add -Wout-array

Methods having array output parameters, like void foo(out String[] ret)
are usually bad because the output array size must be declared and
allocated by the client in Java, and so the size of the array output
cannot be chosen by the server. This undesirable behavior happens
because of how arrays work in Java (they cannot be reallocated). Instead
prefer APIs like String[] foo().

Bug: 168028537
Test: aidl_unittests
Change-Id: Iccee9cf2cb9031f19b45972dadbaaa22670e09b4

1 files changed, 15 insertions, 0 deletions

diff --git a/diagnostics.cpp b/diagnostics.cpp
index 4af90331..9a4cfb8d 100644
--- a/diagnostics.cpp
+++ b/diagnostics.cpp
@@ -224,6 +224,20 @@ struct DiagnoseMixedOneway : DiagnosticsVisitor {
   }
 };
 
+struct DiagnoseOutArray : DiagnosticsVisitor {
+  DiagnoseOutArray(DiagnosticsContext& diag) : DiagnosticsVisitor(diag) {}
+  void Visit(const AidlMethod& m) override {
+    for (const auto& a : m.GetArguments()) {
+      if (a->GetType().IsArray() && a->IsOut()) {
+        diag.Report(m.GetLocation(), DiagnosticID::out_array)
+            << "The method '" << m.GetName() << "' an array output parameter '" << a->GetName()
+            << "'. Instead prefer APIs like '" << a->GetType().Signature() << " " << m.GetName()
+            << "(...).";
+      }
+    }
+  }
+};
+
 bool Diagnose(const AidlDocument& doc, const DiagnosticMapping& mapping) {
   DiagnosticsContext diag(mapping);
 
@@ -233,6 +247,7 @@ bool Diagnose(const AidlDocument& doc, const DiagnosticMapping& mapping) {
   DiagnoseConstName{diag}.Check(doc);
   DiagnoseExplicitDefault{diag}.Check(doc);
   DiagnoseMixedOneway{diag}.Check(doc);
+  DiagnoseOutArray{diag}.Check(doc);
 
   return diag.ErrorCount() == 0;
 }