improve Scala 2 vs 3 cross-compilation scalac options

Author: mr-git

build.sbt

@@ -10,10 +10,26 @@ startYear := Some(2023)
 
 crossScalaVersions := Seq("2.13.16", "3.3.6")
 scalaVersion := crossScalaVersions.value.head
-scalacOptions := Seq(
-  "-release:17",
-  "-Xsource:3",
-  "-deprecation",
+scalacOptions := crossSettings(
+  scalaVersion = scalaVersion.value,
+  if2 = Seq(
+    "-Xsource:3",
+  ),
+  // Good compiler options for Scala 2.13 are coming from com.evolution:sbt-scalac-opts-plugin:0.0.9,
+  // but its support for Scala 3 is limited, especially what concerns linting options.
+  //
+  // If Scala 3 is made the primary target, good linting scalac options for it should be added first.
+  if3 = Seq(
+    "-Ykind-projector:underscores",
+
+    // disable new brace-less syntax:
+    // https://alexn.org/blog/2022/10/24/scala-3-optional-braces/
+    "-no-indent",
+
+    // improve error messages:
+    "-explain",
+    "-explain-types",
+  ),
 )
 autoAPIMappings := true
 versionScheme := Some("early-semver")
@@ -27,6 +43,13 @@ libraryDependencies ++= Seq(
 
 licenses := Seq(("MIT", url("https://opensource.org/licenses/MIT")))
 
+def crossSettings[T](scalaVersion: String, if3: T, if2: T): T = {
+  scalaVersion match {
+    case version if version.startsWith("3") => if3
+    case _ => if2
+  }
+}
+
 addCommandAlias("fmt", "all scalafmtAll scalafmtSbt")
 addCommandAlias("check", "all versionPolicyCheck scalafmtCheckAll scalafmtSbtCheck")
 addCommandAlias("build", "all compile test")