Do not crash when enum discriminant is not recognized

Sometimes the DWARF can omit information about a discriminant, for example when an Option shares a discriminant slot with an enum that it wraps. In this case, lldb could crash, because the discriminant was not found and because there was no default variant. No test case because this relies on a compiler bug that will soon be fixed. Fixes #16
rust-lang · cuviper · Jul 10, 2016 · Jul 28, 2017 · Jan 24, 2018 · Jun 3, 2017
commit 02ddf7625e27b3c45b7a5c9d79bb2ececd923ec7
diff --git a/lldb/source/Plugins/LanguageRuntime/Rust/RustLanguageRuntime.cpp b/lldb/source/Plugins/LanguageRuntime/Rust/RustLanguageRuntime.cpp
@@ -98,6 +98,9 @@ bool RustLanguageRuntime::GetDynamicTypeAndAddress(
     }
 
     CompilerType variant_type = ast->FindEnumVariant(type, discriminant);
+    if (!variant_type) {
+      return false;
+    }
     class_type_or_name = TypeAndOrName(variant_type);
     // The address doesn't change.
     dynamic_address.SetLoadAddress(original_ptr, exe_ctx.GetTargetPtr());

diff --git a/lldb/source/Symbol/RustASTContext.cpp b/lldb/source/Symbol/RustASTContext.cpp
@@ -625,6 +625,10 @@ class RustEnum : public RustAggregateBase {
     int idx = m_default;
     if (iter != m_discriminants.end()) {
       idx = iter->second;
+    } else if (idx == -1) {
+      // If the DWARF was bad somehow, we could end up not finding the
+      // discriminant and not having a default.
+      return CompilerType();
     }
     return FieldAt(idx)->m_type;
   }