TypeSystem: add a platform hook for library search path additions

Windows does not provide a library search path through a mechanism
similar to `DT_RPATH` or `LC_RPATH`.  Libraries are instead resolved via
the environment variable `Path`.  Add a hook to inject the default
library search path into the repl instance.  This is required to ensure
that system libraries are available.  With this, it is finally possible
to run the REPL on Windows without any additional parameters.

Author: compnerd

lldb/include/lldb/Host/HostInfoBase.h

@@ -147,6 +147,11 @@ class HostInfoBase {
       FileSpec &lldb_shlib_spec, FileSpec &file_spec, bool verify) {
     return false;
   }
+
+  /// Return the default set of library paths to search in.  This allows a
+  /// platform specific extension for system libraries that may need to be
+  /// resolved (e.g. `/usr/lib` on Unicies and `Path` on Windows).
+  static std::vector<std::string> GetSwiftLibrarySearchPaths() { return {}; }
 #endif
 
 protected:

lldb/include/lldb/Host/windows/HostInfoWindows.h

@@ -42,6 +42,7 @@ class HostInfoWindows : public HostInfoBase {
   static bool ComputeSwiftResourceDirectory(FileSpec &lldb_shlib_spec,
                                             FileSpec &file_spec, bool verify);
   static llvm::Expected<llvm::StringRef> GetSDKRoot(SDKOptions options);
+  static std::vector<std::string> GetSwiftLibrarySearchPaths();
 #endif
 
 private:

lldb/source/Host/windows/HostInfoWindowsSwift.cpp

@@ -13,9 +13,15 @@
 #include "lldb/Utility/FileSpec.h"
 #include "lldb/Utility/LLDBLog.h"
 #include "lldb/Utility/Log.h"
+
+#include <llvm/ADT/STLExtras.h>
+#include <llvm/ADT/StringExtras.h>
+#include <llvm/ADT/StringRef.h>
 #include <llvm/Support/ConvertUTF.h>
 
+#include <cstdlib>
 #include <string>
+#include <vector>
 
 using namespace lldb_private;
 
@@ -60,3 +66,17 @@ llvm::Expected<llvm::StringRef> HostInfoWindows::GetSDKRoot(SDKOptions options)
     return g_sdkroot;
   return llvm::make_error<HostInfoError>("`SDKROOT` is unset");
 }
+
+std::vector<std::string> HostInfoWindows::GetSwiftLibrarySearchPaths() {
+  static std::once_flag g_flag;
+  static std::vector<std::string> g_library_paths;
+
+  std::call_once(g_flag, []() {
+    llvm::for_each(llvm::split(std::getenv("Path"), ";"),
+                   [](llvm::StringRef path) {
+      g_library_paths.emplace_back(path);
+    });
+  });
+
+  return g_library_paths;
+}

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -3528,6 +3528,8 @@ GetLibrarySearchPaths(const swift::SearchPathOptions &search_path_opts) {
     paths.push_back(path);
   for (std::string path : search_path_opts.LibrarySearchPaths)
     paths.push_back(path);
+  for (const std::string &path : HostInfo::GetSwiftLibrarySearchPaths())
+    paths.emplace_back(path);
   return paths;
 }