Implement indent rule for match

Cases of a match do not have to be indented relative to the match

Author: odersky

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

@@ -307,7 +307,7 @@ object Scanners {
     var sepRegions: List[Token] = Nil
 
     /** Indentation widths, innermost to outermost */
-    var indent: IndentRegion = IndentRegion(IndentWidth.Zero, Set(), null)
+    var indent: IndentRegion = IndentRegion(IndentWidth.Zero, Set(), EMPTY, null)
 
 // Scala 2 compatibility
 
@@ -538,13 +538,15 @@ object Scanners {
           !isLeadingInfixOperator())
         insert(if (pastBlankLine) NEWLINES else NEWLINE, lineOffset)
       else if (indentIsSignificant) {
-        if (lastWidth < curWidth) {
+        if (lastWidth < curWidth ||
+            lastWidth == curWidth && (lastToken == MATCH || lastToken == CATCH) && token == CASE) {
           if (canStartIndentTokens.contains(lastToken)) {
-            indent = IndentRegion(curWidth, Set(), indent)
+            indent = IndentRegion(curWidth, Set(), lastToken, indent)
             insert(INDENT, offset)
           }
         }
-        else if (curWidth < lastWidth) {
+        else if (curWidth < lastWidth ||
+                 curWidth == lastWidth && (indent.token == MATCH || indent.token == CATCH) && token != CASE) {
           if (!isLeadingInfixOperator()) {
             indent = indent.enclosing
             insert(OUTDENT, offset)
@@ -564,7 +566,7 @@ object Scanners {
                 |This falls between previous widths: ${indent.width} and $lastWidth
                 |Sepregions = $sepRegions / ${getClass}""")
         else
-          indent = IndentRegion(indent.width, indent.others + curWidth, indent.outer)
+          indent = IndentRegion(indent.width, indent.others + curWidth, indent.token, indent.outer)
       }
     }
 
@@ -1291,7 +1293,7 @@ object Scanners {
    *  @param width   The principal indendation width
    *  @param others  Other indendation widths > width of lines in the same region
    */
-  class IndentRegion(val width: IndentWidth, val others: Set[IndentWidth], val outer: IndentRegion | Null) {
+  class IndentRegion(val width: IndentWidth, val others: Set[IndentWidth], val token: Token, val outer: IndentRegion | Null) {
     def enclosing: IndentRegion = outer.asInstanceOf[IndentRegion]
     def isOutermost = outer == null
   }

docs/docs/reference/other-new-features/indentation.md

@@ -71,15 +71,15 @@ The Scala grammar is changed so that colons at end of lines are accepted at all
 where an opening brace is legal, except if the previous token can already start an
 indentation region.
 
-### Special Treatment of Match
+### Special Treatment of Case Clauses
 
-The indentation rules for a `match` expression are modified as follows:
+The indentation rules for `match` expressions and `catch` clauses are refined as follows:
 
- - An indentation region is opened after a `match` also if the following `case`
+ - An indentation region is opened after a `match` or `catch` also if the following `case`
    appears at the indentation width that's current for the `match` itself.
  - In that case, the indentation region closes at the first token at that
    same indentation width that is not a `case`, or at any token with a smaller
-	indentation width, whichever comes first.
+   indentation width, whichever comes first.
 
 The rules allow to write `match` expressions where cases are not indented themselves, as in the example below:
 ```scala

tests/pos/indent.scala

@@ -70,6 +70,17 @@ class Coder(words: List[String]):
     case (digit, str) => true
     case _ => false
 
+  ('1', "1") match
+  case (digit, str) => true
+  case _ => false
+
+  try List(1, 2, 3) match
+  case x :: xs => println(x)
+  case Nil => println("Nil")
+  catch
+  case ex: java.io.IOException => println(ex)
+  case ex: Throwable => throw ex
+
   /** Invert the mnemonics map to give a map from chars 'A' ... 'Z' to '2' ... '9' */
   private val charCode0: Map[Char, Char] =
     mnemonics