Bug 1772006 - Part 2: Support converting nsTStringRepr<T> to std::basic_string_view<T>, r=xpcom-reviewers,barret

This type was introduced in c++17, and can be used as a convenient standard
medium for passing around borrowed substring references. It can be implicitly
converted to from string literals and `const char_type*`, meaning that after
this change it can be used as a convenient catch-all type to replace seperate
overloads for `const self_type&`, `const char_type*` and `const
char_type(&)[N]`.

std::basic_string_view also provides standard implementations of some
algorithms which will be convenient for code cleanup in later parts of this
bug.

Differential Revision: https://phabricator.services.mozilla.com/D148297

layout/style/ServoBindings.toml

@@ -46,6 +46,7 @@ raw-lines = [
 hide-types = [
     ".*char_traits",
     ".*incompatible_char_type",
+    ".*string_view",
     # https://github.com/rust-lang/rust-bindgen/issues/1503
     "mozilla::StyleTimingFunction.*",
     # https://github.com/rust-lang/rust-bindgen/issues/1559

xpcom/string/nsTDependentString.h

@@ -43,6 +43,8 @@ class nsTDependentString : public nsTString<T> {
 
   typedef typename base_string_type::const_char_iterator const_char_iterator;
 
+  typedef typename base_string_type::string_view string_view;
+
   typedef typename base_string_type::index_type index_type;
   typedef typename base_string_type::size_type size_type;
 

xpcom/string/nsTDependentSubstring.h

@@ -44,6 +44,8 @@ class nsTDependentSubstring : public nsTSubstring<T> {
 
   typedef typename substring_type::const_char_iterator const_char_iterator;
 
+  typedef typename substring_type::string_view string_view;
+
   typedef typename substring_type::index_type index_type;
   typedef typename substring_type::size_type size_type;
 

xpcom/string/nsTString.h

@@ -54,6 +54,8 @@ class nsTString : public nsTSubstring<T> {
 
   typedef typename substring_type::const_char_iterator const_char_iterator;
 
+  typedef typename substring_type::string_view string_view;
+
   typedef typename substring_type::index_type index_type;
   typedef typename substring_type::size_type size_type;
 

xpcom/string/nsTStringRepr.h

@@ -8,6 +8,7 @@
 #define nsTStringRepr_h
 
 #include <limits>
+#include <string_view>
 #include <type_traits>  // std::enable_if
 
 #include "mozilla/Char16.h"
@@ -131,6 +132,8 @@ class nsTStringRepr {
 
   typedef const char_type* const_char_iterator;
 
+  typedef std::basic_string_view<char_type> string_view;
+
   typedef size_t index_type;
   typedef size_t size_type;
 
@@ -179,10 +182,14 @@ class nsTStringRepr {
 #endif
 
   // Returns pointer to string data (not necessarily null-terminated)
-  constexpr const typename raw_type<T, int>::type Data() const { return mData; }
+  constexpr typename raw_type<T, int>::type Data() const { return mData; }
 
   constexpr size_type Length() const { return static_cast<size_type>(mLength); }
 
+  constexpr string_view View() const { return string_view(Data(), Length()); }
+
+  constexpr operator string_view() const { return View(); }
+
   constexpr DataFlags GetDataFlags() const { return mDataFlags; }
 
   constexpr bool IsEmpty() const { return mLength == 0; }

xpcom/string/nsTSubstring.h

@@ -312,6 +312,8 @@ class nsTSubstring : public mozilla::detail::nsTStringRepr<T> {
 
   typedef typename base_string_type::const_char_iterator const_char_iterator;
 
+  typedef typename base_string_type::string_view string_view;
+
   typedef typename base_string_type::index_type index_type;
   typedef typename base_string_type::size_type size_type;