Handle array to pointer decay for C literals

In C, as opposed to C++, string literals are char [] instead of const
char [] so cast_mut should be added on ArrayToPointerDecay from char []
to char * instead of NoOp const char * to char *.

Author: lucic71

cpp2rust/converter/converter.cpp

@@ -1725,7 +1725,11 @@ bool Converter::VisitImplicitCastExpr(clang::ImplicitCastExpr *expr) {
   case clang::CastKind::CK_ArrayToPointerDecay:
     if (clang::isa<clang::StringLiteral>(sub_expr) ||
         clang::isa<clang::PredefinedExpr>(sub_expr)) {
-      return Convert(sub_expr);
+      Convert(sub_expr);
+      if (IsConversionFromStringLiteralToCharPtr(expr)) {
+        StrCat(".cast_mut()");
+      }
+      return false;
     }
     // __va_list_tag [1] decays to __va_list_tag *. Just pass through by value
     if (IsVaListType(sub_expr->getType())) {

cpp2rust/converter/converter_lib.cpp

@@ -764,9 +764,12 @@ void Unwrap(std::string &s, std::string_view prefix, std::string_view suffix) {
 }
 
 bool IsConversionFromStringLiteralToCharPtr(clang::Expr *expr) {
-  return clang::isa<clang::StringLiteral>(expr->IgnoreImplicit()) &&
-         expr->getType()->isPointerType() &&
-         expr->getType()->getPointeeType()->isCharType();
+  if (clang::isa<clang::StringLiteral>(expr->IgnoreImplicit()) &&
+      expr->getType()->isPointerType()) {
+    auto pointee = expr->getType()->getPointeeType();
+    return pointee->isCharType() && !pointee.isConstQualified();
+  }
+  return false;
 }
 
 } // namespace cpp2rust