[8.1.0] Mark built-in providers as hashable (#25230)

Built-in providers always have an immutable `hashCode`. Making them
hashable makes it easier to deduplicate a list of providers, which is
sometimes required as the list of provider instances returned by a rule
must not contain two instances for a given provider.

Starlark-defined providers are already hashable, so this brings the
behavior of built-in providers in line with them.

Closes #24848.

PiperOrigin-RevId: 724370319
Change-Id: I68147b1f707249a8b09ad170c5fc5b3da4776ccf

Commit
c94a9bd

Co-authored-by: Fabian Meumertzheim <[email protected]>

src/main/java/com/google/devtools/build/lib/packages/BuiltinProvider.java

@@ -65,6 +65,11 @@ public final int hashCode() {
     return getClass().hashCode();
   }
 
+  @Override
+  public void checkHashable() {
+    // The hash code is based on the class, so it is hashable.
+  }
+
   @Override
   public boolean isExported() {
     return true;

src/test/java/com/google/devtools/build/lib/starlark/StarlarkRuleImplementationFunctionsTest.java

@@ -81,6 +81,7 @@
 import java.util.regex.Pattern;
 import net.starlark.java.annot.Param;
 import net.starlark.java.annot.StarlarkMethod;
+import net.starlark.java.eval.Dict;
 import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Mutability;
 import net.starlark.java.eval.Printer;
@@ -4154,4 +4155,11 @@ def _impl(ctx):
     assertThat(a2.getRoot().getExecPathString())
         .matches(getRelativeOutputPath() + "/[\\w\\-]+\\-exec/bin");
   }
+
+  @Test
+  public void testHashableProviders() throws Exception {
+    ev.execAndExport("p = provider()");
+    Dict<?, ?> dict = (Dict<?, ?>) ev.eval("{k: None for k in [DefaultInfo, p, DefaultInfo, p]}");
+    assertThat(dict.size()).isEqualTo(2);
+  }
 }