Silencer: Scala compiler plugin for warning suppression

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NOTE: Scala 2.13.2 and 2.12.13 introduced configurable warnings. This means that unless you're still cross compiling for Scala 2.11, this plugin is obsolete, and you should use @nowarn.

If you're still cross compiling for 2.11 then this plugin can be used in conjunction with scala-collection-compat in order to suppress warnings in all Scala versions using @nowarn.


If you're using SBT, add this to your project definition:

ThisBuild / libraryDependencies ++= Seq(
  compilerPlugin("com.github.ghik" % "silencer-plugin" % silencerVersion cross CrossVersion.full),
  "com.github.ghik" % "silencer-lib" % silencerVersion % Provided cross CrossVersion.full

If you're using Gradle:

ext {
    scalaVersion = "..." // e.g. "2.13.0"
    silencerVersion = "..." // appropriate silencer version
configurations {
    scalacPlugin {
        transitive = false
dependencies {
    compile "com.github.ghik:silencer-lib_$scalaVersion:$silencerVersion"
    scalacPlugin "com.github.ghik:silencer-plugin_$scalaVersion:$silencerVersion"
tasks.withType(ScalaCompile) {
    scalaCompileOptions.additionalParameters =
            configurations.scalacPlugin.collect { "-Xplugin:" + it.absolutePath }

Note that since both silencer-plugin and silencer-lib are compile time only dependencies, Silencer can be used in ScalaJS and Scala Native without having to be cross compiled for them.

Annotation-based suppression

With the plugin enabled, warnings can be suppressed using the @com.github.ghik.silencer.silent or @scala.annotation.nowarn annotation. It can be applied on a single statement or expression, entire def/val/var definition or entire class/object/trait definition.

import com.github.ghik.silencer.silent

@silent class someClass { ... }
@silent def someMethod() = { ... }
someDeprecatedApi("something"): @silent

Message pattern

By default the @silent annotation suppresses all warnings in some code fragment. You can limit the suppression to some specific classes of warnings by passing a message pattern (regular expression) to the annotation, e.g.

def usesDeprecatedApi(): Unit = {

Using @nowarn

Scala 2.13.2 and 2.12.13 introduced configurable warnings using -Wconf compiler option and @scala.annotation.nowarn. annotation. For Scala 2.11, this annotation is provided by the scala-collection-compat library and interpreted by the silencer plugin.

NOTE: @nowarn in Scala 2.13.2 supports various fine-grained filters (e.g. warning category, message patttern, etc.). Silencer only supports the msg=<pattern> filter - all other filters simply suppress everything, as if there were no filters specified.

Detecting unused annotations

If a @silent annotation does not actually suppress any warnings, you can make silencer report an error in such situation. This can be enabled by passing the checkUnused option to the plugin:

scalacOptions += "-P:silencer:checkUnused"

Global regex-based suppression

You can also suppress warnings globally based on a warning message regex. In order to do that, pass this option to scalac:

scalacOptions += "-P:silencer:globalFilters=<semicolon separated message regexes>"

Line content based suppression

Filtering may also be based on the content of source line that generated the warning. This is particularly useful for suppressing 'unused import' warnings based on what's being imported.

scalacOptions += "-P:silencer:lineContentFilters=<semicolon separated line content regexes>"

Filename based suppression

Another option is to suppress all warnings in selected source files. This can be done by specifying a list of file path regexes:

scalacOptions += "-P:silencer:pathFilters=<semicolon separated file path regexes>"

NOTE: In order to make builds independent of environment, filename separators are normalized to UNIX style (/) before the path is matched against path patterns.

By default, absolute file path is matched against path patterns. In order to make your build independent of where your project is checked out, you can specify a list of source root directories. Source file paths will be relativized with respect to them before being matched against path patterns. Usually it should be enough to pass project base directory as source root (i.e. baseDirectory.value in SBT):

scalacOptions += s"-P:silencer:sourceRoots=${baseDirectory.value.getCanonicalPath}"

Another good choice for source roots may be actual SBT source directories:

scalacOptions += s"-P:silencer:sourceRoots=${";")}"

Searching macro expansions

By default (starting from version 1.6.0) silencer does not look for @silent annotations in macro expansions. If you want to bring back the old behaviour where both macro expansions and expandees are searched, use the -P:silencer:searchMacroExpansions option.