Feature: support legacy_symbols (#67)

private/globals.bzl

@@ -16,3 +16,8 @@ GLOBALS = {
     "DefaultInfo": "0.0.1",
     "__TestingOnly_NeverAvailable": "1000000000.0.0",
 }
+
+# This one works in the reverse, put in the version when the global symbol is removed.
+LEGACY_GLOBALS = {
+    "ProtoInfo": "8.0.0",
+}

private/globals_repo.bzl

@@ -29,6 +29,15 @@ bzl_library(
         value = global_ if bazel_version >= parse_version(version) else "None"
         lines.append("    %s = %s," % (global_, value))
 
+    for global_, version in rctx.attr.legacy_globals.items():
+        if not _is_valid_identifier(global_):
+            fail("Invalid global name: %s" % global_)
+
+        value = global_ if bazel_version < parse_version(version) else "None"
+        # If the legacy_globals is available, we take the value from it.
+        # The value is populated by --incompatible_autoload_externally and may apply to older Bazel versions
+        lines.append("    %s = getattr(getattr(native, 'legacy_globals', None), '%s', %s)," % (global_, global_, value))
+
     lines.append(")")
 
     rctx.file("globals.bzl", "\n".join(lines))
@@ -41,7 +50,10 @@ globals_repo = repository_rule(
         "globals": attr.string_dict(
             mandatory = True,
         ),
-    },
+        "legacy_globals": attr.string_dict(
+            mandatory = True,
+        ),
+     },
 )
 
 def _is_valid_identifier(s):

private/repos.bzl

@@ -1,6 +1,6 @@
 """Contains the macro bazel_features_repos to install internal repositories."""
 
-load(":globals.bzl", "GLOBALS")
+load(":globals.bzl", "GLOBALS", "LEGACY_GLOBALS")
 load(":globals_repo.bzl", "globals_repo")
 load(":version_repo.bzl", "version_repo")
 load("@bazel_tools//tools/build_defs/repo:utils.bzl", "maybe")
@@ -14,4 +14,5 @@ def bazel_features_repos():
         globals_repo,
         name = "bazel_features_globals",
         globals = GLOBALS,
+        legacy_globals = LEGACY_GLOBALS,
     )

test/test.bzl

@@ -1,6 +1,6 @@
 """Provides a macro to do some loading-time test assertions."""
 
-load("//private:util.bzl", "BAZEL_VERSION", "ge")
+load("//private:util.bzl", "BAZEL_VERSION", "ge", "lt")
 load("//private:parse.bzl", "parse_version")
 load("//:features.bzl", "bazel_features")
 
@@ -49,6 +49,10 @@ def run_test(name):
     if not bazel_features.globals.DefaultInfo == DefaultInfo:
         fail("bazel_features.globals.DefaultInfo != DefaultInfo")
 
+    # TODO: add tests with --incompatible_autoload_symbols
+    if lt("8.0.0") and not bazel_features.globals.ProtoInfo == ProtoInfo:
+        fail("bazel_features.globals.ProtoInfo != ProtoInfo")
+
     if not bazel_features.globals.__TestingOnly_NeverAvailable == None:
         fail("bazel_features.globals.__TestingOnly_NeverAvailable != None")