Deduplicate extra_providers against built-in providers (#147)

This avoids an error if users specify a provider in `extra_providers`
that's already in the list of built-in providers.

Work towards #146

examples/cc_define_test/cc_define_test.bzl

@@ -1,6 +1,10 @@
 load("@with_cfg.bzl", "with_cfg")
 
-_builder = with_cfg(native.cc_test)
+_builder = with_cfg(
+    native.cc_test,
+    # Verify that duplicated providers are handled gracefully.
+    extra_providers = [DefaultInfo, CcInfo],
+)
 _builder.extend(
     "copt",
     select({

with_cfg/private/rule_defaults.bzl

@@ -1,8 +1,15 @@
 visibility("private")
 
-DEFAULT_PROVIDERS = [
+SPECIAL_CASED_PROVIDERS = [
+    DefaultInfo,
+    # Forwarding is handled by coverage_common.instrumented_files_info.
+    InstrumentedFilesInfo,
     # RunEnvironmentInfo can't be returned from a non-executable, non-test rule and thus requires
     # special handling so that it isn't returned by the transitioning alias.
+    RunEnvironmentInfo,
+]
+
+DEFAULT_PROVIDERS = [
     AnalysisTestResultInfo,
     CcInfo,
     CcToolchainConfigInfo,

with_cfg/private/with_cfg.bzl

@@ -1,6 +1,6 @@
 load(":builder.bzl", "make_builder")
 load(":providers.bzl", "RuleInfo")
-load(":rule_defaults.bzl", "DEFAULT_PROVIDERS", "IMPLICIT_TARGETS")
+load(":rule_defaults.bzl", "DEFAULT_PROVIDERS", "IMPLICIT_TARGETS", "SPECIAL_CASED_PROVIDERS")
 
 visibility("//with_cfg/...")
 
@@ -125,7 +125,7 @@ def with_cfg(
         executable = executable,
         test = test,
         implicit_targets = implicit_targets,
-        providers = DEFAULT_PROVIDERS + extra_providers,
+        providers = _all_providers(extra_providers),
         native = _is_native(kind),
         supports_inheritance = _supports_inheritance(kind),
         supports_extension = _supports_extension(kind),
@@ -174,3 +174,15 @@ def _supports_extension(kind):
 
 def get_implicit_targets(rule_name):
     return IMPLICIT_TARGETS.get(rule_name, [])
+
+def _all_providers(extra_providers):
+    if not extra_providers:
+        return DEFAULT_PROVIDERS
+    all_providers = list(DEFAULT_PROVIDERS)
+
+    # Providers aren't hashable.
+    # TODO: Improve this after https://github.com/bazelbuild/bazel/pull/24848.
+    for p in extra_providers:
+        if p not in all_providers and p not in SPECIAL_CASED_PROVIDERS:
+            all_providers.append(p)
+    return all_providers