Add the C++20 string_view contructor that uses iterators

https://cppreference.com/w/cpp/string/basic_string_view/basic_string_view.html

libc++: https://github.com/llvm/llvm-project/blob/3ecd331808abe0dae36436170df4468765900e0b/libcxx/include/string_view#L335-L343

libstdc++: https://github.com/gcc-mirror/gcc/blob/11f7d5648c42cd2ef6f0841cde6292b94831e7b6/libstdc%2B%2B-v3/include/std/string_view#L154-L162

PiperOrigin-RevId: 799568810
Change-Id: Iebfd505e887654b0f72bd3ad27e031a8114dee0a

Author: derekmauro

absl/strings/string_view.h

@@ -34,7 +34,9 @@
 #include <iosfwd>
 #include <iterator>
 #include <limits>
+#include <memory>
 #include <string>
+#include <type_traits>
 
 #include "absl/base/attributes.h"
 #include "absl/base/nullability.h"
@@ -208,6 +210,16 @@ class ABSL_ATTRIBUTE_VIEW string_view {
     ABSL_ASSERT(data != nullptr || len == 0);
   }
 
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  template <std::contiguous_iterator It, std::sized_sentinel_for<It> End>
+    requires(std::is_same_v<std::iter_value_t<It>, value_type> &&
+             !std::is_convertible_v<End, size_type>)
+  constexpr string_view(It begin, End end)
+      : ptr_(std::to_address(begin)), length_(end - begin) {
+    ABSL_HARDENING_ASSERT(end >= begin);
+  }
+#endif  // ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
   constexpr string_view(const string_view&) noexcept = default;
   string_view& operator=(const string_view&) noexcept = default;
 

absl/strings/string_view_test.cc

@@ -16,6 +16,7 @@
 
 #include <stdlib.h>
 
+#include <array>
 #include <cstddef>
 #include <cstdlib>
 #include <cstring>
@@ -131,6 +132,23 @@ TEST(StringViewTest, Ctor) {
     EXPECT_EQ(8u, s31.length());
   }
 
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  {
+    // Iterator constructor
+    std::string str = "hello";
+    absl::string_view s1(str.begin(), str.end());
+    EXPECT_EQ(s1, "hello");
+
+    std::array<char, 3> arr = { '1', '2', '3' };
+    absl::string_view s2(arr.begin(), arr.end());
+    EXPECT_EQ(s2, "123");
+
+    const char carr[] = "carr";
+    absl::string_view s3(carr, carr + strlen(carr));
+    EXPECT_EQ(s3, "carr");
+  }
+#endif  // ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
   {
     using mstring =
         std::basic_string<char, std::char_traits<char>, Mallocator<char>>;
@@ -1154,6 +1172,18 @@ TEST(StringViewTest, ConstexprCompiles) {
 
   constexpr size_t sp_npos = sp.npos;
   EXPECT_EQ(sp_npos, static_cast<size_t>(-1));
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+  {
+    static constexpr std::array<char, 3> arr = { '1', '2', '3' };
+    constexpr absl::string_view s2(arr.begin(), arr.end());
+    EXPECT_EQ(s2, "123");
+
+    static constexpr char carr[] = "carr";
+    constexpr absl::string_view s3(carr, carr + 4);
+    EXPECT_EQ(s3, "carr");
+  }
+#endif  // ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
 }
 
 constexpr char ConstexprMethodsHelper() {