Emit contextual error message during incremental m2e builds

lib-extra/src/main/java/com/diffplug/spotless/extra/integration/DiffMessageFormatter.java

@@ -22,11 +22,14 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.AbstractMap;
 import java.util.List;
 import java.util.ListIterator;
+import java.util.Map;
 import java.util.Objects;
 
 import org.eclipse.jgit.diff.DiffFormatter;
+import org.eclipse.jgit.diff.Edit;
 import org.eclipse.jgit.diff.EditList;
 import org.eclipse.jgit.diff.MyersDiff;
 import org.eclipse.jgit.diff.RawText;
@@ -234,6 +237,19 @@ private void addIntendedLine(String indent, String line) {
 	 * sequence (\n, \r, \r\n).
 	 */
 	private String diff(File file) throws IOException {
+		return diff(formatter, file).getKey();
+	}
+
+	/**
+	 * Returns a map entry with key being a git-style diff between the contents of the given file and what those contents would
+	 * look like if formatted using the given formatter. Does not end with any newline
+	 * sequence (\n, \r, \r\n). The value of the map entry is the line where the first difference occurred.
+	 */
+	public static Map.Entry<String, Integer> diff(Formatter formatter, File file) throws IOException {
+		return diff(new CleanProviderFormatter(formatter), file);
+	}
+
+	private static Map.Entry<String, Integer> diff(CleanProvider formatter, File file) throws IOException {
 		String raw = new String(Files.readAllBytes(file.toPath()), formatter.getEncoding());
 		String rawUnix = LineEnding.toUnix(raw);
 		String formatted = formatter.getFormatted(file, rawUnix);
@@ -248,13 +264,13 @@ private String diff(File file) throws IOException {
 	}
 
 	/**
-	 * Returns a git-style diff between the two unix strings.
+	 * Returns a map entry with key being a git-style diff between the two unix strings and value being the line of the first difference (in the dirty string)
 	 * <p>
 	 * Output has no trailing newlines.
 	 * <p>
 	 * Boolean args determine whether whitespace or line endings will be visible.
 	 */
-	private static String diffWhitespaceLineEndings(String dirty, String clean, boolean whitespace, boolean lineEndings) throws IOException {
+	private static Map.Entry<String, Integer> diffWhitespaceLineEndings(String dirty, String clean, boolean whitespace, boolean lineEndings) throws IOException {
 		dirty = visibleWhitespaceLineEndings(dirty, whitespace, lineEndings);
 		clean = visibleWhitespaceLineEndings(clean, whitespace, lineEndings);
 
@@ -271,7 +287,11 @@ private static String diffWhitespaceLineEndings(String dirty, String clean, bool
 
 		// we don't need the diff to show this, since we display newlines ourselves
 		formatted = formatted.replace("\\ No newline at end of file\n", "");
-		return NEWLINE_MATCHER.trimTrailingFrom(formatted);
+		return new AbstractMap.SimpleEntry<>(NEWLINE_MATCHER.trimTrailingFrom(formatted), getLineOfFirstDifference(edits));
+	}
+
+	private static int getLineOfFirstDifference(EditList edits) {
+		return edits.stream().mapToInt(Edit::getBeginA).min().getAsInt();
 	}
 
 	private static final CharMatcher NEWLINE_MATCHER = CharMatcher.is('\n');

plugin-maven/src/main/java/com/diffplug/spotless/maven/SpotlessCheckMojo.java

@@ -19,10 +19,12 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugins.annotations.LifecyclePhase;
 import org.apache.maven.plugins.annotations.Mojo;
+import org.sonatype.plexus.build.incremental.BuildContext;
 
 import com.diffplug.spotless.Formatter;
 import com.diffplug.spotless.PaddedCell;
@@ -54,13 +56,17 @@ protected void process(Iterable<File> files, Formatter formatter, UpToDateChecke
 				PaddedCell.DirtyState dirtyState = PaddedCell.calculateDirtyState(formatter, file);
 				if (!dirtyState.isClean() && !dirtyState.didNotConverge()) {
 					problemFiles.add(file);
+					if (buildContext.isIncremental()) {
+						Map.Entry<String, Integer> diffEntry = DiffMessageFormatter.diff(formatter, file);
+						buildContext.addMessage(file, diffEntry.getValue(), 0, diffEntry.getKey(), BuildContext.SEVERITY_ERROR, null);
+					}
 					counter.cleaned();
 				} else {
 					counter.checkedButAlreadyClean();
 					upToDateChecker.setUpToDate(file.toPath());
 				}
 			} catch (IOException | RuntimeException e) {
-				throw new MojoExecutionException("Unable to format file " + file, e);
+				throw new MojoExecutionException("Unable to check file " + file, e);
 			}
 		}