Fix #6432: Refine positions of string interpolator string parts

From
```scala
s"abc${"123"}xyz${1}fgh"
 ^^^^^       ^^^^   ^^^^
```
to
```scala
s"abc${"123"}xyz${1}fgh"
  ^^^        ^^^    ^^^
```

Author: nicolasstucki

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -717,7 +717,7 @@ object Parsers {
      *  @param negOffset   The offset of a preceding `-' sign, if any.
      *                     If the literal is not negated, negOffset = in.offset.
      */
-    def literal(negOffset: Int = in.offset, inPattern: Boolean = false): Tree = {
+    def literal(negOffset: Int = in.offset, inPattern: Boolean = false, inStringInterpolation: Boolean = false): Tree = {
 
       def literalOf(token: Token): Literal = {
         val isNegated = negOffset < in.offset
@@ -738,7 +738,19 @@ object Parsers {
         Literal(Constant(value))
       }
 
-      atSpan(negOffset) {
+      if (inStringInterpolation) {
+        val t = in.token match {
+          case STRINGLIT | STRINGPART =>
+            val value = in.strVal
+            atSpan(negOffset, negOffset, negOffset + value.length) { Literal(Constant(value)) }
+          case _ =>
+            syntaxErrorOrIncomplete(IllegalLiteral())
+            atSpan(negOffset) { Literal(Constant(null)) }
+        }
+        in.nextToken()
+        t
+      }
+      else atSpan(negOffset) {
         if (in.token == QUOTEID) {
           if ((staged & StageKind.Spliced) != 0 && isIdentifierStart(in.name(0))) {
             val t = atSpan(in.offset + 1) {
@@ -775,10 +787,11 @@ object Parsers {
     private def interpolatedString(inPattern: Boolean = false): Tree = atSpan(in.offset) {
       val segmentBuf = new ListBuffer[Tree]
       val interpolator = in.name
+      val isTripleQuoted = in.buf(in.charOffset) == '"' && in.buf(in.charOffset + 1) == '"'
       in.nextToken()
-      while (in.token == STRINGPART) {
+      def nextSegment(literalOffset: Offset) =
         segmentBuf += Thicket(
-            literal(inPattern = inPattern),
+            literal(literalOffset, inPattern = inPattern, inStringInterpolation = true),
             atSpan(in.offset) {
               if (in.token == IDENTIFIER)
                 termIdent()
@@ -798,8 +811,14 @@ object Parsers {
                 EmptyTree
               }
             })
-      }
-      if (in.token == STRINGLIT) segmentBuf += literal(inPattern = inPattern)
+
+      if (in.token == STRINGPART)
+        nextSegment(in.offset + (if (isTripleQuoted) 3 else 1))
+      while (in.token == STRINGPART)
+        nextSegment(in.offset)
+      if (in.token == STRINGLIT)
+        segmentBuf += literal(inPattern = inPattern, negOffset = in.offset, inStringInterpolation = true)
+
       InterpolatedString(interpolator, segmentBuf.toList)
     }
 

tests/neg-macros/i6432.check

@@ -0,0 +1,6 @@
+[61..64] in Test_2.scala
+fgh
+[50..53] in Test_2.scala
+xyz
+[39..42] in Test_2.scala
+abc

tests/neg-macros/i6432/Macro_1.scala

@@ -0,0 +1,18 @@
+
+import scala.quoted._
+import scala.quoted.autolift._
+import scala.quoted.matching._
+
+object Macro {
+  inline def (sc: => StringContext) foo (args: String*): Unit = ${ impl('sc) }
+
+  def impl(sc: Expr[StringContext])(implicit reflect: tasty.Reflection): Expr[Unit] = {
+    import reflect._
+    sc match {
+      case '{ StringContext(${ExprSeq(parts)}: _*) } =>
+        for (part @ Const(s) <- parts)
+          error(s, part.unseal.pos)
+    }
+    '{}
+  }
+}

tests/neg-macros/i6432/Test_2.scala

@@ -0,0 +1,5 @@
+
+object TestA {
+  import Macro._
+  foo"abc${"123"}xyz${"456"}fgh" // error // error // error
+}

tests/neg-macros/i6432b.check

@@ -0,0 +1,6 @@
+[63..66] in Test_2.scala
+fgh
+[52..55] in Test_2.scala
+xyz
+[41..44] in Test_2.scala
+abc

tests/neg-macros/i6432b/Macro_1.scala

@@ -0,0 +1,18 @@
+
+import scala.quoted._
+import scala.quoted.autolift._
+import scala.quoted.matching._
+
+object Macro {
+  inline def (sc: => StringContext) foo (args: String*): Unit = ${ impl('sc) }
+
+  def impl(sc: Expr[StringContext])(implicit reflect: tasty.Reflection): Expr[Unit] = {
+    import reflect._
+    sc match {
+      case '{ StringContext(${ExprSeq(parts)}: _*) } =>
+        for (part @ Const(s) <- parts)
+          error(s, part.unseal.pos)
+    }
+    '{}
+  }
+}

tests/neg-macros/i6432b/Test_2.scala

@@ -0,0 +1,5 @@
+
+object TestA {
+  import Macro._
+  foo"""abc${"123"}xyz${"456"}fgh""" // error // error // error
+}