Improve error position and recovery for bad dollar

Author: som-snytt

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

@@ -1168,7 +1168,9 @@ object Scanners {
           finishNamedToken(IDENTIFIER, target = next)
         }
         else
-          error("invalid string interpolation: `$$`, `$\"`, `$`ident or `$`BlockExpr expected")
+          error("invalid string interpolation: `$$`, `$\"`, `$`ident or `$`BlockExpr expected", off = charOffset - 2)
+          putChar('$')
+          getStringPart(multiLine)
       }
       else {
         val isUnclosedLiteral = !isUnicodeEscape && (ch == SU || (!multiLine && (ch == CR || ch == LF)))

compiler/test/dotty/tools/repl/ReplTest.scala

@@ -63,8 +63,9 @@ extends ReplDriver(options, new PrintStream(out, true, StandardCharsets.UTF_8.na
         case "" => Nil
         case nonEmptyLine => nonEmptyLine :: Nil
       }
+    def nonBlank(line: String): Boolean = line.exists(!Character.isWhitespace(_))
 
-    val expectedOutput = lines.flatMap(filterEmpties)
+    val expectedOutput = lines.filter(nonBlank)
     val actualOutput = {
       val opts = toolArgsParse(lines.take(1))
       val (optsLine, inputLines) = if opts.isEmpty then ("", lines) else (lines.head, lines.drop(1))
@@ -80,7 +81,7 @@ extends ReplDriver(options, new PrintStream(out, true, StandardCharsets.UTF_8.na
         out.linesIterator.foreach(buf.append)
         nstate
       }
-      (optsLine :: buf.toList).flatMap(filterEmpties)
+      (optsLine :: buf.toList).filter(nonBlank)
     }
 
     if !FileDiff.matches(actualOutput, expectedOutput) then

tests/neg/interpolator-dollar.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/interpolator-dollar.scala:5:20 ---------------------------------------------------------------------
+5 |  def oops = f"$s%s $<s" // error: invalid string interpolation
+  |                    ^
+  |                    invalid string interpolation: `$$`, `$"`, `$`ident or `$`BlockExpr expected

tests/neg/interpolator-dollar.scala

@@ -0,0 +1,6 @@
+
+trait T {
+  def s = { println("SSS") ; "hello" }
+  def ok = String.format("%s, %<s", s)
+  def oops = f"$s%s $<s" // error: invalid string interpolation
+}