Add native.package_relative_label function

This essentially brings back the old `relative_to_caller_repository` param to the Label constructor, but with an arguably better API. It converts a label string into a Label object using the context of the calling package; this is useful for macro authors that want to convert string inputs into labels for reasons such as deduping.

Also made `Label()` accept relative labels (like `:foo`) because there really isn't a good reason not to do so.

Short design doc about the proposed API: https://docs.google.com/document/d/1_kMVWRHSBVkSsw1SLWPf3e-3RNSt55ZZYlzFzJkkwMo/edit

Fixes bazelbuild#17260

RELNOTES: Added a `native.package_relative_label()` function, which converts a label string to a Label object in the context of the calling package, in contrast to `Label()`, which does so in the context of the current .bzl file. Both functions now also accept relative labels such as `:foo`, and are idempotent.
PiperOrigin-RevId: 507836895
Change-Id: Ic870fe564d96a77f05dd7258d32c031fca8cacb1

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java

@@ -997,8 +997,11 @@ public boolean isImmutable() {
           .build();
 
   @Override
-  public Label label(String labelString, StarlarkThread thread) throws EvalException {
-    // The label string is interpeted with respect to the .bzl module containing the call to
+  public Label label(Object input, StarlarkThread thread) throws EvalException {
+    if (input instanceof Label) {
+      return (Label) input;
+    }
+    // The label string is interpreted with respect to the .bzl module containing the call to
     // `Label()`. An alternative to this approach that avoids stack inspection is to have each .bzl
     // module define its own copy of the `Label()` builtin embedding the module's own name. This
     // would lead to peculiarities like foo.bzl being able to call bar.bzl's `Label()` symbol to
@@ -1007,9 +1010,9 @@ public Label label(String labelString, StarlarkThread thread) throws EvalExcepti
     BazelModuleContext moduleContext =
         BazelModuleContext.of(Module.ofInnermostEnclosingStarlarkFunction(thread));
     try {
-      return Label.parseWithRepoContext(labelString, moduleContext.packageContext());
+      return Label.parseWithPackageContext((String) input, moduleContext.packageContext());
     } catch (LabelSyntaxException e) {
-      throw Starlark.errorf("Illegal absolute label syntax: %s", e.getMessage());
+      throw Starlark.errorf("invalid label in Label(): %s", e.getMessage());
     }
   }
 

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

@@ -621,6 +621,20 @@ public String repositoryName(StarlarkThread thread) throws EvalException {
     return packageId.getRepository().getNameWithAt();
   }
 
+  @Override
+  public Label packageRelativeLabel(Object input, StarlarkThread thread) throws EvalException {
+    BazelStarlarkContext.from(thread).checkLoadingPhase("native.package_relative_label");
+    if (input instanceof Label) {
+      return (Label) input;
+    }
+    try {
+      String s = (String) input;
+      return PackageFactory.getContext(thread).getBuilder().getLabelConverter().convert(s);
+    } catch (LabelSyntaxException e) {
+      throw Starlark.errorf("invalid label in native.package_relative_label: %s", e.getMessage());
+    }
+  }
+
   private static Dict<String, Object> getRuleDict(Rule rule, Mutability mu) throws EvalException {
     Dict.Builder<String, Object> values = Dict.builder();
 

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/StarlarkNativeModuleApi.java

@@ -15,6 +15,7 @@
 package com.google.devtools.build.lib.starlarkbuildapi;
 
 import com.google.devtools.build.docgen.annot.DocCategory;
+import com.google.devtools.build.lib.cmdline.Label;
 import net.starlark.java.annot.Param;
 import net.starlark.java.annot.ParamType;
 import net.starlark.java.annot.StarlarkBuiltin;
@@ -241,6 +242,38 @@ NoneType exportsFiles(Sequence<?> srcs, Object visibility, Object licenses, Star
       useStarlarkThread = true)
   String repositoryName(StarlarkThread thread) throws EvalException;
 
+  @StarlarkMethod(
+      name = "package_relative_label",
+      doc =
+          "Converts the input string into a <a href='Label'>Label</a> object, in the context of the"
+              + " package currently being initialized (that is, the <code>BUILD</code> file for"
+              + " which the current macro is executing). If the input is already a"
+              + " <code>Label</code>, it is returned unchanged.<p>This function may only be called"
+              + " while evaluating a BUILD file and the macros it directly or indirectly calls; it"
+              + " may not be called in (for instance) a rule implementation function. <p>The result"
+              + " of this function is the same <code>Label</code> value as would be produced by"
+              + " passing the given string to a label-valued attribute of a target declared in the"
+              + " BUILD file. <p><i>Usage note:</i> The difference between this function and <a"
+              + " href='Label#Label'>Label()</a></code> is that <code>Label()</code> uses the"
+              + " context of the package of the <code>.bzl</code> file that called it, not the"
+              + " package of the <code>BUILD</code> file. Use <code>Label()</code> when you need to"
+              + " refer to a fixed target that is hardcoded into the macro, such as a compiler. Use"
+              + " <code>package_relative_label()</code> when you need to normalize a label string"
+              + " supplied by the BUILD file to a <code>Label</code> object. (There is no way to"
+              + " convert a string to a <code>Label</code> in the context of a package other than"
+              + " the BUILD file or the calling .bzl file. For that reason, outer macros should"
+              + " always prefer to pass Label objects to inner macros rather than label strings.)",
+      parameters = {
+        @Param(
+            name = "input",
+            allowedTypes = {@ParamType(type = String.class), @ParamType(type = Label.class)},
+            doc =
+                "The input label string or Label object. If a Label object is passed, it's"
+                    + " returned as is.")
+      },
+      useStarlarkThread = true)
+  Label packageRelativeLabel(Object input, StarlarkThread thread) throws EvalException;
+
   @StarlarkMethod(
       name = "subpackages",
       doc =

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/StarlarkRuleFunctionsApi.java

@@ -701,18 +701,26 @@ StarlarkAspectApi aspect(
   @StarlarkMethod(
       name = "Label",
       doc =
-          "Creates a Label referring to a BUILD target. Use this function when you want to give a"
-              + " default value for the label attributes of a rule or when referring to a target"
-              + " via an absolute label from a macro. The argument must refer to an absolute label."
-              + " The repo part of the label (or its absence) is interpreted in the context of the"
-              + " repo where this Label() call appears. Example: <br><pre"
-              + " class=language-python>Label(\"//tools:default\")</pre>",
+          "Converts a label string into a <code>Label</code> object, in the context of the package"
+              + " where the calling <code>.bzl</code> source file lives. If the given value is"
+              + " already a <code>Label</code>, it is returned unchanged."
+              + "<p>For macros, a related function,"
+              + " <code><a"
+              + " href='native#package_relative_label'>native.package_relative_label()</a></code>,"
+              + " converts the input into a <code>Label</code> in the context of the package"
+              + " currently being constructed. Use that function to mimic the string-to-label"
+              + " conversion that is automatically done by label-valued rule attributes.",
       parameters = {
-        @Param(name = "label_string", doc = "the label string."),
+        @Param(
+            name = "input",
+            allowedTypes = {@ParamType(type = String.class), @ParamType(type = Label.class)},
+            doc =
+                "The input label string or Label object. If a Label object is passed, it's"
+                    + " returned as is.")
       },
       useStarlarkThread = true)
   @StarlarkConstructor
-  Label label(String labelString, StarlarkThread thread) throws EvalException;
+  Label label(Object input, StarlarkThread thread) throws EvalException;
 
   @StarlarkMethod(
       name = "exec_group",

src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeStarlarkNativeModuleApi.java

@@ -16,6 +16,7 @@
 
 import com.google.common.collect.ImmutableCollection;
 import com.google.common.collect.ImmutableList;
+import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.starlarkbuildapi.StarlarkNativeModuleApi;
 import javax.annotation.Nullable;
 import net.starlark.java.eval.Dict;
@@ -77,6 +78,11 @@ public String repositoryName(StarlarkThread thread) {
     return "";
   }
 
+  @Override
+  public Label packageRelativeLabel(Object input, StarlarkThread thread) throws EvalException {
+    return Label.parseCanonicalUnchecked("//:fake_label");
+  }
+
   @Override
   public Sequence<?> subpackages(
       Sequence<?> include, Sequence<?> exclude, boolean allowEmpty, StarlarkThread thread)

src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeStarlarkRuleFunctionsApi.java

@@ -171,11 +171,14 @@ public StarlarkCallable rule(
   }
 
   @Override
-  public Label label(String labelString, StarlarkThread thread) throws EvalException {
+  public Label label(Object input, StarlarkThread thread) throws EvalException {
+    if (input instanceof Label) {
+      return (Label) input;
+    }
     try {
-      return Label.parseCanonical(labelString);
+      return Label.parseCanonical((String) input);
     } catch (LabelSyntaxException e) {
-      throw Starlark.errorf("Illegal absolute label syntax: %s", labelString);
+      throw Starlark.errorf("Illegal absolute label syntax: %s", input);
     }
   }
 

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

@@ -1119,6 +1119,13 @@ public void testLabel() throws Exception {
     assertThat(result.toString()).isEqualTo("//foo/foo:foo");
   }
 
+  @Test
+  public void testLabelIdempotence() throws Exception {
+    Object result = ev.eval("Label(Label('//foo/foo:foo'))");
+    assertThat(result).isInstanceOf(Label.class);
+    assertThat(result.toString()).isEqualTo("//foo/foo:foo");
+  }
+
   @Test
   public void testLabelSameInstance() throws Exception {
     Object l1 = ev.eval("Label('//foo/foo:foo')");

src/test/py/bazel/bzlmod/bazel_module_test.py

@@ -884,5 +884,62 @@ def testJavaRunfilesLibraryRepoMapping(self):
         env_add={'RUNFILES_LIB_DEBUG': '1'})
     self.AssertExitCode(exit_code, 0, stderr, stdout)
 
+  def testNativePackageRelativeLabel(self):
+    self.ScratchFile(
+        'MODULE.bazel',
+        [
+            'module(name="foo")',
+            'bazel_dep(name="bar")',
+            'local_path_override(module_name="bar",path="bar")',
+        ],
+    )
+    self.ScratchFile('WORKSPACE')
+    self.ScratchFile('BUILD')
+    self.ScratchFile(
+        'defs.bzl',
+        [
+            'def mac(name):',
+            '  native.filegroup(name=name)',
+            '  print("1st: " + str(native.package_relative_label(":bleb")))',
+            '  print("2nd: " + str(native.package_relative_label('
+            + '"//bleb:bleb")))',
+            '  print("3rd: " + str(native.package_relative_label('
+            + '"@bleb//bleb:bleb")))',
+            '  print("4th: " + str(native.package_relative_label("//bleb")))',
+            '  print("5th: " + str(native.package_relative_label('
+            + '"@@bleb//bleb:bleb")))',
+            '  print("6th: " + str(native.package_relative_label(Label('
+            + '"//bleb"))))',
+        ],
+    )
+
+    self.ScratchFile(
+        'bar/MODULE.bazel',
+        [
+            'module(name="bar")',
+            'bazel_dep(name="foo", repo_name="bleb")',
+        ],
+    )
+    self.ScratchFile('bar/WORKSPACE')
+    self.ScratchFile(
+        'bar/quux/BUILD',
+        [
+            'load("@bleb//:defs.bzl", "mac")',
+            'mac(name="book")',
+        ],
+    )
+
+    _, _, stderr = self.RunBazel(
+        ['build', '@bar//quux:book'], allow_failure=False
+    )
+    stderr = '\n'.join(stderr)
+    self.assertIn('1st: @@bar~override//quux:bleb', stderr)
+    self.assertIn('2nd: @@bar~override//bleb:bleb', stderr)
+    self.assertIn('3rd: @@//bleb:bleb', stderr)
+    self.assertIn('4th: @@bar~override//bleb:bleb', stderr)
+    self.assertIn('5th: @@bleb//bleb:bleb', stderr)
+    self.assertIn('6th: @@//bleb:bleb', stderr)
+
+
 if __name__ == '__main__':
   unittest.main()