Skip cast insertion in unwritable files with a warning.

clang/lib/3C/CastPlacement.cpp

@@ -177,6 +177,21 @@ CastPlacementVisitor::getCastString(ConstraintVariable *Dst,
 
 void CastPlacementVisitor::surroundByCast(ConstraintVariable *Dst,
                                           CastNeeded CastKind, Expr *E) {
+  PersistentSourceLoc PSL = PersistentSourceLoc::mkPSL(E, *Context);
+  if (!canWrite(PSL.getFileName())) {
+    // 3C has known bugs that can cause attempted cast insertion in
+    // unwritable files in common use cases. Until they are fixed, report a
+    // warning rather than letting the main "unwritable change" error trigger
+    // later.
+    clang::DiagnosticsEngine &DE = Writer.getSourceMgr().getDiagnostics();
+    unsigned ErrorId = DE.getCustomDiagID(
+      DiagnosticsEngine::Warning,
+      "3C internal error: tried to insert a cast into an unwritable file "
+      "(https://github.com/correctcomputation/checkedc-clang/issues/454)");
+    DE.Report(E->getBeginLoc(), ErrorId);
+    return;
+  }
+
   auto CastStrs = getCastString(Dst, CastKind);
 
   // If E is already a cast expression, we will try to rewrite the cast instead

clang/test/3C/canwrite_constraints.h

@@ -19,3 +19,17 @@ int *foo_var = ((void *)0);
 // dedicated test for it.
 inline void no_op() {}
 // CHECK_HIGHER: inline void no_op() _Checked {}
+
+// Test the unwritable cast internal warning
+// (https://github.com/correctcomputation/checkedc-clang/issues/454) using the
+// known bug with itypes and function pointers
+// (https://github.com/correctcomputation/checkedc-clang/issues/423) as an
+// example.
+void unwritable_cast(void ((*g)(int *q)) : itype(_Ptr<void(_Ptr<int>)>)) {
+  // expected-warning@+1 {{Declaration in non-writable file}}
+  int *p = 0;
+  // Now 3C thinks it needs to insert _Assume_bounds_cast<_Ptr<int>> around `p`
+  // because it forgets that it is allowed to use the original type of `g`.
+  // expected-warning@+1 {{3C internal error: tried to insert a cast into an unwritable file}}
+  (*g)(p);
+}