Allow setting supports_dynamic_linker crosstool capability using feature

This cl allows toolchain owners to express that toolchain supports creating dynamic libraries.

This is in theory a breaking change, for the crosstools that don't use `dynamic_library_linker_flag`, and don't specify `linking_mode_flags { mode: DYNAMIC }`, and they do specify feature { name: 'dynamic_linking_mode' }. This currently means the toolchain will generate shared libraries, but with this change it will not.

But since this only happens to toolchains that don't use legacy fields, and I don't think there are such toolchains in the wild, I'll go ahead and do this change without following the incompatible change process.

This cl is a step towards #5883. Also
see the rollout doc here:
https://docs.google.com/document/d/1uv4c1zag6KvdI31qdx8C6jiTognXPQrxgsUpVefm9fM/edit#.

Flag removing legacy behavior is #6861

RELNOTES: None.
PiperOrigin-RevId: 226631573

site/docs/crosstool-reference.md

@@ -1035,4 +1035,11 @@ conditions.
      libraries. This makes incremental relinking faster.
    </td>
   </tr>
+  <tr>
+   <td><strong><code>supports_dynamic_linker</code></strong>
+   </td>
+   <td>If enabled, C++ rules will know the toolchain can produce shared
+     libraries.
+   </td>
+  </tr>
 </table>

src/main/java/com/google/devtools/build/lib/rules/cpp/CcLibrary.java

@@ -163,7 +163,7 @@ public static void init(
             .addLinkstamps(ruleContext.getPrerequisites("linkstamp", Mode.TARGET));
 
     Artifact soImplArtifact = null;
-    boolean supportsDynamicLinker = ccToolchain.supportsDynamicLinker();
+    boolean supportsDynamicLinker = ccToolchain.supportsDynamicLinker(featureConfiguration);
     // TODO(djasper): This is hacky. We should actually try to figure out whether we generate
     // ccOutputs.
     boolean createDynamicLibrary =

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProvider.java

@@ -553,11 +553,10 @@ public boolean supportsGoldLinker() {
     return toolchainInfo.supportsGoldLinker();
   }
 
-  /**
-   * Returns whether the toolchain supports dynamic linking.
-   */
-  public boolean supportsDynamicLinker() {
-    return toolchainInfo.supportsDynamicLinker();
+  /** Returns whether the toolchain supports dynamic linking. */
+  public boolean supportsDynamicLinker(FeatureConfiguration featureConfiguration) {
+    return toolchainInfo.supportsDynamicLinker()
+        || featureConfiguration.isEnabled(CppRuleClasses.SUPPORTS_DYNAMIC_LINKER);
   }
 
   /**

src/main/java/com/google/devtools/build/lib/rules/cpp/CppRuleClasses.java

@@ -391,6 +391,11 @@ public static Label ccToolchainTypeAttribute(RuleDefinitionEnvironment env) {
   /** A feature marking that the toolchain can use --start-lib/--end-lib flags */
   public static final String SUPPORTS_START_END_LIB = "supports_start_end_lib";
 
+  /**
+   * A feature marking that the toolchain can produce binaries that load shared libraries at
+   * runtime.
+   */
+  public static final String SUPPORTS_DYNAMIC_LINKER = "supports_dynamic_linker";
   /** Ancestor for all rules that do include scanning. */
   public static final class CcIncludeScanningRule implements RuleDefinition {
     @Override

src/main/java/com/google/devtools/build/lib/rules/cpp/CppToolchainInfo.java

@@ -332,11 +332,7 @@ public static CppToolchainInfo create(
     this.supportsFission = supportsFission;
     this.supportsStartEndLib = supportsStartEndLib;
     this.supportsEmbeddedRuntimes = supportsEmbeddedRuntimes;
-    this.supportsDynamicLinker =
-        supportsDynamicLinker
-            || toolchainFeatures
-                .getActivatableNames()
-                .contains(CppRuleClasses.DYNAMIC_LINKING_MODE);
+    this.supportsDynamicLinker = supportsDynamicLinker;
     this.supportsInterfaceSharedLibraries = supportsInterfaceSharedLibraries;
     this.supportsGoldLinker = supportsGoldLinker;
     this.toolchainNeedsPic = toolchainNeedsPic;

src/main/java/com/google/devtools/build/lib/rules/cpp/proto/CcProtoAspect.java

@@ -308,7 +308,7 @@ private CcLinkingHelper initializeLinkingHelper(
       helper.addDeps(deps);
       // TODO(dougk): Configure output artifact with action_config
       // once proto compile action is configurable from the crosstool.
-      if (!toolchain.supportsDynamicLinker()) {
+      if (!toolchain.supportsDynamicLinker(featureConfiguration)) {
         helper.setShouldCreateDynamicLibrary(false);
       }
       return helper;

src/test/java/com/google/devtools/build/lib/packages/util/MockCcSupport.java

@@ -62,6 +62,9 @@ public boolean apply(Artifact artifact) {
   public static final String DYNAMIC_LINKING_MODE_FEATURE =
       "feature { name: '" + CppRuleClasses.DYNAMIC_LINKING_MODE + "'}";
 
+  public static final String SUPPORTS_DYNAMIC_LINKER_FEATURE =
+      "feature { name: '" + CppRuleClasses.SUPPORTS_DYNAMIC_LINKER + " enabled: true'}";
+
   /** Feature expected by the C++ rules when pic build is requested */
   public static final String PIC_FEATURE =
       ""

src/test/java/com/google/devtools/build/lib/rules/cpp/CcToolchainTest.java

@@ -774,7 +774,7 @@ private void writeStarlarkRule() throws IOException {
   }
 
   @Test
-  public void testSupportsDynamicLinkerCheckFeatures() throws Exception {
+  public void testSupportsDynamicLinkerIsFalseWhenFeatureNotSet() throws Exception {
     scratch.file("a/BUILD", "cc_toolchain_alias(name = 'b')");
 
     getAnalysisMock()
@@ -789,7 +789,29 @@ public void testSupportsDynamicLinkerCheckFeatures() throws Exception {
     CcToolchainProvider toolchainProvider =
         (CcToolchainProvider) target.get(ToolchainInfo.PROVIDER);
 
-    assertThat(toolchainProvider.supportsDynamicLinker()).isTrue();
+    assertThat(toolchainProvider.supportsDynamicLinker(FeatureConfiguration.EMPTY)).isFalse();
+  }
+
+  @Test
+  public void testSupportsDynamicLinkerIsTrueWhenFeatureSet() throws Exception {
+    scratch.file("a/BUILD", "cc_toolchain_alias(name = 'b')");
+
+    getAnalysisMock()
+        .ccSupport()
+        .setupCrosstool(
+            mockToolsConfig,
+            MockCcSupport.DYNAMIC_LINKING_MODE_FEATURE,
+            MockCcSupport.SUPPORTS_DYNAMIC_LINKER_FEATURE);
+
+    // To make sure the toolchain doesn't define linking_mode_flags { mode: DYNAMIC } as that would
+    // also result in supportsDynamicLinker returning true
+    useConfiguration("--compiler=compiler_no_dyn_linker");
+
+    ConfiguredTarget target = getConfiguredTarget("//a:b");
+    CcToolchainProvider toolchainProvider =
+        (CcToolchainProvider) target.get(ToolchainInfo.PROVIDER);
+
+    assertThat(toolchainProvider.supportsDynamicLinker(FeatureConfiguration.EMPTY)).isFalse();
   }
 
   // Tests CcCommon::enableStaticLinkCppRuntimesFeature when supports_embedded_runtimes is not