From 268b325760f56588da9a3f74c5ab10372f2256b7 Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Tue, 8 Apr 2025 08:47:27 -0700
Subject: [PATCH] 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.
---
 .../dotty/tools/dotc/parsing/Scanners.scala   | 26 +++++++++++++++----
 .../src/dotty/tools/dotc/util/Chars.scala     |  2 +-
 .../dotty/tools/repl/ReplCompilerTests.scala  | 10 +++++++
 3 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/compiler/src/dotty/tools/dotc/parsing/Scanners.scala b/compiler/src/dotty/tools/dotc/parsing/Scanners.scala
index f7050cec41fd..31f074c3f633 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Scanners.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Scanners.scala
@@ -603,6 +603,20 @@ 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)
@@ -615,9 +629,8 @@ 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 =>
@@ -671,13 +684,16 @@ object Scanners {
         reset()
         if atEOL then token = COLONeol
 
-    // consume => and insert <indent> if applicable
+    // consume => and insert <indent> if applicable. Used to detect colon arrow: x =>
     def observeArrowIndented(): Unit =
       if isArrow && indentSyntax then
         peekAhead()
-        val atEOL = isAfterLineEnd || token == EOF
+        val atEOL = isAfterLineEnd
+        val atEOF = token == EOF
         reset()
-        if atEOL then
+        if atEOF then
+          token = EOF
+        else if atEOL then
           val nextWidth = indentWidth(next.offset)
           val lastWidth = currentRegion.indentWidth
           if lastWidth < nextWidth then
diff --git a/compiler/src/dotty/tools/dotc/util/Chars.scala b/compiler/src/dotty/tools/dotc/util/Chars.scala
index 916bdfa9dca3..e68c48903a63 100644
--- a/compiler/src/dotty/tools/dotc/util/Chars.scala
+++ b/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? */
diff --git a/compiler/test/dotty/tools/repl/ReplCompilerTests.scala b/compiler/test/dotty/tools/repl/ReplCompilerTests.scala
index d32b28647c32..0592cbbed1be 100644
--- a/compiler/test/dotty/tools/repl/ReplCompilerTests.scala
+++ b/compiler/test/dotty/tools/repl/ReplCompilerTests.scala
@@ -511,6 +511,16 @@ class ReplCompilerTests extends ReplTest:
     val all = lines()
     assertTrue(hints.forall(hint => all.exists(_.contains(hint))))
 
+  @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");