[clang] Reapply "[clang] Introduce [[clang::lifetime_capture_by(X)]] (PR #115823)

[Haojian Wu via cfe-commits]

================
@@ -3867,6 +3868,112 @@ static void handleCallbackAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
       S.Context, AL, EncodingIndices.data(), EncodingIndices.size()));
 }
 
+LifetimeCaptureByAttr *Sema::ParseLifetimeCaptureByAttr(const ParsedAttr &AL,
+                                                        StringRef ParamName) {
+  // Atleast one capture by is required.
+  if (AL.getNumArgs() == 0) {
+    Diag(AL.getLoc(), diag::err_capture_by_attribute_no_entity)
+        << AL.getRange();
+    return nullptr;
+  }
+  SmallVector<IdentifierInfo *> ParamIdents;
+  SmallVector<SourceLocation> ParamLocs;
+  for (unsigned I = 0; I < AL.getNumArgs(); ++I) {
+    if (AL.isArgExpr(I)) {
+      Expr *E = AL.getArgAsExpr(I);
+      Diag(E->getExprLoc(), diag::err_capture_by_attribute_argument_unknown)
+          << E << E->getExprLoc();
+      continue;
+    }
+    assert(AL.isArgIdent(I));
+    IdentifierLoc *IdLoc = AL.getArgAsIdent(I);
+    if (IdLoc->Ident->getName() == ParamName) {
+      Diag(IdLoc->Loc, diag::err_capture_by_references_itself) << IdLoc->Loc;
+      continue;
+    }
+    ParamIdents.push_back(IdLoc->Ident);
+    ParamLocs.push_back(IdLoc->Loc);
+  }
+  SmallVector<int> FakeParamIndices(ParamIdents.size(),
+                                    LifetimeCaptureByAttr::INVALID);
+  LifetimeCaptureByAttr *CapturedBy = ::new (Context) LifetimeCaptureByAttr(
+      Context, AL, FakeParamIndices.data(), FakeParamIndices.size());
+  CapturedBy->setArgs(std::move(ParamIdents), std::move(ParamLocs));
+  return CapturedBy;
+}
+
+static void handleLifetimeCaptureByAttr(Sema &S, Decl *D,
+                                        const ParsedAttr &AL) {
+  // Do not allow multiple attributes.
+  if (D->hasAttr<LifetimeCaptureByAttr>()) {
+    S.Diag(AL.getLoc(), diag::err_capture_by_attribute_multiple)
+        << AL.getRange();
+    return;
+  }
+  auto *PVD = dyn_cast<ParmVarDecl>(D);
+  assert(PVD);
+  auto *CaptureByAttr = S.ParseLifetimeCaptureByAttr(AL, PVD->getName());
+  if (CaptureByAttr)
+    D->addAttr(CaptureByAttr);
+}
+
+void Sema::LazyProcessLifetimeCaptureByParams(FunctionDecl *FD) {
+  bool HasImplicitThisParam = isInstanceMethod(FD);
+  SmallVector<LifetimeCaptureByAttr *, 1> Attrs;
+  for (ParmVarDecl *PVD : FD->parameters())
+    if (auto *A = PVD->getAttr<LifetimeCaptureByAttr>())
+      Attrs.push_back(A);
+  if (HasImplicitThisParam) {
+    TypeSourceInfo *TSI = FD->getTypeSourceInfo();
+    if (!TSI)
+      return;
+    AttributedTypeLoc ATL;
+    for (TypeLoc TL = TSI->getTypeLoc();
+         (ATL = TL.getAsAdjusted<AttributedTypeLoc>());
+         TL = ATL.getModifiedLoc()) {
+      if (auto *A = ATL.getAttrAs<LifetimeCaptureByAttr>())
+        Attrs.push_back(const_cast<LifetimeCaptureByAttr *>(A));
+    }
+  }
+  if (Attrs.empty())
+    return;
+  llvm::StringMap<int> NameIdxMapping;
+  NameIdxMapping["global"] = LifetimeCaptureByAttr::GLOBAL;
+  NameIdxMapping["unknown"] = LifetimeCaptureByAttr::UNKNOWN;
----------------
hokein wrote:

we can inline these two `key-value`s in the `NameIdxMapping` initialization, this can save some lookup cost. 

```
llvm::StringMap<int> NameIdxMapping = {{"global", LifetimeCaptureByAttr::GLOBAL}, ...}
```

https://github.com/llvm/llvm-project/pull/115823