[clang] extend external_source_symbol attribute with USR clause

Allow the user to specify a concrete USR in the external_source_symbol attribute.
That will let Clang's indexer to use Swift USRs for Swift declarations that are
represented with C++ declarations.

This new clause is used by Swift when generating a C++ header representation
of a Swift module:
https://github.com/apple/swift/pull/63002

Differential Revision: https://reviews.llvm.org/D141324

1 files changed, 9 insertions, 0 deletions

diff --git a/clang/lib/Index/USRGeneration.cpp b/clang/lib/Index/USRGeneration.cpp
index 77b05b6..06a3717 100644
--- a/clang/lib/Index/USRGeneration.cpp
+++ b/clang/lib/Index/USRGeneration.cpp
@@ -1141,6 +1141,15 @@ bool clang::index::generateUSRForDecl(const Decl *D,
   // C++'s operator new function, can have invalid locations but it is fine to
   // create USRs that can identify them.
 
+  // Check if the declaration has explicit external USR specified.
+  auto *CD = D->getCanonicalDecl();
+  if (auto *ExternalSymAttr = CD->getAttr<ExternalSourceSymbolAttr>()) {
+    if (!ExternalSymAttr->getUSR().empty()) {
+      llvm::raw_svector_ostream Out(Buf);
+      Out << ExternalSymAttr->getUSR();
+      return false;
+    }
+  }
   USRGenerator UG(&D->getASTContext(), Buf);
   UG.Visit(D);
   return UG.ignoreResults();