Keep \t and \f escapes in UseTextBlocks to avoid incidental whitespace stripping (#1160)

Fixes #1158

Author: timtebeek

src/main/java/org/openrewrite/java/migrate/lang/UseTextBlocks.java

@@ -168,6 +168,11 @@ private J.Literal toTextBlock(J.Binary binary, String content, List<J.Literal> s
                 content = sb.toString();
                 // escape backslashes
                 content = content.replace("\\", "\\\\");
+                // re-escape tab and form feed characters as escape sequences. A literal tab or form feed is
+                // indistinguishable from incidental leading whitespace, so javac may strip it during text block
+                // whitespace processing (JLS 3.10.6), silently changing the string's value.
+                content = content.replace("\t", "\\t");
+                content = content.replace("\f", "\\f");
                 // escape triple quotes
                 content = content.replace("\"\"\"", "\"\"\\\"");
                 // preserve trailing spaces before a newline

src/test/java/org/openrewrite/java/migrate/lang/UseTextBlocksTest.java

@@ -560,6 +560,54 @@ class Test {
         );
     }
 
+    @Issue("https://github.com/openrewrite/rewrite-migrate-java/issues/1158")
+    @Test
+    void keepTabEscapeToAvoidIncidentalWhitespaceStripping() {
+        rewriteRun(
+          //language=java
+          java(
+            """
+              class Main {
+                  String s =
+                      ""
+                          + "\\tb";
+              }
+              """,
+            """
+              class Main {
+                  String s =
+                      \"""
+                      \\tb\""";
+              }
+              """
+          )
+        );
+    }
+
+    @Issue("https://github.com/openrewrite/rewrite-migrate-java/issues/1158")
+    @Test
+    void keepFormFeedEscapeToAvoidIncidentalWhitespaceStripping() {
+        rewriteRun(
+          //language=java
+          java(
+            """
+              class Main {
+                  String s =
+                      ""
+                          + "\\fb";
+              }
+              """,
+            """
+              class Main {
+                  String s =
+                      \"""
+                      \\fb\""";
+              }
+              """
+          )
+        );
+    }
+
     @Issue("https://github.com/openrewrite/rewrite-migrate-java/issues/195")
     @Test
     void newlinesAlignment() {