Fix case and add test

Author: subhramit

scalafix-rules/src/main/scala/scalafix/internal/rule/DisableSyntax.scala

@@ -113,11 +113,31 @@ final class DisableSyntax(config: DisableSyntaxConfig)
         Diagnostic("noXml", "xml literals should be avoided", token.pos)
       case token: Token.Ident
           if token.value == "asInstanceOf" && config.noAsInstanceOf =>
-        Diagnostic(
-          "asInstanceOf",
-          "asInstanceOf casts are disabled, use pattern matching instead",
-          token.pos
-        )
+        val isMatchableCast = {
+          val tokenIndex = doc.tree.tokens.indexOf(token)
+          if (tokenIndex >= 0) {
+            val tokens = doc.tree.tokens.slice(tokenIndex + 1, tokenIndex + 4)
+            tokens.size >= 3 &&
+              tokens(0).is[Token.LeftBracket] &&
+              tokens(1).is[Token.Ident] && tokens(1).asInstanceOf[Token.Ident].value == "Matchable" &&
+              tokens(2).is[Token.RightBracket]
+          } else false
+        }
+
+        if (isMatchableCast) {
+          Diagnostic(
+            "asInstanceOfMatchable",
+            "asInstanceOf[Matchable] is used here to enable pattern matching on Any. " +
+              "Consider using the .asMatchable extension method instead for better readability.",
+            token.pos
+          )
+        } else {
+          Diagnostic(
+            "asInstanceOf",
+            "asInstanceOf casts are disabled, use pattern matching instead",
+            token.pos
+          )
+        }
       case token: Token.Ident
           if token.value == "isInstanceOf" && config.noIsInstanceOf =>
         Diagnostic(

scalafix-tests/input/src/main/scala-3/test/disableSyntax/MatchableDisableSyntax.scala

@@ -0,0 +1,35 @@
+/*
+rules = DisableSyntax
+DisableSyntax.noAsInstanceOf = true
+*/
+package test.disableSyntax
+
+object MatchableAsInstanceOf {
+  class Example {
+    override def equals(obj: Any): Boolean =
+      obj.asInstanceOf[Matchable] match { /* assert: DisableSyntax.asInstanceOfMatchable
+         ^^^^^^^^^^^^^
+      asInstanceOf[Matchable] is used here to enable pattern matching on Any. Consider using the .asMatchable extension method instead for better readability.
+      */
+        case that: Example => true
+        case _ => false
+      }
+  }
+
+  def regularCast(x: Any): String =
+    x.asInstanceOf[String] /* assert: DisableSyntax.asInstanceOf
+     ^^^^^^^^^^^^^
+    asInstanceOf casts are disabled, use pattern matching instead
+    */
+
+  // whitespace between tokens
+  class AnotherExample {
+    override def equals(obj: Any): Boolean =
+      obj.asInstanceOf  [  Matchable  ] match { /* assert: DisableSyntax.asInstanceOfMatchable
+         ^^^^^^^^^^^^^
+      asInstanceOf[Matchable] is used here to enable pattern matching on Any. Consider using the .asMatchable extension method instead for better readability.
+      */
+        case _ => true
+      }
+  }
+}