
+                for i ∈ 0:5
+                    pullback = f(Val(i))
+                    @test_throws MethodError pullback()
+                    @test_throws MethodError pullback(1, 2)
+                end
+            end
+        end
+
+        @testset "issue #678: identical pullback objects" begin
+            issue_678_f(::Any) = nothing
+            issue_678_g(::Any) = nothing
+            issue_678_h(::Any...) = nothing
+            @non_differentiable issue_678_f(::Any)
+            @non_differentiable issue_678_g(::Any)
+            @non_differentiable issue_678_h(::Any...)
+            @test (
+                last(rrule(issue_678_f, 0.1)) ===
+                last(rrule(issue_678_g, 0.2)) ===
+                last(rrule(issue_678_h, 0.3))
+            )
+        end
+
         @testset "two input one output function" begin
             nondiff_2_1(x, y) = fill(7.5, 100)[x + y]
             @non_differentiable nondiff_2_1(::Any, ::Any)