JSpecify: Handle @nullable elements for enhanced-for-loops on arrays (#…

…986)

Currently, enhanced-for-loops do not honor nullability annotations on
arrays.

**Current Behavior**
Both these dereferences work fine.
```
static @nullable String[] fizz = {"1"}
for (String s: fizz){
     s.toString();
     if (s!=null){
          s.toString();
     }  
}

```
**New Behavior**
The first one throws an error, as expected.
```
static @nullable String[] fizz = {"1"}
for (String s: fizz){
     // BUG: Diagnostic contains: dereferenced expression s is @nullable
     s.toString();
     if (s!=null){
          s.toString();
     }  
}

```



Fixes #983

nullaway/src/main/java/com/uber/nullaway/dataflow/AccessPathNullnessPropagation.java

@@ -30,6 +30,7 @@
 import com.google.errorprone.suppliers.Supplier;
 import com.google.errorprone.suppliers.Suppliers;
 import com.google.errorprone.util.ASTHelpers;
+import com.sun.source.tree.ExpressionTree;
 import com.sun.source.tree.MethodInvocationTree;
 import com.sun.tools.javac.code.Symbol;
 import com.sun.tools.javac.code.Type;
@@ -791,8 +792,15 @@ public TransferResult<Nullness, NullnessStore> visitArrayAccess(
     Nullness resultNullness;
     // Unsoundly assume @NonNull, except in JSpecify mode where we check the type
     if (config.isJSpecifyMode()) {
-      Symbol arraySymbol = ASTHelpers.getSymbol(node.getArray().getTree());
+      Symbol arraySymbol;
       boolean isElementNullable = false;
+      // For enhanced-for-loops we get the symbol from the array expression as the node is desugared
+      ExpressionTree arrayExpr = node.getArrayExpression();
+      if (arrayExpr != null) {
+        arraySymbol = ASTHelpers.getSymbol(arrayExpr);
+      } else {
+        arraySymbol = ASTHelpers.getSymbol(node.getArray().getTree());
+      }
       if (arraySymbol != null) {
         isElementNullable = NullabilityUtil.isArrayElementNullable(arraySymbol, config);
       }

nullaway/src/test/java/com/uber/nullaway/jspecify/ArrayTests.java

@@ -3,7 +3,6 @@
 import com.google.errorprone.CompilationTestHelper;
 import com.uber.nullaway.NullAwayTestsBase;
 import java.util.Arrays;
-import org.junit.Ignore;
 import org.junit.Test;
 
 public class ArrayTests extends NullAwayTestsBase {
@@ -492,7 +491,6 @@ public void loopVariableIndex() {
   }
 
   @Test
-  @Ignore("for-each handling needs to be fixed; see https://github.com/uber/NullAway/issues/983")
   public void forEachLoop() {
     makeHelper()
         .addSourceLines(
@@ -506,8 +504,6 @@ public void forEachLoop() {
             "      if (s != null) {",
             "        s.toString();",
             "      }",
-            "    }",
-            "    for (String s : fizz) {",
             "      // BUG: Diagnostic contains: dereferenced expression s is @Nullable",
             "      s.toString();",
             "    }",