Adding -Xfatal-warnings

[samuelgruetter]

Warnings by the Scala compiler often reveal problems with the code, and there's the -Xfatal-warnings option which makes the build fail if there are warnings. Currently, there are no warnings in this project, so we could add the -Xfatal-warnings option without problems, and be sure that further PRs don't introduce any warnings.

I tried to add the option to build.sbt, but it only applies to the root project but not to the examples project. Apparently scalacOptions in ThisBuild means "really only this project without sub-projects". But that would mean that the other options also don't apply to the examples project, which would be bad... /cc @jmhofer @zsxwing

[headinthebox]

Let's hope we don't have any warnings.

[samuelgruetter]

build.sbt is not set in stone ;-) So if at some point we get a false warning that we can't silence, we can just remove the -Xfatal-warnings option.

[zsxwing]

There are some warnings when compiling with Scala 2.11.

[warn] /Users/zsx/workspace-scala/RxScala/src/test/scala/rx/lang/scala/CompletenessTest.scala:268: method paramss in trait MethodSymbolApi is deprecated: Use `paramLists` instead
[warn]       val paramListStrs = for (paramList <- m.paramss) yield {
[warn]                                               ^
[warn] /Users/zsx/workspace-scala/RxScala/src/test/scala/rx/lang/scala/CompletenessTest.scala:281: method declarations in class TypeApi is deprecated: Use `decls` instead
[warn]     methodMembersToMethodStrings(tp.declarations.filter {
[warn]                                     ^
[warn] /Users/zsx/workspace-scala/RxScala/src/test/scala/rx/lang/scala/CompletenessTest.scala:296: method companionSymbol in trait SymbolApi is deprecated: Use `companion` instead, but beware of possible changes in behavior
[warn]       getPublicInstanceMethods(tp.typeSymbol.companionSymbol.typeSignature)
[warn]                                              ^
[warn] /Users/zsx/workspace-scala/RxScala/src/test/scala/rx/lang/scala/CompletenessTest.scala:319: method companionSymbol in trait SymbolApi is deprecated: Use `companion` instead, but beware of possible changes in behavior
[warn]                    typeOf[rx.Observable[_]].typeSymbol.companionSymbol.typeSignature)
[warn]                                                        ^
[warn] four warnings found

[jbripley]

I tried to fix those in a CompletenessTest cleanup PR that I made, but the suggested method/property changes for the remaining deprecation warnings don't exist in 2.10.

[samuelgruetter]

So this means that there is no solution which emits no warnings and no errors both in 2.10 and 2.11. Which probably means that we better give up the idea of adding the -Xfatal-warnings option...