Improve dentation at EOF

Check trailing blank line at EOF for OUTDENT

If EOF is preceded by only spaces on the last line, do not outdent
because it will complain about alignment during an edit.

Preserve EOF token when probing arrow EOL

For REPL, `: x =>` at EOF sees an EOF in order to detect lambda eol.

Author: som-snytt

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

@@ -601,6 +601,20 @@ object Scanners {
           lastWidth = r.knownWidth
           newlineIsSeparating = r.isInstanceOf[InBraces]
 
+      // can emit OUTDENT if line is not non-empty blank line at EOF
+      inline def isTrailingBlankLine: Boolean =
+        token == EOF && {
+          val end = buf.length - 1 // take terminal NL as empty last line
+          val prev = buf.lastIndexWhere(!isWhitespace(_), end = end)
+          prev < 0 || end - prev > 0 && isLineBreakChar(buf(prev))
+        }
+
+      inline def canDedent: Boolean =
+           lastToken != INDENT
+        && !isLeadingInfixOperator(nextWidth)
+        && !statCtdTokens.contains(lastToken)
+        && !isTrailingBlankLine
+
       if newlineIsSeparating
          && canEndStatTokens.contains(lastToken)
          && canStartStatTokens.contains(token)
@@ -613,9 +627,8 @@ object Scanners {
            || nextWidth == lastWidth && (indentPrefix == MATCH || indentPrefix == CATCH) && token != CASE then
           if currentRegion.isOutermost then
             if nextWidth < lastWidth then currentRegion = topLevelRegion(nextWidth)
-          else if !isLeadingInfixOperator(nextWidth) && !statCtdTokens.contains(lastToken) && lastToken != INDENT then
+          else if canDedent then
             currentRegion match
-              case _ if token == EOF => // no OUTDENT at EOF
               case r: Indented =>
                 insert(OUTDENT, offset)
                 handleNewIndentWidth(r.enclosing, ir =>

compiler/src/dotty/tools/dotc/util/Chars.scala

@@ -50,7 +50,7 @@ object Chars:
   }
 
   /** Is character a whitespace character (but not a new line)? */
-  def isWhitespace(c: Char): Boolean =
+  inline def isWhitespace(c: Char): Boolean =
     c == ' ' || c == '\t' || c == CR
 
   /** Can character form part of a doc comment variable $xxx? */

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

@@ -400,6 +400,16 @@ class ReplCompilerTests extends ReplTest:
     assertTrue(all.head.startsWith("-- [E103] Syntax Error"))
     assertTrue(all.exists(_.trim().startsWith("|  Illegal start of statement: this modifier is not allowed here")))
 
+  @Test def `i22844 regression colon eol`: Unit = initially:
+    run:
+      """|println:
+         |  "hello, world"
+         |""".stripMargin // outdent, but this test does not exercise the bug
+    assertEquals(List("hello, world"), lines())
+
+  @Test def `i22844b regression colon arrow eol`: Unit = contextually:
+    assertTrue(ParseResult.isIncomplete("List(42).map: x =>"))
+
 object ReplCompilerTests:
 
   private val pattern = Pattern.compile("\\r[\\n]?|\\n");