Support: Expose sys::path::is_style_{posix,windows,native}()

Expose three helpers in namespace llvm::sys::path to detect the
path rules followed by sys::path::Style.

- is_style_posix()
- is_style_windows()
- is_style_native()

This are constexpr functions that that will allow a bunch of
path-related code to stop checking `_WIN32`.

Originally I looked at adding system_style(), analogous to
sys::endian::system_endianness(), but future patches (from others) will
add more Windows style variants for slash preferences. These helpers
should be resilient to that change, allowing callers to detect basic
path rules.

Differential Revision: https://reviews.llvm.org/D112288

llvm/include/llvm/Support/Path.h

@@ -27,6 +27,27 @@ namespace path {
 
 enum class Style { windows, posix, native };
 
+/// Check if \p S uses POSIX path rules.
+constexpr bool is_style_posix(Style S) {
+  if (S == Style::posix)
+    return true;
+  if (S != Style::native)
+    return false;
+#if defined(_WIN32)
+  return false;
+#else
+  return true;
+#endif
+}
+
+/// Check if \p S uses Windows path rules.
+constexpr bool is_style_windows(Style S) { return !is_style_posix(S); }
+
+/// Check if \p S uses the same path rules as Style::native.
+constexpr bool is_style_native(Style S) {
+  return is_style_posix(S) == is_style_posix(Style::native);
+}
+
 /// @name Lexical Component Iterator
 /// @{
 

llvm/lib/Support/Path.cpp

@@ -37,11 +37,9 @@ namespace {
   using llvm::sys::path::Style;
 
   inline Style real_style(Style style) {
-#ifdef _WIN32
-    return (style == Style::posix) ? Style::posix : Style::windows;
-#else
-    return (style == Style::windows) ? Style::windows : Style::posix;
-#endif
+    if (is_style_posix(style))
+      return Style::posix;
+    return Style::windows;
   }
 
   inline const char *separators(Style style) {

llvm/unittests/Support/Path.cpp

@@ -69,6 +69,29 @@ struct FileDescriptorCloser {
   int FD;
 };
 
+TEST(is_style_Style, Works) {
+  using namespace llvm::sys::path;
+  // Check platform-independent results.
+  EXPECT_TRUE(is_style_posix(Style::posix));
+  EXPECT_TRUE(is_style_windows(Style::windows));
+  EXPECT_FALSE(is_style_posix(Style::windows));
+  EXPECT_FALSE(is_style_windows(Style::posix));
+
+  // Check platform-dependent results.
+#if defined(_WIN32)
+  EXPECT_FALSE(is_style_posix(Style::native));
+  EXPECT_TRUE(is_style_windows(Style::native));
+#else
+  EXPECT_TRUE(is_style_posix(Style::native));
+  EXPECT_FALSE(is_style_windows(Style::native));
+#endif
+
+  // Check is_style_native().
+  EXPECT_TRUE(is_style_native(Style::native));
+  EXPECT_EQ(is_style_posix(Style::native), is_style_native(Style::posix));
+  EXPECT_EQ(is_style_windows(Style::native), is_style_native(Style::windows));
+}
+
 TEST(is_separator, Works) {
   EXPECT_TRUE(path::is_separator('/'));
   EXPECT_FALSE(path::is_separator('\0'));
@@ -78,11 +101,8 @@ TEST(is_separator, Works) {
   EXPECT_TRUE(path::is_separator('\\', path::Style::windows));
   EXPECT_FALSE(path::is_separator('\\', path::Style::posix));
 
-#ifdef _WIN32
-  EXPECT_TRUE(path::is_separator('\\'));
-#else
-  EXPECT_FALSE(path::is_separator('\\'));
-#endif
+  EXPECT_EQ(path::is_style_windows(path::Style::native),
+            path::is_separator('\\'));
 }
 
 TEST(is_absolute_gnu, Works) {
@@ -107,6 +127,10 @@ TEST(is_absolute_gnu, Works) {
               std::get<1>(Path));
     EXPECT_EQ(path::is_absolute_gnu(std::get<0>(Path), path::Style::windows),
               std::get<2>(Path));
+
+    constexpr int Native = is_style_posix(path::Style::native) ? 1 : 2;
+    EXPECT_EQ(path::is_absolute_gnu(std::get<0>(Path), path::Style::native),
+              std::get<Native>(Path));
   }
 }