use ProjectTo

Author: nmheim

src/rulesets/Base/arraymath.jl

@@ -441,7 +441,12 @@ end
 frule((_, Δx, Δy), ::typeof(-), x::AbstractArray, y::AbstractArray) = x - y, Δx - Δy
 
 function rrule(::typeof(-), x::AbstractArray, y::AbstractArray)
-    subtract_pullback(dy) = (NoTangent(), dy, -dy)
+    xproj = ProjectTo(x)
+    yproj = ProjectTo(y)
+    function subtract_pullback(dy_raw)
+        dy = unthunk(dy_raw)  # projs will otherwise unthunk twice
+        (NoTangent(), xproj(dy), yproj(-dy))
+    end
     return x - y, subtract_pullback
 end