fix var scope of thunk macro

Author: chengchingwen

src/tangent_types/thunks.jl

@@ -144,11 +144,18 @@ Define a [`Thunk`](@ref) wrapping the `expr`, to lazily defer its evaluation.
 macro thunk(body)
     # Basically `:(Thunk(() -> $(esc(body))))` but use the location where it is defined.
     # so we get useful stack traces if it errors.
-    func = Expr(:->, Expr(:tuple), Expr(:block, __source__, body))
+    letargs = Base._lift_one_interp!(body)
+    func = Base.replace_linenums!(:(()->($(esc(body)))), __source__)
     return quote
-        _usethunks() ?
-            Thunk($(esc(func))) :
-            $(esc(body))
+        if _usethunks()
+            let $(letargs...)
+                Thunk($func)
+            end
+        else
+            let $(letargs...)
+                $(esc(body))
+            end
+        end
     end
 end