[improvement] Helpful error output is provided when formatting fails (#…

…366)



## Before this PR
```
java.lang.RuntimeException: com.google.googlejavaformat.java.FormatterException: 5:15: error: ';' expected

	at com.palantir.conjure.java.util.Goethe.formatAndEmit(Goethe.java:55)
	at com.palantir.conjure.java.util.GoetheTest.testFormatAndEmit(GoetheTest.java:38)
Caused by: com.google.googlejavaformat.java.FormatterException: 5:15: error: ';' expected
	at com.google.googlejavaformat.java.FormatterException.fromJavacDiagnostics(FormatterException.java:50)
	at com.google.googlejavaformat.java.Formatter.format(Formatter.java:151)
	at com.google.googlejavaformat.java.Formatter.getFormatReplacements(Formatter.java:258)
	at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:234)
	at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:202)
	at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:189)
	at com.palantir.conjure.java.util.Goethe.formatAndEmit(Goethe.java:52)
	... 25 more
```

## After this PR
```
java.lang.RuntimeException: Failed to format 'com.palantir.foo.Foo'
';' expected
    type oops name = bar;
            ^

	at com.palantir.conjure.java.util.Goethe.formatAndEmit(Goethe.java:55)
	at com.palantir.conjure.java.util.GoetheTest.testFormatAndEmit(GoetheTest.java:38)
Caused by: com.google.googlejavaformat.java.FormatterException: 5:15: error: ';' expected
	at com.google.googlejavaformat.java.FormatterException.fromJavacDiagnostics(FormatterException.java:50)
	at com.google.googlejavaformat.java.Formatter.format(Formatter.java:151)
	at com.google.googlejavaformat.java.Formatter.getFormatReplacements(Formatter.java:258)
	at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:234)
	at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:202)
	at com.google.googlejavaformat.java.Formatter.formatSource(Formatter.java:189)
	at com.palantir.conjure.java.util.Goethe.formatAndEmit(Goethe.java:52)
	... 25 more
```

==COMMIT_MSG==
Helpful error output is provided when formatting fails
==COMMIT_MSG=

conjure-java-core/src/main/java/com/palantir/conjure/java/util/Goethe.java

@@ -16,10 +16,13 @@
 
 package com.palantir.conjure.java.util;
 
+import com.google.common.base.CharMatcher;
 import com.google.common.base.Preconditions;
 import com.google.common.base.Splitter;
+import com.google.common.base.Strings;
 import com.google.common.io.CharSink;
 import com.google.common.io.CharSource;
+import com.google.googlejavaformat.FormatterDiagnostic;
 import com.google.googlejavaformat.java.Formatter;
 import com.google.googlejavaformat.java.FormatterException;
 import com.google.googlejavaformat.java.JavaFormatterOptions;
@@ -29,6 +32,7 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.List;
 
 /** Tools for a better JavaPoet. */
 public final class Goethe {
@@ -46,13 +50,39 @@ public static Path formatAndEmit(JavaFile file, Path baseDir) {
             file.writeTo(code);
 
             CharSink sink = com.google.common.io.Files.asCharSink(outputFile.toFile(), StandardCharsets.UTF_8);
-            JAVA_FORMATTER.formatSource(CharSource.wrap(code), sink);
+            try {
+                JAVA_FORMATTER.formatSource(CharSource.wrap(code), sink);
+            } catch (FormatterException e) {
+                throw new RuntimeException(generateMessage(file, code.toString(), e.diagnostics()), e);
+            }
             return outputFile;
-        } catch (IOException | FormatterException e) {
+        } catch (IOException e) {
             throw new RuntimeException(e);
         }
     }
 
+    private static String generateMessage(
+            JavaFile file, String unformattedSource, List<FormatterDiagnostic> formatterDiagnostics) {
+        try {
+            List<String> lines = Splitter.on('\n').splitToList(unformattedSource);
+            StringBuilder failureText = new StringBuilder();
+            failureText.append("Failed to format '")
+                    .append(file.packageName).append('.').append(file.typeSpec.name).append("'\n");
+            for (FormatterDiagnostic formatterDiagnostic : formatterDiagnostics) {
+                failureText.append(formatterDiagnostic.message()).append("\n")
+                        // Diagnostic values are one-indexed, while our list is zero-indexed.
+                        .append(lines.get(formatterDiagnostic.line() - 1))
+                        .append('\n')
+                        // Offset by two to convert from one-indexed to zero indexed values, and account for the caret.
+                        .append(Strings.repeat(" ", Math.max(0, formatterDiagnostic.column() - 2)))
+                        .append("^\n\n");
+            }
+            return CharMatcher.is('\n').trimFrom(failureText.toString());
+        } catch (RuntimeException e) {
+            return "Failed to format:\n" + unformattedSource;
+        }
+    }
+
     /**
      * Returns the full path for the given Java file and Java base dir. In a nutshell, turns packages into directories,
      * e.g., {@code com.foo.bar.MyClass -> /<baseDir>/com/foo/bar/MyClass.java} and creates all directories.

conjure-java-core/src/test/java/com/palantir/conjure/java/util/GoetheTest.java

@@ -0,0 +1,46 @@
+/*
+ * (c) Copyright 2019 Palantir Technologies Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.palantir.conjure.java.util;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import com.squareup.javapoet.CodeBlock;
+import com.squareup.javapoet.JavaFile;
+import com.squareup.javapoet.TypeSpec;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+public class GoetheTest {
+
+    @Rule
+    public TemporaryFolder folder = new TemporaryFolder();
+
+    @Test
+    public void testFormatAndEmit() {
+        JavaFile javaFile = JavaFile.builder("com.palantir.foo", TypeSpec.classBuilder("Foo")
+                .addStaticBlock(CodeBlock.builder().addStatement("type oops name = bar").build())
+                .build()).build();
+        assertThatThrownBy(() -> Goethe.formatAndEmit(javaFile, folder.getRoot().toPath()))
+                .hasMessageContaining("Failed to format 'com.palantir.foo.Foo'")
+                .hasMessageContaining("';' expected")
+                .hasMessageContaining("" // newline to align the output
+                        + "    type oops name = bar;\n"
+                        + "            ^");
+    }
+
+}