improvement: Eliminate empty positions in generated NIR (#3775)

nir/src/main/scala/scala/scalanative/nir/Defns.scala

@@ -20,18 +20,14 @@ import scala.scalanative.nir.Defn.Define
  *  their semantics (e.g., whether a method may be inlined). Attributes are also
  *  used to mark special-purpose definitions, such as stubs, proxies and FFIs.
  */
-sealed abstract class Defn {
+sealed abstract class Defn extends Positioned {
 
   /** Returns the name of the definition. */
   def name: Global
 
   /** Returns the attributes of the definition. */
   def attrs: Attrs
 
-  /** Returns the site in the program sources corresponding to the definition.
-   */
-  def pos: SourcePosition
-
   /** Returns a textual representation of `this`. */
   final def show: String =
     nir.Show(this)

nir/src/main/scala/scala/scalanative/nir/Insts.scala

@@ -1,9 +1,8 @@
 package scala.scalanative
 package nir
 
-sealed abstract class Inst {
+sealed abstract class Inst extends Positioned {
   final def show: String = nir.Show(this)
-  def pos: SourcePosition
 }
 
 object Inst {

nir/src/main/scala/scala/scalanative/nir/Positioned.scala

@@ -0,0 +1,21 @@
+package scala.scalanative.nir
+
+trait Positioned {
+
+  /** Returns the site in the program sources corresponding to the definition.
+   */
+  def pos: SourcePosition
+
+  if (Positioned.debugEmptyPositions && pos.isEmpty) {
+    System.err.println(s"\nFound empty position in $this, backtrace:")
+    new RuntimeException()
+      .getStackTrace()
+      .take(10)
+      .foreach(println)
+  }
+}
+
+object Positioned {
+  private final val debugEmptyPositions =
+    sys.props.contains("scalanative.debug.nir.positions")
+}

nir/src/main/scala/scala/scalanative/nir/SourcePosition.scala

@@ -11,6 +11,7 @@ sealed case class NIRSource(directory: Path, path: Path) {
 object NIRSource {
   object None extends NIRSource(null, null) {
     override def debugName: String = "<no-source>"
+    override def toString(): String = s"NIRSource($debugName)"
   }
 }
 
@@ -36,6 +37,9 @@ final case class SourcePosition(
 
   def isEmpty: Boolean = this eq SourcePosition.NoPosition
   def isDefined: Boolean = !isEmpty
+  def orElse(other: => SourcePosition): SourcePosition =
+    if (isEmpty) other
+    else this
 }
 
 object SourcePosition {

nscplugin/src/main/scala-2/scala/scalanative/nscplugin/NirGenExports.scala

@@ -168,7 +168,7 @@ trait NirGenExports[G <: nsc.Global with Singleton] {
           val boxedParams = paramTypes
             .zip(entryParams)
             .map((buf.fromExtern _).tupled(_))
-          val argsp = boxedParams.map(ValTree(_))
+          val argsp = boxedParams.map(ValTree(_)(member.pos))
           val res = buf.genApplyModuleMethod(owner, member, argsp)
           val unboxedRes = buf.toExtern(externRetType, res)
           buf.ret(unboxedRes)