Add a feature to require explicitly passing module maps.

Currently, module maps contain both "use <module>" entries that specify which
modules the current module map depends on, and "extern module" entries that
provide paths where to load the dependent module maps from.

This change adds a feature "module_map_without_extern_module", which instructs
blaze to not write the "extern module" entries into the module map. Instead,
the crosstool needs to add -fmodule-file flags for each dependent module file
where needed for the compile via the new build variable
"dependent_module_map_files".

Note that the feature is phrased negatively ("_without_") in order to simplify
the roll-out of this feature: as long as crosstools do not specify any
features, they still want the old behavior.
We cannot make the feature positive and add it to the legacy configuration, as
we currently cannot remove features that have already been set in the crosstool
file.

--
MOS_MIGRATED_REVID=104757413

1 files changed, 12 insertions, 0 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppRuleClasses.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppRuleClasses.java
index b7bfaf85ef..77a9719923 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/CppRuleClasses.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/CppRuleClasses.java
@@ -101,6 +101,18 @@ public class CppRuleClasses {
   public static final String MODULE_MAP_HOME_CWD = "module_map_home_cwd";
 
   /**
+   * A string constant for the module_map_without_extern_module feature.
+   *
+   * <p>This features is a transitional feature; enabling it means that generated module maps
+   * will not have "extern module" declarations inside them; instead, the module maps need
+   * to be passed via the dependent_module_map_files build variable.
+   *
+   * <p>This variable is phrased negatively to aid the roll-out: currently, the default is that
+   * "extern module" declarations are generated.
+   */
+  public static final String MODULE_MAP_WITHOUT_EXTERN_MODULE = "module_map_without_extern_module";
+
+  /**
    * A string constant for the layering_check feature.
    */
   public static final String LAYERING_CHECK = "layering_check";