Disable toString plugin

This is a Scala 2 and 3 compiler plugin to trigger compiler warnings (which can be turned into errors with -Xfatal-warnings), when certain types are converted to Strings via global methods like .toString, string interpolation, and calls to mkString on iterables of that type.

Install

Add the resolver and compiler plugin dependency to your build.sbt:

resolvers += "bondlink-maven-repo" at "https://maven.bondlink-cdn.com"
addCompilerPlugin("bondlink" %% "disable-to-string-plugin" % "0.2.2")

Configure

To configure which types trigger warnings, pass additional flags to scalac. Available options are:

Option	Behavior
-P:disableToString:all	Disable string conversions on all types
-P:disableToString:literal=com.example.Type	Disable string conversions for a type named exactly com.example.Type
-P:disableToString:regex=com\\.example\\.(Example)?Type	Disable string conversions for types matching the given regex

You can add these in SBT with

scalacOptions += "-P:disableToString:all"

Use

Once you've configured types through scalac options, you should see warnings reported when compiling code:

val test1 = 1.toString
         /* ^
            Use a `cats.Show` instance instead of `Int.toString` */

val test2 = s"foo ${1}"
                 /* ^
                    Only strings can be concatenated. Consider defining a `cats.Show[Int]` and using `show"..."` from
                    `cats.syntax.show._` */

val test3 = List(1, 2).mkString(",")
              /* ^
                 Use `cats.Foldable[List].mkString_` instead of `List[Int].mkString`
                 or convert elements to `String`s before calling `mkString` */

The plugin encourages use of cats.Show, but also works with scalaz.Show. All of the cases above are allowed within Show instances so code like this will compile without warnings:

new cats.Show[Int] { def show(i: Int) = i.toString }
cats.Show.show((i: Int) => i.toString)

new scalaz.Show[Int] { def show(i: Int) = scalaz.Cord(i.toString) }
scalaz.Show.show((i: Int) => i.toString)