[nullsafe] Mark @Nullsafe with @TypeQualifierDefault&Co for Kotlin interop

Summary:
Kotlin has an experimental support for [JSR-305 custom nullability
qualifiers](https://github.com/Kotlin/KEEP/blob/master/proposals/jsr-305-custom-nullability-qualifiers.md).

Annotating Nullsafe in a special way makes kotlinc recognize it as
such custom nullability qualifier and therefore treat types coming
from Nullsafe Java classes **not** as platform types, but rather
proper nonnull/nullable, which affects:
1. Generated bytecode (more thorough null-checks).
2. Type inference in the IDE.

NOTE re: p.1: one might expect that with properly annotated Java code
Kotlin would avoid inserting runtime checks. This is not how
Kotlin-Java interop works - in reality Kotlin does even more runtime
checking for Java code annotated as Nonull, which IMO is a good
thing, since you can't trust Java anyway.

Reviewed By: mityal

Differential Revision: D21278440

fbshipit-source-id: d0598738a

infer/annotations/Makefile

@@ -8,6 +8,7 @@ include $(ROOT_DIR)/Makefile.config
 
 CWD = $(shell pwd)
 JSR_JAR = $(DEPENDENCIES_DIR)/java/jsr-305/jsr305.jar
+KOTLIN_ANNOT_JAR = $(DEPENDENCIES_DIR)/java/kotlin-annotations/kotlin-annotations-jvm-1.3.72.jar
 SOURCES_DIR = src/main/java
 ANNOT_SOURCES = $(shell find $(SOURCES_DIR)/com/facebook/infer/annotation -name "*.java")
 ANNOT_CLASSES = 'annot_classes'
@@ -19,7 +20,7 @@ all: $(ANNOTATIONS_JAR) $(SOURCES_JAR)
 
 $(ANNOTATIONS_JAR): $(ANNOT_SOURCES)
 	$(MKDIR_P) $(ANNOT_CLASSES)
-	$(JAVAC) -source 7 -target 7 -cp $(JSR_JAR) $(ANNOT_SOURCES) -d $(ANNOT_CLASSES)
+	$(JAVAC) -source 7 -target 7 -cp $(JSR_JAR):$(KOTLIN_ANNOT_JAR) $(ANNOT_SOURCES) -d $(ANNOT_CLASSES)
 	cd $(ANNOT_CLASSES) && jar cvf $(ANNOTATIONS_JAR) com
 
 $(SOURCES_JAR): $(ANNOT_SOURCES)

infer/annotations/pom.xml

@@ -65,6 +65,12 @@
       <artifactId>jsr305</artifactId>
       <version>3.0.1</version>
     </dependency>
+    <!-- https://mvnrepository.com/artifact/org.jetbrains.kotlin/kotlin-annotations-jvm -->
+    <dependency>
+        <groupId>org.jetbrains.kotlin</groupId>
+        <artifactId>kotlin-annotations-jvm</artifactId>
+        <version>1.3.72</version>
+    </dependency>
   </dependencies>
 
   <build>

infer/annotations/src/main/java/com/facebook/infer/annotation/Nullsafe.java

@@ -11,9 +11,22 @@ import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
+import javax.annotation.Nonnull;
+import javax.annotation.meta.TypeQualifierDefault;
+import kotlin.annotations.jvm.MigrationStatus;
+import kotlin.annotations.jvm.UnderMigration;
 
 @Retention(RetentionPolicy.CLASS)
 @Target({ElementType.TYPE})
+// These 2 annotations are needed for better interop of @Nullsafe with Kotlin,
+// essentially telling it that both params and return values are non-null by
+// default.
+@Nonnull
+@TypeQualifierDefault({ElementType.METHOD, ElementType.PARAMETER})
+// This annotation is needed for kotlinc to recognize {@code
+// TypeQualifierDefault} without explicitly passing -Xjsr305=strict flag (which
+// may be problematic in large codebases).
+@UnderMigration(status = MigrationStatus.STRICT)
 /**
  * Configures nullability checking mode of annotated classes; a more general version of {@link
  * NullsafeStrict}.