[Sema] Add warning for unused lambda captures

Summary:
Warn when a lambda explicitly captures something that is not used in its body.

The warning is part of -Wunused and can be enabled with -Wunused-lambda-capture.

Reviewers: rsmith, arphaman, jbcoe, aaron.ballman

Subscribers: Quuxplusone, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D28467

llvm-svn: 291905

3 files changed, 25 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index d62e8fd68b6..7b404f40a76 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -13916,8 +13916,10 @@ bool Sema::tryCaptureVariable(
 
     // Check whether we've already captured it.
     if (isVariableAlreadyCapturedInScopeInfo(CSI, Var, Nested, CaptureType, 
-                                             DeclRefType)) 
+                                             DeclRefType)) {
+      CSI->getCapture(Var).markUsed(BuildAndDiagnose);
       break;
+    }
     // If we are instantiating a generic lambda call operator body, 
     // we do not want to capture new variables.  What was captured
     // during either a lambdas transformation or initial parsing
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index b2fb33f5343..66bdf1a2da8 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -1106,6 +1106,7 @@ bool Sema::CheckCXXThisCapture(SourceLocation Loc, const bool Explicit,
             dyn_cast<CapturingScopeInfo>(FunctionScopes[idx])) {
       if (CSI->CXXThisCaptureIndex != 0) {
         // 'this' is already being captured; there isn't anything more to do.
+        CSI->Captures[CSI->CXXThisCaptureIndex - 1].markUsed(BuildAndDiagnose);
         break;
       }
       LambdaScopeInfo *LSI = dyn_cast<LambdaScopeInfo>(CSI);
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index a0d574915eb..da2768f2131 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1384,7 +1384,7 @@ static void addBlockPointerConversion(Sema &S,
 }
 
 static ExprResult performLambdaVarCaptureInitialization(
-    Sema &S, LambdaScopeInfo::Capture &Capture, FieldDecl *Field) {
+    Sema &S, const LambdaScopeInfo::Capture &Capture, FieldDecl *Field) {
   assert(Capture.isVariableCapture() && "not a variable capture");
 
   auto *Var = Capture.getVariable();
@@ -1438,6 +1438,21 @@ mapImplicitCaptureStyle(CapturingScopeInfo::ImplicitCaptureStyle ICS) {
   llvm_unreachable("Unknown implicit capture style");
 }
 
+void Sema::DiagnoseUnusedLambdaCapture(const LambdaScopeInfo::Capture &From) {
+  if (!From.isVLATypeCapture()) {
+    Expr *Init = From.getInitExpr();
+    if (Init && Init->HasSideEffects(Context))
+      return;
+  }
+
+  auto diag = Diag(From.getLocation(), diag::warn_unused_lambda_capture);
+  if (From.isThisCapture())
+    diag << "'this'";
+  else
+    diag << From.getVariable();
+  diag << From.isNonODRUsed();
+}
+
 ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
                                  LambdaScopeInfo *LSI) {
   // Collect information from the lambda scope.
@@ -1476,10 +1491,14 @@ ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
     // Translate captures.
     auto CurField = Class->field_begin();
     for (unsigned I = 0, N = LSI->Captures.size(); I != N; ++I, ++CurField) {
-      LambdaScopeInfo::Capture From = LSI->Captures[I];
+      const LambdaScopeInfo::Capture &From = LSI->Captures[I];
       assert(!From.isBlockCapture() && "Cannot capture __block variables");
       bool IsImplicit = I >= LSI->NumExplicitCaptures;
 
+      // Warn about unused explicit captures.
+      if (!CurContext->isDependentContext() && !IsImplicit && !From.isODRUsed())
+        DiagnoseUnusedLambdaCapture(From);
+
       // Handle 'this' capture.
       if (From.isThisCapture()) {
         Captures.push_back(