Don't suggest adding a dependency if testonly incompatible.

bant/explore/query-utils.cc

@@ -90,6 +90,8 @@ class TargetFinder : public BaseVoidVisitor {
         current_.name = scalar->AsString();
       } else if (lhs == "alwayslink") {
         current_.alwayslink = scalar->AsInt();
+      } else if (lhs == "testonly") {
+        current_.testonly = scalar->AsInt();
       } else if (lhs == "include_prefix") {
         current_.include_prefix = scalar->AsString();
       } else if (lhs == "strip_include_prefix") {
@@ -127,6 +129,8 @@ class TargetFinder : public BaseVoidVisitor {
       // But until then, we need to check for the constant symbol manually.
       if (lhs == "alwayslink") {
         current_.alwayslink = (id->id() == "True");
+      } else if (lhs == "testonly") {
+        current_.testonly = (id->id() == "True");
       }
     }
   }

bant/explore/query-utils.h

@@ -49,6 +49,7 @@ struct Result {
   std::string_view include_prefix;        // ... to manipulate the path ...
   std::string_view strip_include_prefix;  // ... files from hdrs are found.
   bool alwayslink = false;
+  bool testonly = false;
 };
 
 // Callback of a query.

bant/tool/dwyu-internal.h

@@ -81,6 +81,8 @@ class DWYUGenerator {
                             const ParsedBuildFile &build_file);
 
   bool IsAlwayslink(const BazelTarget &target) const;
+  bool IsTestonlyMismatch(const BazelTarget &target, const BazelTarget &dep,
+                          const query::Result &dep_detail) const;
   bool CanSee(const BazelTarget &target, const BazelTarget &dep) const;
 
   Session &session_;

bant/tool/dwyu.cc

@@ -227,7 +227,8 @@ void DWYUGenerator::InitKnownLibraries() {
   for (const auto &[_, parsed_package] : project_.ParsedFiles()) {
     const BazelPackage &current_package = parsed_package->package;
     query::FindTargets(parsed_package->ast,
-                       {"cc_library", "cc_proto_library", "alias"},  //
+                       {"cc_library", "cc_proto_library", "alias",  //
+                        "cc_test"},  // also indexing test for testonly check.
                        [&](const query::Result &target) {
                          auto self = BazelTarget::ParseFrom(
                            absl::StrCat(":", target.name), current_package);
@@ -246,18 +247,49 @@ bool DWYUGenerator::IsAlwayslink(const BazelTarget &target) const {
   return found->second.alwayslink;
 }
 
+bool DWYUGenerator::IsTestonlyMismatch(const BazelTarget &target,
+                                       const BazelTarget &dep,
+                                       const query::Result &dep_detail) const {
+  if (!dep_detail.testonly) return false;  // non-testonly always compatible.
+
+  auto target_detail_found = known_libs_.find(target);
+  if (target_detail_found == known_libs_.end()) {
+    return false;  // Should not happen, but let's not flag as issue.
+  }
+  const query::Result &target_detail = target_detail_found->second;
+  if (target_detail.testonly || target_detail.rule == "cc_test") {
+    return false;
+  }
+
+  project_.Loc(session_.info(), target_detail.name)
+    << " '" << target << "' is using headers that would be provided by '" << dep
+    << "', but the latter is marked testonly, the former not. "
+    << "Not adding dependency.\n";
+  return true;
+}
+
 bool DWYUGenerator::CanSee(const BazelTarget &target,
                            const BazelTarget &dep) const {
-  if (target.package == dep.package) {
-    // We can implicitly see all the targets in the same package.
-    return true;
-  }
   auto found = known_libs_.find(dep);
   if (found == known_libs_.end()) return true;  // Unknown ? Be Bold.
   if (!found->second.deprecation.empty()) {
     // Consider a library with a deprecation as not visible.
     return false;
   }
+
+  // If the dependency in question is testonly, but we're not, then
+  // this is equivalent to not visible.
+  if (IsTestonlyMismatch(target, dep, found->second)) {
+    return false;
+  }
+
+  // On to actual visibility checks.
+
+  if (target.package == dep.package) {
+    // We can implicitly see all the targets in the same package.
+    return true;
+  }
+
   List *visibility_list = found->second.visibility;
   if (!visibility_list) return true;
   bool any_valid_visiblity_pattern = false;

bant/tool/dwyu_test.cc

@@ -560,6 +560,34 @@ cc_library(
   tester.RunForTarget("//some/path:bar");
 }
 
+TEST(DWYUTest, DoNotAdd_IfTestonlyMismatch) {
+  ParsedProjectTestUtil pp;
+  pp.Add("//lib/path", R"(
+cc_library(
+  name = "foo",
+  srcs = ["foo.cc"],
+  hdrs = ["foo.h"],
+  testonly = True,
+)
+)");
+
+  pp.Add("//some/path", R"(
+cc_library(
+  name = "bar",
+  srcs = ["bar.cc"],
+  # needed //lib/path:foo dependency not given, but it is testonly.
+)
+)");
+
+  DWYUTestFixture tester(pp.project());
+  // No add expected.
+  tester.AddSource("some/path/bar.cc", R"(
+#include "lib/path/foo.h"
+)");
+  tester.RunForTarget("//some/path:bar");
+  EXPECT_THAT(tester.LogContent(), HasSubstr("is marked testonly"));
+}
+
 TEST(DWYUTest, Remove_SuperfluousDependency) {
   ParsedProjectTestUtil pp;
   pp.Add("//some/path", R"(