Make RedundantStringConversion an error

Six toString() calls on what string concatenation or Path.of converts
anyway are gone: on the diagnostics Main prints, on the paths MainTest
expects, and on an environment variable, which is a String already.

Author: abashev

buildSrc/src/main/groovy/open-java-format.error-prone-conventions.gradle

@@ -28,6 +28,7 @@ tasks.withType(JavaCompile).configureEach {
         error('FormatStringConcatenation')
         error('JUnitClassModifiers')
         error('JUnitMethodDeclaration')
+        error('RedundantStringConversion')
         error('RefactorSwitch')
         error('SameNameButDifferent')
         error('UnusedVariable')

open-java-format-jdk-bootstrap/src/test/java/com/palantir/javaformat/bootstrap/FormatterServicesTest.java

@@ -102,8 +102,7 @@ private static Stream<FormatterService> getFormatters() {
         return Stream.of(
                 new BootstrappingFormatterService(
                         javaBinPath(), Runtime.version().feature(), getClasspath()),
-                new NativeImageFormatterService(
-                        Path.of(System.getenv("NATIVE_IMAGE_CLASSPATH").toString())));
+                new NativeImageFormatterService(Path.of(System.getenv("NATIVE_IMAGE_CLASSPATH"))));
     }
 
     private String getTestResourceContent(String resourceName) {

open-java-format/src/main/java/com/palantir/javaformat/java/Main.java

@@ -165,7 +165,7 @@ private int formatFiles(
             } catch (ExecutionException e) {
                 if (e.getCause() instanceof FormatterException) {
                     for (FormatterDiagnostic diagnostic : ((FormatterException) e.getCause()).diagnostics()) {
-                        errWriter.println(path + ":" + diagnostic.toString());
+                        errWriter.println(path + ":" + diagnostic);
                     }
                 } else {
                     errWriter.println(path + ": error: "
@@ -228,7 +228,7 @@ private int formatStdin(CommandLineOptions parameters, JavaFormatterOptions opti
             }
         } catch (FormatterException e) {
             for (FormatterDiagnostic diagnostic : e.diagnostics()) {
-                errWriter.println(stdinFilename + ":" + diagnostic.toString());
+                errWriter.println(stdinFilename + ":" + diagnostic);
             }
             ok = false;
             // TODO(cpovirk): Catch other types of exception (as we do in the formatFiles case).

open-java-format/src/test/java/com/palantir/javaformat/java/MainTest.java

@@ -439,10 +439,7 @@ void dryRunFiles() throws Exception {
         assertThat(exitCode).isEqualTo(0);
 
         assertThat(out.toString())
-                .isEqualTo(b.toAbsolutePath().toString()
-                        + System.lineSeparator()
-                        + c.toAbsolutePath().toString()
-                        + System.lineSeparator());
+                .isEqualTo(b.toAbsolutePath() + System.lineSeparator() + c.toAbsolutePath() + System.lineSeparator());
         assertThat(err.toString()).isEmpty();
     }
 
@@ -495,7 +492,7 @@ void exitIfChangedFiles() throws Exception {
         Process process = formatterMain(path.toAbsolutePath().toString()).start();
         process.waitFor();
         String out = new String(ByteStreams.toByteArray(process.getInputStream()), UTF_8);
-        assertThat(out).isEqualTo(path.toAbsolutePath().toString() + System.lineSeparator());
+        assertThat(out).isEqualTo(path.toAbsolutePath() + System.lineSeparator());
         assertThat(process.exitValue()).isEqualTo(1);
     }