add safe keyword, opposite of unsafe(left/right)

src/macro.jl

@@ -137,6 +137,7 @@ function parse_options(exs...)
         )
     expr = nothing
     nograd = Symbol[]
+    safe = Symbol[]
     ranges = Tuple[]
     for ex in exs
         # Actual options:
@@ -160,6 +161,16 @@ function parse_options(exs...)
                 throw("this accepts nograd=A or nograd=(A,B,C)")
             end
 
+        # Safe keyword
+        elseif isexpr(ex, :(=)) && ex.args[1] == :safe
+            if ex.args[2] isa Symbol
+                push!(safe, ex.args[2])
+            elseif isexpr(ex.args[2], :tuple)
+                append!(safe, ex.args[2].args)
+            else
+                throw("this accepts safe=i or safe=(i,j,k)")
+            end
+
         # Ranges specified outside:
         elseif isexpr(ex, :call) && ex.args[1] in [:in, :∈]
             push!(ranges, (ex.args[2], ex.args[3]))
@@ -201,6 +212,7 @@ function parse_options(exs...)
         cuda=opts[:cuda],
         tensor=opts[:tensor],
         nograd=nograd,
+        safe=safe,
     ), ranges, expr
 end
 
@@ -586,7 +598,7 @@ detectunsafe(expr, list, store) = MacroTools_postwalk(expr) do ex
             MacroTools_postwalk(i) do x
                 @capture_(x, B_[inner__]) || return x
                 # Now we have found an array which indexes another one, mark its indices unsafe
-                append!(list, filter(j -> j isa Symbol, inner))
+                append!(list, setdiff(filter(j -> j isa Symbol, inner), store.safe))
                 unique!(list)
                 # and don't compute a gradient for the inner array
                 B isa Symbol && push!(store.nograd, B)