Add customizable names for definitions in quotes

This allow naming val/def in quoted code with computed names

Author: nicolasstucki

docs/docs/reference/metaprogramming/macros.md

@@ -564,6 +564,38 @@ while (i < arr.length) {
 }
 sum
 ```
+### Sowing meaningful definition names in quotes
+
+In the `powerCode` example above there is a `'{ val y = $x * $x; ... }` which when printed
+may show several different `val y = ...`. Even though there is no higene issue it may be hard
+to read the code. To overcome this each `y` can be assigned a meeningful name using the
+`scala.quoted.show.showName` annotation. For example `'{ @showName(${Expr("y" + i)}) val y = $x * $x; ... }`
+will assign to each `y` a name `y{i}` where `{i}` is a known String, if `i == 3` then it would be named `x3`.
+
+The `powerCode` can be defined as follows using `showName`
+```scala
+def powerCode(n: Long, x: Expr[Double]))(given QuoteContext): Expr[Double] = '{
+  val x1 = $x
+  ${ powerCode(n, 2, 'x1) }
+}
+def powerCode(n: Long, i: Int, x: Expr[Double])(given QuoteContext): Expr[Double] =
+  if (n == 0) '{1.0}
+  else if (n % 2 == 0) '{ @showName(${Expr("x" + i)}) val y = $x * $x; ${powerCode(n / 2, idx * 2, 'y)} }
+  else '{ $x * ${powerCode(n - 1, idx, x)} }
+```
+then
+```scala
+powerCode(16, '{7}).show
+```
+will show
+```scala
+val x1: scala.Double = 7
+val x2: scala.Double = x1.*(x1)
+val x4: scala.Double = x2.*(x2)
+val x8: scala.Double = x4.*(x4)
+val x16: scala.Double = x8.*(x8)
+x16
+```
 
 ### Find implicits within a macro
 

library/src/scala/quoted/show/showName.scala

@@ -0,0 +1,24 @@
+package scala.quoted.show
+
+/** Annotation used inside a quote to give a custom name to a definition.
+ *  The `name` argument must be a literal String.
+ *
+ *  Usage:
+ *  ```scala
+ *  def let(name: String)(value: Expr[Int])(in: Expr[Int] => Expr[Int]): Expr[Int] = '{
+ *    @showName(${Expr(name)})
+ *    val x = $value
+ *    ${ in('x) }
+ *  }
+ *  ```
+ *  then using it in
+ *  ```scala
+ *  let("myVal")('{4})(x => '{ $x + 1}).show
+ *  ```
+ *  will retuns the code
+ *  ```scala
+ *  val myVal = 4
+ *  myVal + 1
+ *  ```
+ */
+class showName(name: String) extends scala.annotation.Annotation

library/src/scala/tasty/reflect/Printers.scala

@@ -679,7 +679,8 @@ trait Printers
           if (vdef.symbol.flags.is(Flags.Mutable)) this += highlightKeyword("var ")
           else this += highlightKeyword("val ")
 
-          this += highlightValDef(name) += ": "
+          val name1 = splicedName(vdef.symbol).getOrElse(name)
+          this += highlightValDef(name1) += ": "
           printTypeTree(tpt)
           rhs match {
             case Some(tree) =>
@@ -714,7 +715,8 @@ trait Printers
 
           printProtectedOrPrivate(ddef)
 
-          this += highlightKeyword("def ") += highlightValDef((if (isConstructor) "this" else name))
+          val name1: String = if (isConstructor) "this" else splicedName(ddef.symbol).getOrElse(name)
+          this += highlightKeyword("def ") += highlightValDef(name1)
           printTargsDefs(targs.zip(targs))
           val it = argss.iterator
           while (it.hasNext)
@@ -734,8 +736,11 @@ trait Printers
         case Ident("_") =>
           this += "_"
 
-        case IsTerm(tree @ Ident(_)) =>
-          printType(tree.tpe)
+        case IsIdent(tree) =>
+          splicedName(tree.symbol) match {
+            case Some(name) => this += name
+            case _ => printType(tree.tpe)
+          }
 
         case Select(qual, name) =>
           printQualTree(qual)
@@ -1637,12 +1642,15 @@ trait Printers
 
       def printAnnotation(annot: Term)(given elideThis: Option[Symbol]): Buffer = {
         val Annotation(ref, args) = annot
-        this += "@"
-        printTypeTree(ref)
-        if (args.isEmpty)
-          this
-        else
-          inParens(printTrees(args, ", "))
+        if (annot.symbol.owner.fullName == "scala.quoted.show.showName") this
+        else {
+          this += "@"
+          printTypeTree(ref)
+          if (args.isEmpty)
+            this
+          else
+            inParens(printTrees(args, ", "))
+        }
       }
 
       def printDefAnnotations(definition: Definition)(given elideThis: Option[Symbol]): Buffer = {
@@ -1809,6 +1817,14 @@ trait Printers
       private def escapedString(str: String): String = str flatMap escapedChar
     }
 
+    private def splicedName(sym: Symbol)(given ctx: Context): Option[String] = {
+      sym.annots.find(_.symbol.owner.fullName == "scala.quoted.show.showName").flatMap {
+        case Apply(_, Literal(Constant(c: String)) :: Nil) => Some(c)
+        case Apply(_, Inlined(_, _, Literal(Constant(c: String))) :: Nil) => Some(c)
+        case annot => None
+      }
+    }
+
     private object SpecialOp {
       def unapply(arg: Tree)(given ctx: Context): Option[(String, List[Term])] = arg match {
         case IsTerm(arg @ Apply(fn, args)) =>

tests/run-staging/quoted-show-name.check

@@ -0,0 +1,13 @@
+((x1: scala.Double) => x1.*({
+   val x2: scala.Double = x1.*(x1)
+   val x4: scala.Double = x2.*(x2)
+  x4.*({
+     val x8: scala.Double = x4.*(x4)
+    x8.*({
+       val x16: scala.Double = x8.*(x8)
+       val x32: scala.Double = x16.*(x16)
+       val x64: scala.Double = x32.*(x32)
+      x64
+    })
+  })
+}))

tests/run-staging/quoted-show-name.scala

@@ -0,0 +1,21 @@
+import scala.quoted._
+import scala.quoted.show.showName
+import scala.quoted.staging._
+import scala.reflect.ClassTag
+
+object Test {
+  given Toolbox = Toolbox.make(getClass.getClassLoader)
+  def main(args: Array[String]): Unit = withQuoteContext {
+    println(powerCode(77).show)
+  }
+
+  def powerCode(n: Long)(given QuoteContext): Expr[Double => Double] =
+    '{ x1 => ${powerCode(n, 2, 'x1)} }
+
+  def powerCode(n: Long, idx: Int, x: Expr[Double])(given QuoteContext): Expr[Double] =
+    if (n == 0) '{1.0}
+    else if (n == 1) x
+    else if (n % 2 == 0) '{ @showName(${Expr("x" + idx)}) val y = $x * $x; ${powerCode(n / 2, idx * 2, '{y})} }
+    else '{ $x * ${powerCode(n - 1, idx, x)} }
+
+}