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include/swift/Basic/PathRemapper.h

@@ -46,6 +46,11 @@ public:
   /// Returns a remapped `Path` if it starts with a prefix in the map; otherwise
   /// the original path is returned.
   std::string remapPath(StringRef Path) const {
+    // Clang's implementation of this feature also compares the path string
+    // directly instead of treating path segments as indivisible units. The
+    // latter would arguably be more accurate, but we choose to preserve
+    // compatibility with Clang (especially because we propagate the flag to
+    // ClangImporter as well).
     for (const auto &Mapping : PathMappings)
       if (Path.startswith(Mapping.first))
         return (Twine(Mapping.second) +

include/swift/Option/Options.td

@@ -472,7 +472,7 @@ def gdwarf_types : Flag<["-"], "gdwarf-types">,
   HelpText<"Emit full DWARF type info.">;
 def debug_prefix_map : Separate<["-"], "debug-prefix-map">,
     Flags<[FrontendOption]>,
-    HelpText<"remap source paths and search paths in debug info">;
+    HelpText<"Remap source paths and search paths in debug info">;
 
 def debug_info_format : Joined<["-"], "debug-info-format=">,
   Flags<[FrontendOption]>,

lib/Frontend/CompilerInvocation.cpp

@@ -397,6 +397,9 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
     // Forward -debug-prefix-map arguments from Swift to Clang as
     // -fdebug-prefix-map. This is required to ensure DIFiles created there,
     // like "<swift-imported-modules>", have their paths remapped properly.
+    // (Note, however, that Clang's usage of std::map means that the remapping
+    // may not be applied in the same order, which can matter if one mapping is
+    // a prefix of another.)
     Opts.ExtraArgs.push_back("-fdebug-prefix-map=" + A);
   }