[lldb] improve the heuristics for checking if a terminal supports Unicode

lldb/include/lldb/Host/Terminal.h

@@ -68,6 +68,18 @@ class Terminal {
 
   llvm::Error SetHardwareFlowControl(bool enabled);
 
+  /// Returns whether or not the current terminal supports Unicode rendering.
+  ///
+  /// The value is cached after the first computation.
+  ///
+  /// On POSIX systems, we check if the LANG environment variable contains the
+  /// substring "UTF-8", case insensitive.
+  ///
+  /// On Windows, we always return true since we use the `WriteConsoleW` API
+  /// internally. Note that the default Windows codepage (437) does not support
+  /// all Unicode characters. This function does not check the codepage.
+  static bool SupportsUnicode();
+
 protected:
   struct Data;
 

lldb/include/lldb/Host/common/DiagnosticsRendering.h

@@ -59,10 +59,27 @@ struct DiagnosticDetail {
 
 StructuredData::ObjectSP Serialize(llvm::ArrayRef<DiagnosticDetail> details);
 
+/// Renders an array of DiagnosticDetail instances.
+///
+/// \param[in] stream
+///     The stream to render the diagnostics to.
+/// \param offset_in_command
+///     An optional offset to the column position of the diagnostic in the
+///     source.
+/// \param show_inline
+///     Whether to show the diagnostics inline.
+/// \param details
+///     The array of DiagnosticsDetail to render.
+/// \param force_ascii
+///     Whether to force ascii rendering. If false, Unicode characters will be
+///     used if the output file supports them.
+///
+/// \see lldb_private::Terminal::SupportsUnicode
 void RenderDiagnosticDetails(Stream &stream,
                              std::optional<uint16_t> offset_in_command,
                              bool show_inline,
-                             llvm::ArrayRef<DiagnosticDetail> details);
+                             llvm::ArrayRef<DiagnosticDetail> details,
+                             bool force_ascii = false);
 
 class DiagnosticError
     : public llvm::ErrorInfo<DiagnosticError, CloneableECError> {

lldb/source/Host/common/DiagnosticsRendering.cpp

@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "lldb/Host/common/DiagnosticsRendering.h"
+#include "lldb/Host/Terminal.h"
+
 #include <cstdint>
 
 using namespace lldb_private;
@@ -85,7 +87,8 @@ static llvm::raw_ostream &PrintSeverity(Stream &stream,
 void RenderDiagnosticDetails(Stream &stream,
                              std::optional<uint16_t> offset_in_command,
                              bool show_inline,
-                             llvm::ArrayRef<DiagnosticDetail> details) {
+                             llvm::ArrayRef<DiagnosticDetail> details,
+                             bool force_ascii) {
   if (details.empty())
     return;
 
@@ -97,12 +100,8 @@ void RenderDiagnosticDetails(Stream &stream,
     return;
   }
 
-  // Since there is no other way to find this out, use the color
-  // attribute as a proxy for whether the terminal supports Unicode
-  // characters.  In the future it might make sense to move this into
-  // Host so it can be customized for a specific platform.
   llvm::StringRef cursor, underline, vbar, joint, hbar, spacer;
-  if (stream.AsRawOstream().colors_enabled()) {
+  if (Terminal::SupportsUnicode() && !force_ascii) {
     cursor = "˄";
     underline = "˜";
     vbar = "│";

lldb/source/Host/common/Terminal.cpp

@@ -400,6 +400,22 @@ llvm::Error Terminal::SetHardwareFlowControl(bool enabled) {
 #endif // LLDB_ENABLE_TERMIOS
 }
 
+bool Terminal::SupportsUnicode() {
+  static std::optional<bool> g_result;
+  if (g_result)
+    return g_result.value();
+#ifdef _WIN32
+  return true;
+#else
+  const char *lang_var = std::getenv("LANG");
+  if (!lang_var)
+    return false;
+  g_result =
+      llvm::StringRef(lang_var).lower().find("utf-8") != std::string::npos;
+#endif
+  return g_result.value();
+}
+
 TerminalState::TerminalState(Terminal term, bool save_process_group)
     : m_tty(term) {
   Save(term, save_process_group);

lldb/unittests/Host/common/DiagnosticsRenderingTest.cpp

@@ -10,7 +10,7 @@ class ErrorDisplayTest : public ::testing::Test {};
 
 std::string Render(std::vector<DiagnosticDetail> details) {
   StreamString stream;
-  RenderDiagnosticDetails(stream, 0, true, details);
+  RenderDiagnosticDetails(stream, 0, true, details, /*force_ascii=*/true);
   return stream.GetData();
 }
 } // namespace