Scala CLI Deprecating Features

Deprecating features (Scala CLI)

All deprecation mechanisms emit aggregated warnings (single consolidated message) via Logger.deprecationWarning / Logger.flushDeprecationWarnings, respecting suppression via --suppress-deprecated-warnings or config suppress-warning.deprecated-features true.

Warning format

The formatter always prefixes with the exact name used and the feature type:

• Single: [warn] `--foo` option is deprecated. Use --bar instead.\nDeprecated features may be removed in a future version.
• Multiple: consolidated bullet-point list with each entry prefixed by name and type

The message/detail passed by callers should NOT repeat the feature name — only provide the reason or migration hint. Pass "" for no extra detail.

1. Deprecate a CLI option (entire option)

In the options case class, add @Tag(tags.deprecatedOption("detail")) or @Tag(tags.deprecatedOption) (no detail):

@Tag(tags.deprecatedOption("Use --bar instead."))
  foo: Option[Boolean] = None,

• Fires for any name/alias of the option — the exact alias used is shown in the warning
• Detected in RestrictedCommandsParser via arg.isDeprecatedOption

2. Deprecate a CLI option alias

Add @Tag(tags.deprecated("aliasName")) alongside the @Name("aliasName"):

@Name("oldAlias")
@Tag(tags.deprecated("oldAlias"))
  myOption: Option[Boolean] = None,

• Fires only when the specific alias is used

3. Deprecate a using directive (key or value)

Add an entry to DeprecatedDirectives.deprecatedCombinationsAndReplacements in modules/build/.../preprocessing/DeprecatedDirectives.scala:

// Key swap (e.g. lib → dep):
DirectiveTemplate(Seq("oldKey"), None) -> keyReplacement("newKey")(
  deprecatedWarning("oldKey", "newKey")
)

// Deprecated for removal (no replacement):
DirectiveTemplate(Seq("removedKey"), None) -> noReplacement(
  deprecatedWarningForRemoval("removedKey")
)

• keyReplacement / valueReplacement — swap to a new key or value, emits a TextEdit for IDE quick-fix
• noReplacement — deprecated for removal, no TextEdit offered
• Emitted as a positioned Diagnostic (supports BSP with source locations)
• Not aggregated (kept as individual diagnostics for IDE support)

4. Deprecate a sub-command

Override deprecationMessage in the command object (detail only, name is auto-prefixed):

object MyCommand extends ScalaCommand[MyOptions] {
  override def deprecationMessage: Option[String] =
    Some("Use other-command instead.")
}

For deprecating only a specific command alias, override deprecatedNames:

override def deprecatedNames: Set[List[String]] = Set(List("old-alias"))

5. Deprecate a config key

Pass deprecationMessage to the Key constructor (currently supported on BooleanEntry):

val myKey = new Key.BooleanEntry(
  prefix = Seq("my"),
  name = "key",
  specificationLevel = SpecificationLevel.IMPLEMENTATION,
  description = "...",
  deprecationMessage = Some("Use my.new-key instead.")
)

• Warning emitted in Config.scala when the key is accessed

Post-deprecation checklist

1. Run ./mill -i __.compile
2. Run relevant tests
3. Run ./mill -i 'generate-reference-doc[]'.run (deprecated options/aliases are marked in reference docs)
4. Update user-facing documentation if needed