Fix scala#6622: Add code interpolation

Allows to get string representations for code passed in the interpolated
values

```scala
inline def logged(p1: => Any) = {
  val c = code"code: $p1"
  val res = p1
  (c, p1)
}
logged(indentity("foo"))
```

is equivalent to:
```scala
("code: indentity("foo")", indentity("foo"))
```

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -234,6 +234,8 @@ class Definitions {
     def Compiletime_constValue(implicit ctx: Context): Symbol = Compiletime_constValueR.symbol
     @threadUnsafe lazy val Compiletime_constValueOptR: TermRef = CompiletimePackageObjectRef.symbol.requiredMethodRef("constValueOpt")
     def Compiletime_constValueOpt(implicit ctx: Context): Symbol = Compiletime_constValueOptR.symbol
+    @threadUnsafe lazy val Compiletime_codeR: TermRef = CompiletimePackageObjectRef.symbol.requiredMethodRef("code")
+    def Compiletime_code(implicit ctx: Context): Symbol = Compiletime_codeR.symbol
 
   /** The `scalaShadowing` package is used to safely modify classes and
    *  objects in scala so that they can be used from dotty. They will

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -472,7 +472,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
     val expansion = inliner.transform(rhsToInline)
 
     def issueError() = callValueArgss match {
-      case (msgArg :: rest) :: Nil =>
+      case (msgArg :: Nil) :: Nil =>
         msgArg.tpe match {
           case ConstantType(Constant(msg: String)) =>
             // Usually `error` is called from within a rewrite method. In this
@@ -482,23 +482,49 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
             val callToReport = if (enclosingInlineds.nonEmpty) enclosingInlineds.last else call
             val ctxToReport = ctx.outersIterator.dropWhile(enclosingInlineds(_).nonEmpty).next
             def issueInCtx(implicit ctx: Context) = {
-              def decompose(arg: Tree): String = arg match {
-                case Typed(arg, _) => decompose(arg)
-                case SeqLiteral(elems, _) => elems.map(decompose).mkString(", ")
-                case arg =>
-                  arg.tpe.widenTermRefExpr match {
-                    case ConstantType(Constant(c)) => c.toString
-                    case _ => arg.show
-                  }
-              }
-              ctx.error(s"$msg${rest.map(decompose).mkString(", ")}", callToReport.sourcePos)
+              ctx.error(msg, callToReport.sourcePos)
             }
             issueInCtx(ctxToReport)
           case _ =>
         }
       case _ =>
     }
 
+    def issueCode()(implicit ctx: Context): Literal = {
+      def decompose(arg: Tree): String = arg match {
+        case Typed(arg, _) => decompose(arg)
+        case SeqLiteral(elems, _) => elems.map(decompose).mkString(", ")
+        case Block(Nil, expr) => decompose(expr)
+        case Inlined(_, Nil, expr) => decompose(expr)
+        case arg =>
+          arg.tpe.widenTermRefExpr match {
+            case ConstantType(Constant(c)) => c.toString
+            case _ => arg.show
+          }
+      }
+
+     def malformedString(): String = {
+       ctx.error("Malformed part `code` string interpolator", call.sourcePos)
+       ""
+      }
+
+      callValueArgss match {
+        case List(List(Apply(_,List(Typed(SeqLiteral(Literal(headConst) :: parts,_),_)))), List(Typed(SeqLiteral(interpolatedParts,_),_)))
+            if parts.size == interpolatedParts.size =>
+          val constantParts = parts.map {
+            case Literal(const) => const.stringValue
+            case _ => malformedString()
+          }
+          val decomposedInterpolations = interpolatedParts.map(decompose)
+          val constantString = decomposedInterpolations.zip(constantParts)
+              .foldLeft(headConst.stringValue) { case (acc, (p1, p2)) => acc + p1 + p2 }
+
+          Literal(Constant(constantString)).withSpan(call.span)
+        case _ =>
+          Literal(Constant(malformedString()))
+      }
+    }
+
     trace(i"inlining $call", inlining, show = true) {
 
       // The normalized bindings collected in `bindingsBuf`
@@ -522,9 +548,13 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(implicit ctx: Context) {
 
       if (inlinedMethod == defn.Compiletime_error) issueError()
 
-      // Take care that only argument bindings go into `bindings`, since positions are
-      // different for bindings from arguments and bindings from body.
-      tpd.Inlined(call, finalBindings, finalExpansion)
+      if (inlinedMethod == defn.Compiletime_code) {
+        issueCode()(ctx.fresh.setSetting(ctx.settings.color, "never"))
+      } else {
+        // Take care that only argument bindings go into `bindings`, since positions are
+        // different for bindings from arguments and bindings from body.
+        tpd.Inlined(call, finalBindings, finalExpansion)
+      }
     }
   }
 

library/src-3.x/scala/compiletime/package.scala

@@ -4,7 +4,23 @@ package object compiletime {
 
   erased def erasedValue[T]: T = ???
 
-  inline def error(inline msg: String, objs: Any*): Nothing = ???
+  inline def error(inline msg: String): Nothing = ???
+
+  /** Returns the string representations for code passed in the interpolated values
+   *  ```scala
+   *  inline def logged(p1: => Any) = {
+   *    val c = code"code: $p1"
+   *    val res = p1
+   *    (c, p1)
+   *  }
+   *  logged(indentity("foo"))
+   *  ```
+   *  is equivalent to:
+   *  ```scala
+   *  ("code: indentity("foo")", indentity("foo"))
+   *  ```
+   */
+  inline def (self: => StringContext) code (args: => Any*): String = ???
 
   inline def constValueOpt[T]: Option[T] = ???
 

tests/neg/i6622.scala

@@ -0,0 +1,9 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    println(StringContext("abc ", "", "").code(println(34))) // error
+  }
+
+}

tests/neg/i6622a.scala

@@ -0,0 +1,11 @@
+import scala.compiletime._
+
+object Test {
+
+  def nonConstant: String = ""
+
+  def main(args: Array[String]): Unit = {
+    println(StringContext("abc ", nonConstant).code(println(34))) // error
+  }
+
+}

tests/neg/i6622b.scala

@@ -0,0 +1,9 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    println(StringContext("abc ").code(println(34), 34)) // error
+  }
+
+}

tests/neg/i6622c.scala

@@ -0,0 +1,9 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    println(StringContext(Seq.empty[String]:_*).code(println(34))) // error
+  }
+
+}

tests/neg/i6622d.scala

@@ -0,0 +1,9 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    println(StringContext("abc").code(Seq.empty[Any]:_*)) // error
+  }
+
+}

tests/neg/i6622e.scala

@@ -0,0 +1,9 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    println(StringContext(Seq.empty[String]:_*).code(Seq.empty[Any]:_*)) // error
+  }
+
+}

tests/neg/i6622f.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i6622f.scala:6:8 -----------------------------------------------------------------------------------
+6 |    fail(println("foo")) // error
+  |    ^^^^^^^^^^^^^^^^^^^^
+  |    failed: println("foo") ...

tests/neg/i6622f.scala

@@ -0,0 +1,11 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    fail(println("foo")) // error
+  }
+
+  inline def fail(p1: => Any) = error(code"failed: $p1 ...")
+
+}

tests/run/i6622.scala

@@ -0,0 +1,15 @@
+import scala.compiletime._
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    assert(code"abc ${println(34)} ..." == "abc println(34) ...")
+    assert(code"abc ${println(34)}" == "abc println(34)")
+    assert(code"${println(34)} ..." == "println(34) ...")
+    assert(code"${println(34)}" == "println(34)")
+    assert(code"..." == "...")
+    assert(testConstant(code"") == "")
+  }
+
+  inline def testConstant(inline msg: String): String = msg
+}