Improve warnings

Author: josevalim

CHANGELOG.md

@@ -111,7 +111,7 @@
 #### Elixir
 
   * [Kernel] Using `start..stop` to match on ranges is soft-deprecated and will warn on future Elixir versions. Use `start..stop//step` instead
-  * [Kernel] Using `start..stop` to create decreasing ranges is soft-deprecated and will warn on future versions. However, note this will only warn if the right-side is a variable. For example, `1..-2` and `x..-1` won't warn, but `1..x` or `x..y` will
+  * [Kernel] Using `start..stop` to create decreasing ranges is soft-deprecated and will warn on future versions. Use `start..stop//-1` instead
 
 ### 4. Hard-deprecations
 

lib/elixir/src/elixir_parser.yrl

@@ -718,7 +718,7 @@ build_op(AST, {_Kind, Location, '//'}, Right) ->
       {'..//', Meta, [Left, Middle, Right]};
 
     _ ->
-      return_error(meta_from_location(Location), "the range step operator (//) must immediately follow the range definition operator (..), for example: 1..9//2. Syntax error before: ", "'//'")
+      return_error(meta_from_location(Location), "the range step operator (//) must immediately follow the range definition operator (..), for example: 1..9//2. If you wanted to define a default argument, use (\\\\) instead. Syntax error before: ", "'//'")
   end;
 
 build_op({UOp, _, [Left]}, {_Kind, Location, 'in'}, Right) when ?rearrange_uop(UOp) ->