Fix spacing, i21577.check, and warn comment changes

Author: SuperCl4sh

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -969,7 +969,6 @@ object SpaceEngine {
 
   def checkMatch(m: Match)(using Context): Unit =
     if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
-
     if reachabilityCheckable(m.selector) then
       // With explicit nulls, even if the selector type is non-nullable,
       // we still need to consider the possibility of null value,

tests/explicit-nulls/neg/pattern-matching.scala

@@ -51,4 +51,4 @@ class Foo:
     case Cat => 200
     case null => 300 // ok
 
-// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/explicit-nulls/run/pattern-matching.scala

@@ -34,4 +34,4 @@ object Test:
     val r6 = s2 match
       case null => 200
       case s2: String => 100
-    assert(r6 == 200)
+    assert(r6 == 200)

tests/explicit-nulls/warn/i21577.check

@@ -1,32 +1,32 @@
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:5:9 --------------------------------------------
-5 |    case _ => // warn
-  |         ^
-  |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:12:9 -------------------------------------------
-12 |    case _ => // warn
-   |         ^
-   |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:16:7 -------------------------------------------
-16 |  case _ => // warn
-   |       ^
-   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E121] Pattern Match Warning: tests\explicit-nulls\warn\i21577.scala:20:7 -------------------------------------------
-20 |  case _ => // warn
-   |       ^
-   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
--- [E029] Pattern Match Exhaustivity Warning: tests\explicit-nulls\warn\i21577.scala:29:27 -----------------------------
-29 |def f7(s: String | Null) = s match // warn
-   |                           ^
-   |                           match may not be exhaustive.
-   |
-   |                           It would fail on pattern case: _: Null
-   |
-   | longer explanation available when compiling with `-explain`
--- [E029] Pattern Match Exhaustivity Warning: tests\explicit-nulls\warn\i21577.scala:36:33 -----------------------------
-36 |def f9(s: String | Int | Null) = s match // warn
-   |                                 ^
-   |                                 match may not be exhaustive.
-   |
-   |                                 It would fail on pattern case: _: Int
-   |
-   | longer explanation available when compiling with `-explain`
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:5:9 --------------------------------------------
+5 |    case _ => // warn: null only
+  |         ^
+  |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:12:9 -------------------------------------------
+12 |    case _ => // warn: null only
+   |         ^
+   |         Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:16:7 -------------------------------------------
+16 |  case _ => // warn: null only
+   |       ^
+   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E121] Pattern Match Warning: tests/explicit-nulls/warn/i21577.scala:20:7 -------------------------------------------
+20 |  case _ => // warn: unreachable
+   |       ^
+   |       Unreachable case except for null (if this is intentional, consider writing case null => instead).
+-- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:29:27 -----------------------------
+29 |def f7(s: String | Null) = s match // warn: not exhaustive
+   |                           ^
+   |                           match may not be exhaustive.
+   |
+   |                           It would fail on pattern case: _: Null
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E029] Pattern Match Exhaustivity Warning: tests/explicit-nulls/warn/i21577.scala:36:33 -----------------------------
+36 |def f9(s: String | Int | Null) = s match // warn: not exhaustive
+   |                                 ^
+   |                                 match may not be exhaustive.
+   |
+   |                                 It would fail on pattern case: _: Int
+   |
+   | longer explanation available when compiling with `-explain`

tests/explicit-nulls/warn/i21577.scala

@@ -2,22 +2,22 @@ def f(s: String) =
   val s2 = s.trim()
   s2 match
     case s3: String =>
-    case _ => // warn
+    case _ => // warn: null only
 
 
 def f2(s: String | Null) =
   val s2 = s.nn.trim()
   s2 match
     case s3: String =>
-    case _ => // warn
+    case _ => // warn: null only
 
 def f3(s: String | Null) = s match
   case s2: String =>
-  case _ => // warn
+  case _ => // warn: null only
 
 def f5(s: String) = s match
   case _: String =>
-  case _ => // warn
+  case _ => // warn: unreachable
 
 def f6(s: String) = s.trim() match
   case _: String =>
@@ -26,13 +26,13 @@ def f6(s: String) = s.trim() match
 def f61(s: String) = s.trim() match
   case _: String =>
 
-def f7(s: String | Null) = s match // warn
+def f7(s: String | Null) = s match // warn: not exhaustive
   case _: String =>
 
 def f8(s: String | Null) = s match
   case _: String =>
   case null =>
 
-def f9(s: String | Int | Null) = s match // warn
+def f9(s: String | Int | Null) = s match // warn: not exhaustive
   case _: String =>
-  case null =>
+  case null =>