Make list EMPTY instance work like tuples

I.e., use an accessor for type inference. The EMPTY field will be made private in a future CL.

RELNOTES: None
PiperOrigin-RevId: 163843569

src/main/java/com/google/devtools/build/lib/syntax/SkylarkList.java

@@ -29,7 +29,7 @@
 import java.util.RandomAccess;
 import javax.annotation.Nullable;
 
-/** A class to handle lists and tuples in Skylark. */
+/** A Skylark list or tuple. */
 @SkylarkModule(
   name = "sequence",
   documented = false,
@@ -332,6 +332,26 @@ public MutableList(Mutability mutability) {
       this(Collections.EMPTY_LIST, mutability);
     }
 
+    /**
+     * A shared instance for the empty list with immutable mutability.
+     *
+     * <p>Other immutable empty list objects can exist, e.g. lists that were once mutable but whose
+     * environments were then frozen. This instance is for empty lists that were always frozen from
+     * the beginning.
+     *
+     * @deprecated Prefer {@link #empty()} instead, since that includes a cast for the element type.
+     *     This field will be made private in the near future.
+     */
+    @Deprecated
+    public static final MutableList<?> EMPTY =
+        new MutableList<>(ImmutableList.of(), Mutability.IMMUTABLE);
+
+    /** Returns an empty frozen list, cast to have an arbitrary content type. */
+    @SuppressWarnings("unchecked")
+    public static <E> MutableList<E> empty() {
+      return (MutableList<E>) EMPTY;
+    }
+
     /**
      * Builds a Skylark list from a variable number of arguments.
      * @param env an Environment from which to inherit Mutability, or null for immutable
@@ -493,11 +513,6 @@ public Mutability mutability() {
     public boolean isTuple() {
       return false;
     }
-
-    /**
-     * An empty IMMUTABLE MutableList.
-     */
-    public static final MutableList<?> EMPTY = new MutableList<>(Tuple.EMPTY, Mutability.IMMUTABLE);
   }
 
   /** An immutable tuple, e.g. in (1, 2, 3) */
@@ -528,16 +543,14 @@ private Tuple(ImmutableList<E> contents) {
       this.contents = contents;
     }
 
-    @Override
-    public Mutability mutability() {
-      return Mutability.IMMUTABLE;
-    }
-
     /**
-     * THE empty Skylark tuple.
+     * A shared instance for the empty tuple.
+     *
+     * <p>This instance should be the only empty tuple.
      */
     private static final Tuple<?> EMPTY = new Tuple<>(ImmutableList.of());
 
+    /** Returns the empty tuple, cast to have an arbitrary content type. */
     @SuppressWarnings("unchecked")
     public static <E> Tuple<E> empty() {
       return (Tuple<E>) EMPTY;
@@ -553,6 +566,11 @@ public static<E> Tuple<E> create(ImmutableList<E> contents) {
       return new Tuple<>(contents);
     }
 
+    @Override
+    public Mutability mutability() {
+      return Mutability.IMMUTABLE;
+    }
+
     /**
      * Creates a Tuple from an Iterable.
      */

src/test/java/com/google/devtools/build/lib/skylark/SkylarkIntegrationTest.java

@@ -932,7 +932,7 @@ public void testRuleClassNonMandatoryEmptyOutputs() throws Exception {
 
     ConfiguredTarget target = getConfiguredTarget("//test/skylark:cr");
     assertThat(target.get("o1")).isEqualTo(Runtime.NONE);
-    assertThat(target.get("o2")).isEqualTo(MutableList.EMPTY);
+    assertThat(target.get("o2")).isEqualTo(MutableList.empty());
   }
 
   @Test

src/test/java/com/google/devtools/build/lib/skylark/SkylarkRuleImplementationFunctionsTest.java

@@ -1538,7 +1538,7 @@ public void testBinDirPath() throws Exception {
   public void testEmptyLabelListTypeAttrInCtx() throws Exception {
     SkylarkRuleContext ctx = createRuleContext("//foo:baz");
     Object result = evalRuleContextCode(ctx, "ruleContext.attr.srcs");
-    assertThat(result).isEqualTo(MutableList.EMPTY);
+    assertThat(result).isEqualTo(MutableList.empty());
   }
 
   @Test

src/test/java/com/google/devtools/build/lib/syntax/EvaluationTest.java

@@ -460,13 +460,13 @@ public void testListMultiply() throws Exception {
         .testStatement("[1, 2] * 3", MutableList.of(env, 1, 2, 1, 2, 1, 2))
         .testStatement("[1, 2] * 4", MutableList.of(env, 1, 2, 1, 2, 1, 2, 1, 2))
         .testStatement("[8] * 5", MutableList.of(env, 8, 8, 8, 8, 8))
-        .testStatement("[    ] * 10", MutableList.EMPTY)
-        .testStatement("[1, 2] * 0", MutableList.EMPTY)
-        .testStatement("[1, 2] * -4", MutableList.EMPTY)
+        .testStatement("[    ] * 10", MutableList.empty())
+        .testStatement("[1, 2] * 0", MutableList.empty())
+        .testStatement("[1, 2] * -4", MutableList.empty())
         .testStatement(" 2 * [1, 2]", MutableList.of(env, 1, 2, 1, 2))
-        .testStatement("10 * []", MutableList.EMPTY)
-        .testStatement(" 0 * [1, 2]", MutableList.EMPTY)
-        .testStatement("-4 * [1, 2]", MutableList.EMPTY);
+        .testStatement("10 * []", MutableList.empty())
+        .testStatement(" 0 * [1, 2]", MutableList.empty())
+        .testStatement("-4 * [1, 2]", MutableList.empty());
   }
 
   @SuppressWarnings("unchecked")