Permit conversion to string of std::string_view. (#730)

Fixes #728.

I've been reluctant to implement this because it could pose a larger
runtime cost than the caller might expect.  We need to copy the whole
underlying string into the available buffer just to ensure termination.

But as the bug shows, users may need the conversion in order to pass
arrays of `string_view` as statement parameters.

NEWS

@@ -1,6 +1,7 @@
 7.8.2
  - Fix broken `to_buf()` on `zview`. (#728)
  - Support `PQfsize()` and `PQfmod()`. (#727)
+ - Implement conversion from `string_view` to string. (#728)
 7.8.1
  - Regenerate build files. Should fix ARM Mac build. (#715)
  - Reinstate `<ciso646>` that MSVC can't live with or without. (#713)

include/pqxx/internal/conversions.hxx

@@ -651,6 +651,13 @@ template<> struct string_traits<std::string_view>
     return begin + std::size(value) + 1;
   }
 
+  static zview to_buf(char *begin, char *end, std::string_view const &value)
+  {
+    // You'd think we could just return the same view but alas, there's no
+    // zero-termination on a string_view.
+    return generic_to_buf(begin, end, value);
+  }
+
   /// Don't convert to this type; it has nowhere to store its contents.
   static std::string_view from_string(std::string_view) = delete;
 };

test/unit/test_string_conversion.cxx

@@ -6,6 +6,9 @@
 
 #include "../test_helpers.hxx"
 
+using namespace std::literals;
+
+
 // Some enums with string conversions.
 enum EnumA
 {
@@ -171,8 +174,43 @@ void test_convert_null()
 }
 
 
+void test_string_view_conversion()
+{
+  using traits = pqxx::string_traits<std::string_view>;
+
+  PQXX_CHECK_EQUAL(
+    pqxx::to_string("view here"sv),
+    "view here"s,
+    "Bad conversion from string_view.");
+
+  char buf[200];
+
+  char *end{traits::into_buf(std::begin(buf), std::end(buf), "more view"sv)};
+  PQXX_CHECK(
+    std::begin(buf) < end and end < std::end(buf),
+    "string_view into_buf did not stay within its buffer.");
+  PQXX_CHECK(
+    *(end - 1) == '\0', "string_view into_buf did not zero-terminate.");
+  PQXX_CHECK_EQUAL(
+    (std::string{buf, static_cast<std::size_t>(end - std::begin(buf) - 1)}),
+    "more view"s,
+    "string_view into_buf wrote wrong data.");
+  PQXX_CHECK(
+    *(end - 2) == 'w', "string_view into_buf is in the wrong place.");
+
+  std::string_view org{"another!"sv};
+  pqxx::zview out{traits::to_buf(std::begin(buf), std::end(buf), org)};
+  PQXX_CHECK_EQUAL(
+    std::string{out}, "another!"s, "string_view to_buf returned wrong data.");
+  PQXX_CHECK(
+    std::data(out) != std::data(org),
+    "string_view to_buf returned original view, which may not be terminated.");
+}
+
+
 PQXX_REGISTER_TEST(test_string_conversion);
 PQXX_REGISTER_TEST(test_convert_variant_to_string);
 PQXX_REGISTER_TEST(test_integer_conversion);
 PQXX_REGISTER_TEST(test_convert_null);
+PQXX_REGISTER_TEST(test_string_view_conversion);
 } // namespace