SIL: make a linkage mismatch during de-serialization a human readable diagnostic

Although this error can only happen when tinkering with the stdlib's runtime functions, it's nice to get a readable error message. So far, the compiler just crashed later without a hint to the original problem.

Author: eeckstein

include/swift/AST/DiagnosticsSIL.def

@@ -142,6 +142,10 @@ ERROR(deserialize_function_type_mismatch,Fatal,
       "type mismatch of function '%0', declared as %1 but used in a swift module as %2",
       (StringRef, Type, Type))
 
+ERROR(deserialize_function_linkage_mismatch,Fatal,
+      "linkage mismatch of function '%0', declared as %1 but expected as %2",
+      (StringRef, StringRef, StringRef))
+
 ERROR(without_actually_escaping_on_isolated_any,none,
       "withoutActuallyEscaping is currently unimplemented for '@isolated(any)' "
       "function values", ())

lib/SILOptimizer/UtilityPasses/Link.cpp

@@ -10,7 +10,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/AST/DiagnosticsSIL.h"
 #include "swift/AST/ProtocolConformance.h"
+#include "swift/Demangling/Demangle.h"
 #include "swift/SILOptimizer/PassManager/Passes.h"
 #include "swift/SILOptimizer/PassManager/Transforms.h"
 #include "swift/SIL/SILModule.h"
@@ -158,8 +160,25 @@ linkEmbeddedRuntimeFunctionByName(#NAME, EFFECT, StringRef(#CC) == "C_CC");    \
     if (auto *Fn = M.lookUpFunction(name, byAsmName)) return Fn;
 
     SILFunction *Fn =
-        M.getSILLoader()->lookupSILFunction(name, Linkage, byAsmName);
-    if (!Fn) return nullptr;
+        M.getSILLoader()->lookupSILFunction(name, Linkage,byAsmName);
+
+    if (!Fn) {
+      SILFunction *fnWithWrongLinkage =
+          M.getSILLoader()->lookupSILFunction(name, {}, byAsmName);
+
+      if (fnWithWrongLinkage) {
+        auto fnName = Demangle::demangleSymbolAsString(name,
+                        Demangle::DemangleOptions::SimplifiedUIDemangleOptions());
+        auto &diags = getModule()->getASTContext().Diags;
+        diags.diagnose(fnWithWrongLinkage->getLocation().getSourceLoc(),
+                       diag::deserialize_function_linkage_mismatch,
+                       fnName, getLinkageString(fnWithWrongLinkage->getLinkage()),
+                       getLinkageString(*Linkage));
+        diags.flushConsumers();
+        exit(1);
+      }
+      return nullptr;
+    }
 
     if (M.linkFunction(Fn, LinkMode))
       invalidateAnalysis(Fn, SILAnalysis::InvalidationKind::Everything);