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)))

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
+}