Add string_view API for c++17

Author: roligugus

include/cxx.h

@@ -15,6 +15,45 @@
 #include <basetsd.h>
 #endif
 
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+#error "string_view gets auto-detected"
+#endif
+
+// clang-format off
+// detect if string_view supported (C++17 required)
+#ifdef __has_include
+  #if __has_include(<version>)
+    // gcc >= 9, clang >= 9, msvc >= 19.22
+    #include <version>
+    #if __cpp_lib_string_view >= 201603L
+      #define CXXBRIDGE_HAS_STRING_VIEW
+    #endif
+  #else
+    // no <version> include
+    #if defined(__cpp_lib_string_view) && __cpp_lib_string_view >= 201603L
+      // gcc: 8, clang: 5 - 8, msvc: 19.15 - 19.21
+      #define CXXBRIDGE_HAS_STRING_VIEW
+    #else
+      // only include if compiled with c++17
+      #if (__GNUC__ >= 7 && __cplusplus >= 201703L) || (_MSVC_LANG >= 201703L)
+        // gcc: 7, msvc: 19.14
+        #define CXXBRIDGE_HAS_STRING_VIEW
+      #endif
+    #endif
+  #endif
+#else
+  // no __has_include
+  #if _MSC_VER >= 1910L && _MSVC_LANG > 201402L
+    // msvc: 19.10 requires c++latest (!)
+    #define CPPBRIDGE_HAS_STRING_VIEW
+  #endif
+#endif
+// clang-format on
+
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+#include <string_view>
+#endif
+
 namespace rust {
 inline namespace cxxbridge1 {
 
@@ -34,6 +73,9 @@ class String final {
   String(String &&) noexcept;
   ~String() noexcept;
 
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+  String(std::string_view sv) : String(sv.data(), sv.length()) {}
+#endif
   String(const std::string &);
   String(const char *);
   String(const char *, size_t);
@@ -42,6 +84,11 @@ class String final {
   String &operator=(String &&) noexcept;
 
   explicit operator std::string() const;
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+  explicit operator std::string_view() const noexcept {
+    return {this->data(), this->size()};
+  }
+#endif
 
   // Note: no null terminator.
   const char *data() const noexcept;
@@ -71,13 +118,21 @@ class String final {
 class Str final {
 public:
   Str() noexcept;
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+  Str(std::string_view sv) : Str(sv.data(), sv.length()) {}
+#endif
   Str(const std::string &);
   Str(const char *);
   Str(const char *, size_t);
 
   Str &operator=(const Str &) noexcept = default;
 
   explicit operator std::string() const;
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+  explicit operator std::string_view() const noexcept {
+    return {this->data(), this->size()};
+  }
+#endif
 
   // Note: no null terminator.
   const char *data() const noexcept;

tests/ffi/lib.rs

@@ -216,8 +216,10 @@ pub mod ffi {
         fn r_take_ref_r(r: &R);
         fn r_take_ref_c(c: &C);
         fn r_take_str(s: &str);
+        fn r_take_empty_str(s: &str);
         fn r_take_slice_char(s: &[c_char]);
         fn r_take_rust_string(s: String);
+        fn r_take_empty_rust_string(s: String);
         fn r_take_unique_ptr_string(s: UniquePtr<CxxString>);
         fn r_take_ref_vector(v: &CxxVector<u8>);
         fn r_take_ref_empty_vector(v: &CxxVector<u64>);
@@ -470,10 +472,18 @@ fn r_take_str(s: &str) {
     assert_eq!(s, "2020");
 }
 
+fn r_take_empty_str(s: &str) {
+    assert_eq!(s.len(), 0);
+}
+
 fn r_take_rust_string(s: String) {
     assert_eq!(s, "2020");
 }
 
+fn r_take_empty_rust_string(s: String) {
+    assert_eq!(s.len(), 0);
+}
+
 fn r_take_slice_char(s: &[c_char]) {
     assert_eq!(s.len(), 5);
     let s = cast::c_char_to_unsigned(s);

tests/ffi/tests.cc

@@ -624,8 +624,24 @@ extern "C" const char *cxx_run_test() noexcept {
   r_take_unique_ptr(std::unique_ptr<C>(new C{2020}));
   r_take_ref_c(C{2020});
   r_take_str(rust::Str("2020"));
+  r_take_empty_str(rust::Str{""});
+  r_take_empty_str(rust::Str{nullptr, 0});
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+  r_take_str(std::string_view{"2020"});
+  r_take_empty_str(std::string_view{});
+  r_take_empty_str(std::string_view{""});
+  ASSERT(static_cast<std::string_view>(rust::Str{"2020"}) == "2020");
+#endif
   r_take_slice_char(rust::Slice<const char>(SLICE_DATA, sizeof(SLICE_DATA)));
   r_take_rust_string(rust::String("2020"));
+  r_take_empty_rust_string(rust::String{""});
+  r_take_empty_rust_string(rust::String{nullptr, 0});
+#ifdef CXXBRIDGE_HAS_STRING_VIEW
+  r_take_rust_string(std::string_view{"2020"});
+  r_take_empty_rust_string(std::string_view{});
+  r_take_empty_rust_string(std::string_view{""});
+  ASSERT(static_cast<std::string_view>(rust::String{"2020"}) == "2020");
+#endif
   r_take_unique_ptr_string(
       std::unique_ptr<std::string>(new std::string("2020")));
   r_take_ref_vector(std::vector<uint8_t>{20, 2, 0});