make @non_differentiable use identical pullbacks when possible (#679)

* make `@non_differentiable` use identical pullbacks when possible

Fixes #678

* simpler

* bump version

Author: nsajko

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.23.0"
+version = "1.24.0"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"

src/ChainRulesCore.jl

@@ -2,7 +2,7 @@ module ChainRulesCore
 using Base.Broadcast: broadcasted, Broadcasted, broadcastable, materialize, materialize!
 using Base.Meta
 using LinearAlgebra
-using Compat: hasfield, hasproperty, ismutabletype
+using Compat: hasfield, hasproperty, ismutabletype, Returns
 
 export frule, rrule  # core function
 # rule configurations

src/rule_definition_tools.jl

@@ -436,9 +436,7 @@ function _nondiff_rrule_expr(__source__, primal_sig_parts, primal_invoke)
     tup_expr = tuple_expression(primal_sig_parts)
     primal_name = first(primal_invoke.args)
     pullback_expr = @strip_linenos quote
-        function $(esc(propagator_name(primal_name, :pullback)))(@nospecialize(_))
-            return $(tup_expr)
-        end
+        Returns($(tup_expr))
     end
 
     @gensym kwargs

test/rule_definition_tools.jl

@@ -42,6 +42,20 @@ end
 
 @testset "rule_definition_tools.jl" begin
     @testset "@non_differentiable" begin
+        @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)