Add template deduction guide for Slice constructor

PR #1367 added an explicit Slice constructor which allows constructing
from any C++ container.

This adds a template deduction guide for that constructor which allows
the Slice type to be inferred from the container's contained type.

This applies to C++17 only.

Note that, because C can be any type of container, we need to use a
template template parameter.

Test: cargo test
Test: cargo test -F c++14
Test: cargo test -F c++17
Test: cargo test -F c++20

include/cxx.h

@@ -220,6 +220,13 @@ class Slice final
   std::array<std::uintptr_t, 2> repr;
 };
 
+// Slice<T> template deduction guides
+#ifdef __cpp_deduction_guides
+template <typename C>
+explicit Slice(C &c)
+    -> Slice<std::remove_reference_t<decltype(*std::declval<C>().data())>>;
+#endif // __cpp_deduction_guides
+
 template <typename T>
 class Slice<T>::iterator final {
 public:

tests/ffi/tests.cc

@@ -891,11 +891,36 @@ extern "C" const char *cxx_run_test() noexcept {
   rust::String bad_utf16_rstring = rust::String::lossy(bad_utf16_literal);
   ASSERT(bad_utf8_rstring == bad_utf16_rstring);
 
-  std::vector<int> cpp_vec{1, 2, 3};
-  rust::Slice<int> slice_of_cpp_vec(cpp_vec);
-  ASSERT(slice_of_cpp_vec.data() == cpp_vec.data());
-  ASSERT(slice_of_cpp_vec.size() == cpp_vec.size());
-  ASSERT(slice_of_cpp_vec[0] == 1);
+  // Test Slice<T> explicit constructor from container
+  {
+    std::vector<int> cpp_vec{1, 2, 3};
+    rust::Slice<int> slice_of_cpp_vec(cpp_vec);
+    ASSERT(slice_of_cpp_vec.data() == cpp_vec.data());
+    ASSERT(slice_of_cpp_vec.size() == cpp_vec.size());
+    ASSERT(slice_of_cpp_vec[0] == 1);
+  }
+
+  // Test Slice<T> template deduction guides
+#ifdef __cpp_deduction_guides
+  {
+    // C<T> -> Slice<T>
+    std::vector<int> cpp_vec{1, 2, 3};
+    auto auto_slice_of_cpp_vec = rust::Slice(cpp_vec);
+    static_assert(
+        std::is_same_v<decltype(auto_slice_of_cpp_vec), rust::Slice<int>>);
+    ASSERT(auto_slice_of_cpp_vec.data() == cpp_vec.data());
+    ASSERT(auto_slice_of_cpp_vec.size() == cpp_vec.size());
+  }
+  {
+    // const C<T> -> Slice<const T>
+    const std::vector<int> cpp_vec{1, 2, 3};
+    auto auto_slice_of_cpp_vec = rust::Slice(cpp_vec);
+    static_assert(std::is_same_v<decltype(auto_slice_of_cpp_vec),
+                                 rust::Slice<const int>>);
+    ASSERT(auto_slice_of_cpp_vec.data() == cpp_vec.data());
+    ASSERT(auto_slice_of_cpp_vec.size() == cpp_vec.size());
+  }
+#endif // __cpp_deduction_guides
 
   rust::Vec<int> vec1{1, 2};
   rust::Vec<int> vec2{3, 4};