[clang] [LifetimeSafety] Introduce buildOriginFlowChain for use-after-scope (PR #199345)

[Yuan Suo via cfe-commits]

================
@@ -471,13 +474,50 @@ class LifetimeSafetySemaHelperImpl : public LifetimeSafetySemaHelper {
   }
 
   std::string getDiagSubjectDescription(const Expr *E) {
+    // FIXME: Ideally, this should use IgnoreParenImpCasts().
+    // However, according to the comment on IgnoreParenImpCasts(),
+    // it is not fully equivalent to IgnoreImpCasts() + IgnoreParens().
+    // Once the FIXME in IgnoreParenImpCasts() is resolved,
+    // this can be switched to use IgnoreParenImpCasts().
+    E = E->IgnoreImpCasts()->IgnoreParens();
     if (isa<MaterializeTemporaryExpr>(E))
       return "local temporary object";
 
     if (const auto *DRE = dyn_cast<DeclRefExpr>(E))
       return getDiagSubjectDescription(DRE->getDecl());
     // TODO: Handle other expression types.
-    return "";
+    return "expression";
+  }
+
+  bool shouldShowInAliasChain(const Expr *CurrExpr, const Expr *LastExpr) {
+    CurrExpr = CurrExpr->IgnoreImpCasts()->IgnoreParens();
+    LastExpr = LastExpr->IgnoreImpCasts()->IgnoreParens();
+
+    if (CurrExpr->getSourceRange() == LastExpr->getSourceRange())
+      return false;
+    if (isa<MaterializeTemporaryExpr>(CurrExpr) &&
+        isa<MaterializeTemporaryExpr>(LastExpr))
----------------
suoyuan666 wrote:

Originally, I thought the goal was only to avoid diagnostics like `local temporary object aliases the storage of local temporary object`, while still allowing cases such as `local temporary object aliases the storage of local variable 'a'` to be displayed.

However, I realize my reasoning here was a bit confused. Even if we want that behavior, it shouldn't depend on `LastExpr` in the first place.

That's my mistake. I'll change the implementation to only inspect `CurrExpr`.


https://github.com/llvm/llvm-project/pull/199345