Implement 3 prominent feature requests:

<rdar://problem/15975935> warning that you can use 'let' not 'var'
<rdar://problem/18876585> Compiler should warn me if I set a parameter as 'var' but never modify it
<rdar://problem/17224539> QoI: warn about unused variables

This uses a simple pass in MiscDiagnostics that walks the body of an
AbstractFunctionDecl.  This means that it doesn't warn about unused properties (etc),
but it captures a vast majority of the cases.

It also does not warn about unused parameters (as a policy decision) because it is too noisy,
there are a variety of other refinements that could be done as well, thoughts welcome.



Swift SVN r28412

include/swift/AST/DiagnosticsSema.def

@@ -2265,6 +2265,28 @@ ERROR(availability_declaration_less_available_than_protocol, sema_avail,
 NOTE(availability_protocol_requirement_here, sema_avail, none,
      "protocol requirement here", ())
 
+
+//------------------------------------------------------------------------------
+// Variable usage diagnostics
+//------------------------------------------------------------------------------
+
+WARNING(pbd_never_used, sema_varusage, none,
+        "initialization of %select{variable|immutable value}1 %0 was never used"
+        "; consider replacing with assignment to '_' or removing it",
+        (Identifier, unsigned))
+
+WARNING(variable_never_used, sema_varusage, none,
+        "%select{variable|immutable value}1 %0 was never used; "
+        "consider replacing with '_' or removing it",
+        (Identifier, unsigned))
+WARNING(variable_never_mutated, sema_varusage, none,
+        "%select{variable|parameter}1 %0 was never mutated; "
+        "consider changing to 'let' constant",
+        (Identifier, unsigned))
+WARNING(variable_never_read, sema_varusage, none,
+        "%select{variable|parameter}1 %0 was written to, but never read",
+        (Identifier, unsigned))
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

lib/Sema/MiscDiagnostics.cpp

@@ -19,6 +19,7 @@
 #include "swift/Basic/SourceManager.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/Parse/Lexer.h"
+#include "llvm/ADT/MapVector.h"
 using namespace swift;
 
 //===--------------------------------------------------------------------===//
@@ -774,6 +775,364 @@ void swift::performStmtDiagnostics(TypeChecker &TC, const Stmt *S) {
   return diagUnreachableCode(TC, S);
 }
 
+//===--------------------------------------------------------------------===//
+// Per func/init diagnostics
+//===--------------------------------------------------------------------===//
+
+namespace {
+class VarDeclUsageChecker : public ASTWalker {
+  TypeChecker &TC;
+  
+  // Keep track of some information about a variable.
+  enum {
+    RK_Read    = 1,      ///< Whether it was ever read.
+    RK_Written = 2,      ///< Whether it was ever written or passed inout.
+  };
+  
+  /// These are all of the variables that we are tracking.  VarDecls get added
+  /// to this when the declaration is seen.  We use a MapVector to keep the
+  /// diagnostics emission in deterministic order.
+  llvm::SmallMapVector<VarDecl*, unsigned, 32> VarDecls;
+  
+  bool sawError = false;
+  
+public:
+  VarDeclUsageChecker(TypeChecker &TC, AbstractFunctionDecl *AFD) : TC(TC) {
+    // Track the parameters of the function.
+    for (auto P : AFD->getBodyParamPatterns())
+      P->forEachVariable([&](VarDecl *VD) {
+        if (shouldTrackVarDecl(VD))
+          VarDecls[VD] = 0;
+      });
+  }
+
+  // After we have scanned the entire region, diagnose variables that could be
+  // declared with a narrower usage kind.
+  ~VarDeclUsageChecker();
+  
+  bool shouldTrackVarDecl(VarDecl *VD) {
+    // If the variable is implicit, ignore it.
+    if (VD->isImplicit() || VD->getLoc().isInvalid())
+      return false;
+    
+    // If the variable was invalid, ignore it and notice that the code is
+    // malformed.
+    if (VD->isInvalid() || !VD->hasType()) {
+      sawError = true;
+      return false;
+    }
+    
+    // If the variable is already unnamed, ignore it.
+    if (!VD->hasName() || VD->getName().str() == "_")
+      return false;
+    
+    return true;
+  }
+
+  void addMark(Decl *D, unsigned Flag) {
+    auto *vd = dyn_cast<VarDecl>(D);
+    if (!vd) return;
+
+    auto vdi = VarDecls.find(vd);
+    if (vdi != VarDecls.end())
+      vdi->second |= Flag;
+  }
+  
+  void markBaseOfAbstractStorageDeclStore(Expr *E, ConcreteDeclRef decl);
+  
+  void markStoredOrInOutExpr(Expr *E, unsigned Flags);
+  
+  // We generally walk into declarations, other than types and nested functions.
+  // FIXME: peek into capture lists of nested functions.
+  bool walkToDeclPre(Decl *D) override {
+    if (isa<TypeDecl>(D))
+      return false;
+      
+    // If this is a VarDecl, then add it to our list of things to track.
+    if (auto *vd = dyn_cast<VarDecl>(D))
+      if (shouldTrackVarDecl(vd))
+        VarDecls[vd] = 0;
+
+    if (auto *afd = dyn_cast<AbstractFunctionDecl>(D)) {
+      // If this is a nested function with a capture list, mark any captured
+      // variables.
+      if (afd->isBodyTypeChecked()) {
+        for (const auto &capture : afd->getCaptureInfo().getCaptures())
+          addMark(capture.getDecl(), RK_Read|RK_Written);
+      } else {
+        // If the body hasn't been type checked yet, be super-conservative and
+        // mark all variables as used.  This can be improved later, e.g. by
+        // walking the untype-checked body to look for things that could
+        // possibly be used.
+        VarDecls.clear();
+      }
+      
+      // Don't walk into it though, it may not even be type checked yet.
+      return false;
+    }
+
+    
+    // Note that we ignore the initialization behavior of PatternBindingDecls,
+    // but we do want to walk into them, because we want to see any uses or
+    // other things going on in the initializer expressions.
+    return true;
+  }
+  
+  /// The heavy lifting happens when visiting expressions.
+  std::pair<bool, Expr *> walkToExprPre(Expr *E) override;
+};
+}
+
+
+// After we have scanned the entire region, diagnose variables that could be
+// declared with a narrower usage kind.
+VarDeclUsageChecker::~VarDeclUsageChecker() {
+  // If we saw an ErrorExpr somewhere in the body, then we have a malformed AST
+  // and we know stuff got dropped.  Instead of producing these diagnostics,
+  // lets let the bigger issues get resolved first.
+  if (sawError)
+    return;
+  
+  for (auto elt : VarDecls) {
+    auto *var = elt.first;
+    unsigned access = elt.second;
+
+    // If this is a 'let' value, any stores to it are actually initializations,
+    // not mutations.
+    if (var->isLet())
+      access &= ~RK_Written;
+    
+    // If this variable has WeakStorageType, then it can be mutated in ways we
+    // don't know.
+    if (var->getType()->is<WeakStorageType>())
+      access |= RK_Written;
+    
+    // If this is a vardecl with 'inout' type, then it is an inout argument to a
+    // function, never diagnose anything related to it.
+    if (var->getType()->is<InOutType>())
+      continue;    
+    
+    // Consider parameters to always have been read.  It is common to name a
+    // parameter and not use it (e.g. because you are an override or want the
+    // named keyword, etc).  Warning to rewrite it to _ is more annoying than
+    // it is useful.
+    if (isa<ParamDecl>(var))
+      access |= RK_Read;
+    
+    // Diagnose variables that were never used (other than their
+    // initialization).
+    //
+    if (access == 0) {
+      // If the source of the VarDecl is a trivial PatternBinding with only a
+      // single binding, rewrite the whole thing into an assignment.
+      //    let x = foo()
+      //  ->
+      //    _ = foo()
+      if (auto *pbd = var->getParentPatternBinding())
+        if (pbd->getSingleVar() == var && pbd->getInit(0) != nullptr) {
+          unsigned varKind = var->isLet();
+          TC.diagnose(var->getLoc(), diag::pbd_never_used,
+                      var->getName(), varKind)
+            .fixItReplace(SourceRange(pbd->getLoc(), var->getLoc()), "_");
+          continue;
+        }
+      
+      // Otherwise, this is something more complex, perhaps
+      //    let (a,b) = foo()
+      // Just rewrite the one variable with a _.
+      unsigned varKind = var->isLet();
+      TC.diagnose(var->getLoc(), diag::variable_never_used,
+                  var->getName(), varKind)
+        .fixItReplace(var->getLoc(), "_");
+      continue;
+    }
+    
+    // If this is a mutable 'var', and it was never written to, suggest
+    // upgrading to 'let'.  We do this even for a parameter.
+    if (!var->isLet() && (access & RK_Written) == 0) {
+      SourceLoc FixItLoc;
+      
+      // Try to find the location of the 'var' so we can produce a fixit.  If
+      // this is a simple PatternBinding, use its location.
+      if (auto *PBD = var->getParentPatternBinding())
+        if (PBD->getSingleVar() == var)
+          FixItLoc = PBD->getLoc();
+      
+      // If this is a parameter explicitly marked 'var', remove it.
+      if (auto *param = dyn_cast<ParamDecl>(var))
+        if (auto *pattern = param->getParamParentPattern())
+          if (auto *vp = dyn_cast<VarPattern>(pattern)) {
+            TC.diagnose(var->getLoc(), diag::variable_never_mutated,
+                        var->getName(), /*param*/1)
+              .fixItRemove(vp->getLoc());
+            continue;
+          }
+      
+      unsigned varKind = isa<ParamDecl>(var);
+      // FIXME: fixit when we can find a pattern binding.
+      if (FixItLoc.isInvalid())
+        TC.diagnose(var->getLoc(), diag::variable_never_mutated,
+                    var->getName(), varKind);
+      else
+        TC.diagnose(var->getLoc(), diag::variable_never_mutated,
+                    var->getName(), varKind)
+          .fixItReplace(FixItLoc, "let");
+      continue;
+    }
+    
+    // If this is a variable that was only written to, emit a warning.
+    if ((access & RK_Read) == 0) {
+      TC.diagnose(var->getLoc(), diag::variable_never_read, var->getName(),
+                  isa<ParamDecl>(var));
+      continue;
+    }
+  }
+}
+
+/// Handle a store to "x.y" where 'base' is the expression for x and 'decl' is
+/// the decl for 'y'.
+void VarDeclUsageChecker::
+markBaseOfAbstractStorageDeclStore(Expr *base, ConcreteDeclRef decl) {
+  // If the base is a class or an rvalue, then this store just loads the base.
+  if (base->getType()->isAnyClassReferenceType() ||
+      !(base->getType()->isLValueType() || base->getType()->is<InOutType>())) {
+    base->walk(*this);
+    return;
+  }
+
+  // If the store is to a non-mutating member, then this is just a load, even
+  // if the base is an inout expr.
+  auto *ASD = cast<AbstractStorageDecl>(decl.getDecl());
+  if ((ASD->hasAccessorFunctions() && ASD->getSetter() &&
+       !ASD->getSetter()->isMutating()) ||
+      (ASD->hasAddressors() && ASD->getMutableAddressor() &&
+       !ASD->getMutableAddressor()->isMutating())) {
+    // Sema conservatively converts the base to inout expr when it is an lvalue.
+    // Look through it because we know it isn't actually doing a load/store.
+    if (auto *ioe = dyn_cast<InOutExpr>(base))
+      base = ioe->getSubExpr();
+    base->walk(*this);
+    return;
+  }
+  
+  // Otherwise this is a read and write of the base.
+  return markStoredOrInOutExpr(base, RK_Written|RK_Read);
+}
+
+
+ void VarDeclUsageChecker::markStoredOrInOutExpr(Expr *E, unsigned Flags) {
+   // Sema leaves some subexpressions null, which seems really unfortunate.  It
+   // should replace them with ErrorExpr.
+   if (E == nullptr || !E->getType() || E->getType()->is<ErrorType>()) {
+     sawError = true;
+     return;
+   }
+   
+   // Ignore parens and other easy cases.
+   E = E->getSemanticsProvidingExpr();
+   
+   // If we found a decl that is being assigned to, then mark it.
+   if (auto *DRE = dyn_cast<DeclRefExpr>(E)) {
+     addMark(DRE->getDecl(), Flags);
+     return;
+   }
+
+   if (auto *TE = dyn_cast<TupleExpr>(E)) {
+     for (auto &elt : TE->getElements())
+       markStoredOrInOutExpr(elt, Flags);
+     return;
+   }
+   
+   // If this is an assignment into a mutating subscript lvalue expr, then we
+   // are mutating the base expression.  We also need to visit the index
+   // expressions as loads though.
+   if (auto *SE = dyn_cast<SubscriptExpr>(E)) {
+     // The index of the subscript is evaluted as an rvalue.
+     SE->getIndex()->walk(*this);
+     if (SE->hasDecl())
+       markBaseOfAbstractStorageDeclStore(SE->getBase(), SE->getDecl());
+     else  // FIXME: Should not be needed!
+       markStoredOrInOutExpr(SE->getBase(), RK_Written|RK_Read);
+     
+     return;
+   }
+   
+   if (auto *ioe = dyn_cast<InOutExpr>(E))
+     return markStoredOrInOutExpr(ioe->getSubExpr(), RK_Written|RK_Read);
+   
+   if (auto *MRE = dyn_cast<MemberRefExpr>(E)) {
+     markBaseOfAbstractStorageDeclStore(MRE->getBase(), MRE->getMember());
+     return;
+   }
+   
+   if (auto *TEE = dyn_cast<TupleElementExpr>(E))
+     return markStoredOrInOutExpr(TEE->getBase(), Flags);
+
+
+   // If we don't know what kind of expression this is, assume it's a reference
+   // and mark it as a read.
+   E->walk(*this);
+}
+
+   
+
+/// The heavy lifting happens when visiting expressions.
+std::pair<bool, Expr *> VarDeclUsageChecker::walkToExprPre(Expr *E) {
+  // Sema leaves some subexpressions null, which seems really unfortunate.  It
+  // should replace them with ErrorExpr.
+  if (E == nullptr || !E->getType() || E->getType()->is<ErrorType>()) {
+    sawError = true;
+    return { false, E };
+  }
+
+  // If this is a DeclRefExpr found in a random place, it is a load of the
+  // vardecl.
+  if (auto *DRE = dyn_cast<DeclRefExpr>(E))
+    addMark(DRE->getDecl(), RK_Read);
+
+  // If this is an AssignExpr, see if we're mutating something that we know
+  // about.
+  if (auto *assign = dyn_cast<AssignExpr>(E)) {
+    markStoredOrInOutExpr(assign->getDest(), RK_Written);
+    
+    // Don't walk into the LHS of the assignment, only the RHS.
+    assign->getSrc()->walk(*this);
+    return { false, E };
+  }
+  
+  // '&x' is a read and write of 'x'.
+  if (auto *io = dyn_cast<InOutExpr>(E)) {
+    markStoredOrInOutExpr(io->getSubExpr(), RK_Read|RK_Written);
+    // Don't bother walking into this.
+    return { false, E };
+  }
+  
+  // If we saw an ErrorExpr, take note of this.
+  if (isa<ErrorExpr>(E))
+    sawError = true;
+  
+  return { true, E };
+}
+
+
+
+/// Perform diagnostics for func/init/deinit declarations.
+void swift::performAbstractFuncDeclDiagnostics(TypeChecker &TC,
+                                               AbstractFunctionDecl *AFD) {
+  assert(AFD->getBody() && "Need a body to check");
+  
+  // Don't produce these diagnostics for implicitly generated code.
+  if (AFD->getLoc().isInvalid() || AFD->isImplicit() || AFD->isInvalid())
+    return;
+  
+  // Check for unused variables, as well as variables that are could be
+  // declared as constants.
+  AFD->getBody()->walk(VarDeclUsageChecker(TC, AFD));
+}
+
+
+
+
 //===--------------------------------------------------------------------===//
 // Utility functions
 //===--------------------------------------------------------------------===//

lib/Sema/MiscDiagnostics.h

@@ -16,6 +16,7 @@
 #include "swift/AST/Attr.h"
 
 namespace swift {
+  class AbstractFunctionDecl;
   class DeclContext;
   class Expr;
   class InFlightDiagnostic;
@@ -29,6 +30,9 @@ void performExprDiagnostics(TypeChecker &TC, const Expr *E,
 /// \brief Emit diagnostics for a given statement.
 void performStmtDiagnostics(TypeChecker &TC, const Stmt *S);
 
+void performAbstractFuncDeclDiagnostics(TypeChecker &TC,
+                                        AbstractFunctionDecl *AFD);
+  
 /// Emit a fix-it to set the accessibility of \p VD to \p desiredAccess.
 ///
 /// This actually updates \p VD as well.

lib/Sema/TypeCheckStmt.cpp

@@ -1120,7 +1120,11 @@ bool TypeChecker::typeCheckAbstractFunctionBody(AbstractFunctionDecl *AFD) {
   if (DebugTimeFunctionBodies)
     timer.emplace(AFD);
 
-  return typeCheckAbstractFunctionBodyUntil(AFD, SourceLoc());
+  if (typeCheckAbstractFunctionBodyUntil(AFD, SourceLoc()))
+    return true;
+  
+  performAbstractFuncDeclDiagnostics(*this, AFD);
+  return false;
 }
 
 // Type check a function body (defined with the func keyword) that is either a

test/Parse/switch.swift

@@ -28,7 +28,7 @@ func parseError4(x: Int) {
 
 func parseError5(x: Int) {
   switch x {
-  case var z // expected-error {{expected ':' after 'case'}}
+  case let z // expected-error {{expected ':' after 'case'}} expected-warning {{immutable value 'z' was never used}}
   }
 }
 

test/SILPasses/definite_init_diagnostics.swift

@@ -12,6 +12,7 @@ func markUsed<T>(t: T) {}
 // memory promotion pass.
 
 func test1() -> Int {
+  // expected-warning @+1 {{variable 'a' was never mutated; consider changing to 'let' constant}}
   var a : Int  // expected-note {{variable defined here}}
   return a     // expected-error {{variable 'a' used before being initialized}}
 }
@@ -54,6 +55,7 @@ func test2() {
 
   // Closures.
 
+  // expected-warning @+1 {{variable 'b1' was never mutated}}
   var b1 : Int        // expected-note {{variable defined here}}
   takes_closure {     // expected-error {{variable 'b1' used before being initialized}}
     markUsed(b1)
@@ -63,6 +65,7 @@ func test2() {
   takes_closure {     // ok.
     markUsed(b2)
   }
+  b2 = 1
 
   var b3 : Int
   b3 = 4
@@ -73,17 +76,19 @@ func test2() {
   // Structs
   var s1 : SomeStruct
   s1 = SomeStruct()   // ok
+  _ = s1
 
   var s2 : SomeStruct  // expected-note {{variable defined here}}
   s2.x = 1             // expected-error {{struct 's2' must be completely initialized before a member is stored to}}
 
 
   // Classes
+  // expected-warning @+1 {{variable 'c1' was never mutated}}
   var c1 : SomeClass   // expected-note {{variable defined here}}
   markUsed(c1.x)          // expected-error {{variable 'c1' used before being initialized}}
 
 
-  var c2 = SomeClass()
+  let c2 = SomeClass()
   markUsed(c2.x)          // ok
   
   
@@ -97,10 +102,11 @@ func test2() {
   
   // Unowned.  This is immediately crashing code (it causes a retain of a
   // released object) so it should be diagnosed with a warning someday.
+  // expected-warning @+1 {{variable 'u1' was never mutated; consider changing to 'let' constant}}
   unowned var u1 : SomeClass // expected-note {{variable defined here}}
   var _ = u1                // expected-error {{variable 'u1' used before being initialized}}
 
-  unowned var u2 = SomeClass()
+  unowned let u2 = SomeClass()
   var _ = u2                // ok
 }
 
@@ -108,10 +114,12 @@ func test2() {
 
 // Tuple field sensitivity.
 func test4() {
+  // expected-warning @+1 {{variable 't1' was never mutated; consider changing to 'let' constant}}
   var t1 = (1, 2, 3)
   markUsed(t1.0 + t1.1 + t1.2)  // ok
 
 
+  // expected-warning @+1 {{variable 't2' was never mutated; consider changing to 'let' constant}}
   var t2 : (Int, Int, Int)   // expected-note 3 {{variable defined here}}
   markUsed(t2.0)   // expected-error {{variable 't2.0' used before being initialized}}
   markUsed(t2.1)   // expected-error {{variable 't2.1' used before being initialized}}
@@ -156,7 +164,7 @@ func test5<T>(x: T, y: T) {
   var a : ((T, T), T)  // expected-note {{variable defined here}}
   a.0 = (x, y)
   
-  var b = a     // expected-error {{variable 'a.1' used before being initialized}}
+  _ = a     // expected-error {{variable 'a.1' used before being initialized}}
 }
 
 
@@ -190,24 +198,32 @@ protocol SomeProtocol {
 protocol DerivedProtocol : SomeProtocol {}
 
 func existentials(i: Int, dp: DerivedProtocol) {
+  // expected-warning @+1 {{variable 'a' was written to, but never read}}
   var a : Any = ()
   a = i
 
+  // expected-warning @+1 {{variable 'b' was written to, but never read}}
   var b : Any
   b = ()
 
+  // expected-warning @+1 {{variable 'c' was never used}}
   var c : Any   // no uses.
   
+  // expected-warning @+1 {{variable 'd1' was never mutated}}
   var d1 : Any  // expected-note {{variable defined here}}
-  var d2 = d1   // expected-error {{variable 'd1' used before being initialized}}
+  _ = d1   // expected-error {{variable 'd1' used before being initialized}}
   
+
+  // expected-warning @+1 {{variable 'e' was never mutated}}
   var e : SomeProtocol  // expected-note {{variable defined here}}
   e.protoMe()           // expected-error {{variable 'e' used before being initialized}}
   
   var f : SomeProtocol = dp  // ok, init'd by existential upcast.
   
+  // expected-warning @+1 {{variable 'g' was never mutated}}
   var g : DerivedProtocol   // expected-note {{variable defined here}}
   f = g                     // expected-error {{variable 'g' used before being initialized}}
+  _ = f
 }
 
 
@@ -235,9 +251,11 @@ struct EmptyStruct {}
 func useEmptyStruct(a: EmptyStruct) {}
 
 func emptyStructTest() {
-  var a : EmptyStruct  // expected-note {{variable defined here}}
+  let a : EmptyStruct  // expected-note {{variable defined here}}
   useEmptyStruct(a)    // expected-error {{variable 'a' used before being initialized}}
 
+  // expected-warning @+2 {{variable 'b' was never mutated}}
+  // expected-warning @+1 {{variable 'd' was never mutated}}
   var (b,c,d) : (EmptyStruct,EmptyStruct,EmptyStruct) // expected-note 2 {{variable defined here}}
   
   c = EmptyStruct()
@@ -271,6 +289,7 @@ func partialInit() {
       var x : Int
       if (n > 2) { continue }
       x = 1
+      _ = x
     }
   }
   
@@ -278,6 +297,7 @@ func partialInit() {
   func trivial_tuple() {
     var a : (Int, Int)
     a.1 = 1
+    _ = a.1
   }
 
 
@@ -288,6 +308,7 @@ func partialInit() {
       x.0 = SomeClass()
     } else {
       x.1 = SomeClass()
+      _ = x.1
     }
   }
 }
@@ -305,6 +326,7 @@ func conditionalInitOrAssign(c : Bool, x : Int) {
     t = x
   }
   t = 2
+  _ = t
   
   // Nontrivial type
   var sc : SomeClass
@@ -312,6 +334,7 @@ func conditionalInitOrAssign(c : Bool, x : Int) {
     sc = SomeClass()
   }
   sc = SomeClass()
+  _ = sc
   
   // Tuple element types
   var tt : (SomeClass, SomeClass)
@@ -354,12 +377,12 @@ func tuple_test() -> Int {
 
   t.1 = 4
 
-  for i in 0..<45 {
+  for _ in 0..<45 {
   }
 
   t.0 = 1
 
-  for i in 0..<45 {
+  for _ in 0..<45 {
   }
   
   return t.1+t.0  // No diagnostic, everything is fully initialized.
@@ -443,7 +466,7 @@ class DelegatingCtorClass {
 
   convenience init(x: EmptyStruct) {
     self.init()
-    var tmp = ivar // okay: ivar has been initialized by the delegation above
+    _ = ivar // okay: ivar has been initialized by the delegation above
   }
   
   convenience init(x: EmptyStruct, y: EmptyStruct) {
@@ -480,11 +503,11 @@ struct DelegatingCtorStruct {
 
   init(a : Double) {
     self.init()
-    var tmp = ivar // okay: ivar has been initialized by the delegation above
+    _ = ivar // okay: ivar has been initialized by the delegation above
   }
   
   init(a : Int) {
-    var tmp = ivar // expected-error {{use of 'self' in delegating initializer before self.init is called}}
+    _ = ivar // expected-error {{use of 'self' in delegating initializer before self.init is called}}
     self.init()
   }
 
@@ -511,12 +534,12 @@ enum DelegatingCtorEnum {
 
   init(a : Double) {
     self.init()
-    var tmp = self // okay: self has been initialized by the delegation above
+    _ = self // okay: self has been initialized by the delegation above
     self = .Dinosaur
   }
   
   init(a : Int) {
-    var tmp = self // expected-error {{use of 'self' in delegating initializer before self.init is called}}
+    _ = self // expected-error {{use of 'self' in delegating initializer before self.init is called}}
     self.init()
   }
 
@@ -577,7 +600,7 @@ class rdar16119509_Derived : rdar16119509_Base {
 // <rdar://problem/16797372> Bogus error: self.init called multiple times in initializer
 extension Foo {
   convenience init() {
-    for i in 0..<42 {
+    for _ in 0..<42 {
     }
     self.init(a: 4)
   }
@@ -928,7 +951,7 @@ struct MyMutabilityImplementation : TestMutabilityProtocol {
 func testLocalProperties(b : Int) -> Int {
   // expected-note @+1 {{change 'let' to 'var' to make it mutable}}
   let x : Int
-  let y : Int // never assigned is ok
+  let y : Int // never assigned is ok    expected-warning {{immutable value 'y' was never used}}
 
   x = b
   x = b    // expected-error {{immutable value 'x' may only be initialized once}}
@@ -941,6 +964,7 @@ func testLocalProperties(b : Int) -> Int {
     z = b
   }
 
+  _ = z
   return x
 }
 
@@ -949,7 +973,7 @@ func testAddressOnlyProperty<T>(b : T) -> T {
   // expected-note @+1 {{change 'let' to 'var' to make it mutable}}
   let x : T
   let y : T
-  let z : T   // never assigned is ok.
+  let z : T   // never assigned is ok.  expected-warning {{immutable value 'z' was never used}}
   x = b
   y = b
   x = b   // expected-error {{immutable value 'x' may only be initialized once}}
@@ -1023,7 +1047,7 @@ class MyClassTestExample {
 
   init(){
     clientFormat = MyClassWithAnInt()
-    let channels = clientFormat.channelCount
+    let _ = clientFormat.channelCount
   }
 }
 
@@ -1101,6 +1125,7 @@ func testReassignment() {
   let c : Int  // expected-note {{change 'let' to 'var' to make it mutable}}
   c = 12
   c = 32  // expected-error {{immutable value 'c' may only be initialized once}}
+  _ = c
 }
 
 

test/attr/attr_autoclosure.swift

@@ -12,7 +12,7 @@ func func2(@autoclosure fp : () -> Int) { func2(4)}
 
 func func3(@autoclosure fp fpx : () -> Int) {func3(fp: 0)}
 func func4(@autoclosure fp fp : () -> Int) {func4(fp: 0)}
-func func5(@autoclosure var fp fp : () -> Int) {func5(fp: 0)}
+func func5(@autoclosure var fp fp : () -> Int) {func5(fp: 0)} // expected-warning {{parameter 'fp' was never mutated}}
 func func6(@autoclosure () -> Int) {func6(0)}
 
 // declattr and typeattr on the argument.

test/decl/class/override.swift

@@ -144,12 +144,13 @@ class H : G {
 
   func oneA(_: AnyObject) {}
   func oneB(x x: AnyObject) {}
-  func oneC(var x x: AnyObject) {}
+  func oneC(var x x: AnyObject) {} // expected-warning {{parameter 'x' was never mutated}}
   func oneD(x: AnyObject) {}
 
   func manyA(_: AnyObject, _: AnyObject) {}
   func manyB(a: AnyObject, b: AnyObject) {}
-  func manyC(var a: AnyObject, var b: AnyObject) {}
+  func manyC(var a: AnyObject,  // expected-warning {{parameter 'a' was never mutated}}
+             var b: AnyObject) {} // expected-warning {{parameter 'b' was never mutated}}
 
   func result() -> AnyObject? { return nil }
   func both(x: AnyObject) -> AnyObject? { return x }
@@ -166,7 +167,7 @@ class IUOTestSubclass : IUOTestBaseClass {
   override func oneB(x x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
   // expected-note@-1 {{remove '!' to make the parameter required}} {{36-37=}}
   // expected-note@-2 {{add parentheses to silence this warning}} {{27-27=(}} {{37-37=)}}
-  override func oneC(var x x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
+  override func oneC(var x x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}} expected-warning {{parameter 'x' was never mutated}}
   // expected-note@-1 {{remove '!' to make the parameter required}} {{40-41=}}
   // expected-note@-2 {{add parentheses to silence this warning}} {{31-31=(}} {{41-41=)}}
   override func oneD(x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
@@ -179,7 +180,7 @@ class IUOTestSubclass : IUOTestBaseClass {
   override func manyB(a: AnyObject!, b: AnyObject!) {} // expected-warning 2 {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
   // expected-note@-1 2 {{remove '!' to make the parameter required}}
   // expected-note@-2 2 {{add parentheses to silence this warning}}
-  override func manyC(var a: AnyObject!, var b: AnyObject!) {} // expected-warning 2 {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
+  override func manyC(var a: AnyObject!, var b: AnyObject!) {} // expected-warning 2 {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}} expected-warning {{parameter 'a' was never mutated}} expected-warning {{parameter 'b' was never mutated}}
   // expected-note@-1 2 {{remove '!' to make the parameter required}}
   // expected-note@-2 2 {{add parentheses to silence this warning}}
 
@@ -205,7 +206,7 @@ class IUOTestSubclass2 : IUOTestBaseClass {
   override func oneA(x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
   // expected-note@-1 {{remove '!' to make the parameter required}} {{34-35=}}
   // expected-note@-2 {{add parentheses to silence this warning}} {{25-25=(}} {{35-35=)}}
-  override func oneB(var x x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}
+  override func oneB(var x x: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}} expected-warning {{parameter 'x' was never mutated}}
   // expected-note@-1 {{remove '!' to make the parameter required}} {{40-41=}}
   // expected-note@-2 {{add parentheses to silence this warning}} {{31-31=(}} {{41-41=)}}
   override func oneD(_: AnyObject!) {} // expected-warning {{overriding instance method parameter of type 'AnyObject' with implicitly unwrapped optional type 'AnyObject!'}}

test/decl/func/functions.swift

@@ -134,7 +134,7 @@ func !!!<T>(lhs: UnsafePointer<T>, rhs: UnsafePointer<T>) -> Bool { return false
 
 // <rdar://problem/16786168> Functions currently permit 'var inout' parameters
 func var_inout_error(inout var x : Int) {} // expected-error {{parameter may not have multiple 'inout', 'var', or 'let' specifiers}}
-func var_inout_error(var inout x : Int) {} // expected-error {{parameter may not have multiple 'inout', 'var', or 'let' specifiers}}
+func var_inout_error(var inout x : Int) {} // expected-error {{parameter may not have multiple 'inout', 'var', or 'let' specifiers}} expected-warning {{parameter 'x' was never mutated}}
 
 
 

test/decl/func/trailing_closures.swift

@@ -5,7 +5,7 @@
 func f1(f: () -> (), bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
 func f2(f: (() -> ())!, bar: Int = 10, wibble: Int = 17) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
 func f3(f: (() -> ())?, bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
-func f4(var f: (() -> ())?, bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}}
+func f4(var f: (() -> ())?, bar: Int = 10) { } // expected-warning{{closure parameter prior to parameters with default arguments will not be treated as a trailing closure}} expected-warning {{parameter 'f' was never mutated}}
 
 // An extra set of parentheses suppresses the warning.
 func g1(f: (() -> ()), bar: Int = 10) { } // no-warning

test/decl/var/properties.swift

@@ -113,7 +113,7 @@ var x15: Int {
   // For the purpose of this test we need to use an attribute that can not be
   // applied to the getter.
   weak
-  var foo: SomeClass? = SomeClass()
+  var foo: SomeClass? = SomeClass()  // expected-warning {{variable 'foo' was written to, but never read}}
   return 0
 }
 
@@ -179,7 +179,7 @@ func disambiguateGetSet3Attr() {
 // Disambiguated as stored property with a trailing closure in the initializer.
 func disambiguateGetSet4() {
   func set(x: Int, fn: () -> ()) {}
-  var newValue: Int = 0
+  let newValue: Int = 0
   var a: Int = takeTrailingClosure {
     set(newValue) {}
   }
@@ -425,7 +425,7 @@ extension ProtocolWithExtension1 {
 }
 
 func getS() -> S {
-  var s: S
+  let s: S
   return s
 }
 
@@ -491,7 +491,8 @@ func accept_int(c: Int) { }
 
 func test_settable_of_nonsettable(a: Aleph) {
   a.b.c = 1 // expected-error{{cannot assign}}
-  var x:Int = a.b.c
+  let x:Int = a.b.c
+  _ = x
 
   accept_int(a.b.c)
   accept_int_inout(&a.b.c) // expected-error {{cannot pass immutable value of type 'Int' as inout argument}}
@@ -597,7 +598,7 @@ class SelfRefProperties {
     }
     set {
       markUsed(setter) // no-warning
-      var unused = setter + setter
+      var unused = setter + setter // expected-warning {{initialization of variable 'unused' was never used; consider replacing with assignment to '_' or removing it}}
       setter = newValue // expected-warning {{attempting to modify 'setter' within its own setter}}
       // expected-note@-1 {{access 'self' explicitly to silence this warning}} {{7-7=self.}}
     }

test/decl/var/usage.swift

@@ -0,0 +1,135 @@
+// RUN: %target-parse-verify-swift
+
+// <rdar://problem/20872721> QoI: warn about unused variables
+// <rdar://problem/15975935> warning that you can use 'let' not 'var'
+// <rdar://problem/18876585> Compiler should warn me if I set a parameter as 'var' but never modify it
+
+func basicTests() -> Int {
+  let x = 42 // expected-warning {{immutable value 'x' was never used; consider replacing with assignment to '_' or removing it}}
+  var y = 12 // expected-warning {{variable 'y' was never mutated; consider changing to 'let' constant}}
+  let _ = 42 // ok
+  _ = 42 // ok
+  return y
+}
+
+func mutableParameter(a : Int, h : Int, var i : Int, var j: Int,
+       var g : Int) -> Int { // expected-warning {{parameter 'g' was never mutated; consider changing to 'let' constant}}
+  swap(&i, &j)
+  return i+g
+}
+
+struct X {
+  func f() {}
+  mutating func g() {}
+}
+
+
+func testStruct() {
+  let a = X()
+  a.f()
+  
+  var b = X()
+  b.g()
+  
+  var c = X()  // expected-warning {{variable 'c' was never mutated; consider changing to 'let' constant}}
+  c.f()
+}
+
+func takeClosure(fn : () -> ()) {}
+
+class TestClass {
+
+  func f() {
+   
+    takeClosure { [weak self] in  // self is mutable but never mutated.  Ok because it is weak
+      self?.f()
+    }
+  }
+}
+
+
+func nestedFunction() -> Int {
+  var x = 42  // No warning about being never-set.
+  
+  func g() {
+    x = 97
+    var q = 27  // expected-warning {{variable 'q' was never used}}
+  }
+  g()
+  
+  return x
+}
+
+func neverRead() {
+  var x = 42  // expected-warning {{variable 'x' was written to, but never read}}
+  
+  x = 97
+  x = 123
+}
+
+func property() -> Int {
+  var p : Int {  // everything ok
+    return 42
+  }
+  return p
+}
+
+
+func testInOut(inout x : Int) {  // Ok.
+}
+
+struct TestStruct {
+  var property = 42
+}
+
+
+func testStructMember() -> TestStruct {
+  var x = TestStruct()  // ok
+  x.property = 17
+  return x
+}
+
+
+func testSubscript() -> [Int] {
+  var x = [1,2,3] // ok
+  x[1] = 27
+  return x
+}
+
+
+func testTuple(var x : Int) -> Int {
+  var y : Int  // Ok, stored by a tuple
+  
+  (x, y) = (1,2)
+  return y
+}
+  
+
+
+ struct TestComputedPropertyStruct {
+  
+  var x : Int {
+    get {}
+    nonmutating set {}
+  }
+}
+
+func test() {
+  let v = TestComputedPropertyStruct()
+  v.x = 42
+  
+  var v2 = TestComputedPropertyStruct()  // expected-warning {{variable 'v2' was never mutated; consider changing to 'let' constant}}
+  v2.x = 42
+}
+
+func test4() {
+  // expected-warning @+1 {{variable 'dest' was never mutated; consider changing to 'let' constant}}
+  var dest = UnsafeMutablePointer<Int>(bitPattern: 0)
+
+  dest[0] = 0
+}
+
+func testTuple() {
+  var tup : (x:Int, y:Int)  // expected-warning {{variable 'tup' was written to, but never read}}
+  tup.x = 1
+}

test/expr/closure/closures.swift

@@ -92,8 +92,8 @@ func closure_no_body(p: () -> ()) {
 
 // rdar://12019415
 func t() {
-  var u8 : UInt8
-  var x : Bool
+  let u8 : UInt8 = 1
+  let x : Bool = true
 
   if 0xA0..<0xBF ~= Int(u8) && x {
   }
@@ -139,9 +139,11 @@ class ExplicitSelfRequiredTest {
 
     // <rdar://problem/18877391> "self." shouldn't be required in the initializer expression in a capture list
     // This should not produce an error, "x" isn't being captured by the closure.
+    // expected-warning @+1 {{initialization of immutable value 'myX' was never used}}
     doStuff({ [myX = x] in 4 })
 
     // This should produce an error, since x is used within the inner closure.
+    // expected-warning @+1 {{initialization of immutable value 'myX' was never used}}
     doStuff({ [myX = {x}] in 4 })    // expected-error {{reference to property 'x' in closure requires explicit 'self.' to make capture semantics explicit}}
 
     return 42
@@ -161,9 +163,9 @@ class SomeClass {
 func testCaptureBehavior(ptr : SomeClass) {
   // Test normal captures.
   weak var wv : SomeClass? = ptr
-  unowned var uv : SomeClass = ptr
-  unowned(unsafe) var uv1 : SomeClass = ptr
-  unowned(safe) var uv2 : SomeClass = ptr
+  unowned let uv : SomeClass = ptr
+  unowned(unsafe) let uv1 : SomeClass = ptr
+  unowned(safe) let uv2 : SomeClass = ptr
   doStuff { wv!.foo() }
   doStuff { uv.foo() }
   doStuff { uv1.foo() }
@@ -171,10 +173,11 @@ func testCaptureBehavior(ptr : SomeClass) {
 
   
   // Capture list tests
-  var v1 : SomeClass? = ptr
-  var v2 : SomeClass = ptr
+  let v1 : SomeClass? = ptr
+  let v2 : SomeClass = ptr
 
   doStuff { [weak v1] in v1!.foo() }
+  // expected-warning @+2 {{variable 'v1' was written to, but never read}}
   doStuff { [weak v1,                 // expected-note {{previous}}
              weak v1] in v1!.foo() }  // expected-error {{definition conflicts with previous value}}
   doStuff { [unowned v2] in v2.foo() }
@@ -182,7 +185,8 @@ func testCaptureBehavior(ptr : SomeClass) {
   doStuff { [unowned(safe) v2] in v2.foo() }
   doStuff { [weak v1, weak v2] in v1!.foo() + v2!.foo() }
 
-  var i = 42
+  let i = 42
+  // expected-warning @+1 {{variable 'i' was never mutated}}
   doStuff { [weak i] in i! }   // expected-error {{'weak' cannot be applied to non-class type 'Int'}}
 }
 
@@ -194,13 +198,14 @@ extension SomeClass {
 
     // rdar://16889886 - Assert when trying to weak capture a property of self in a lazy closure
     doStuff { [weak self.field] in field!.foo() }   // expected-error {{fields may only be captured by assigning to a specific name}} expected-error {{reference to property 'field' in closure requires explicit 'self.' to make capture semantics explicit}}
+    // expected-warning @+1 {{variable 'self' was written to, but never read}}
     doStuff { [weak self&field] in 42 }  // expected-error {{expected ']' at end of capture list}}
 
   }
 
   func strong_in_capture_list() {
     // <rdar://problem/18819742> QOI: "[strong self]" in capture list generates unhelpful error message
-    var fn = {[strong self] () -> () in return }  // expected-error {{expected 'weak', 'unowned', or no specifier in capture list}}
+    var _ = {[strong self] () -> () in return }  // expected-error {{expected 'weak', 'unowned', or no specifier in capture list}}
   }
 }
 
@@ -209,10 +214,10 @@ extension SomeClass {
 var closureWithObservedProperty: () -> () = {
   var a: Int = 42 {
   willSet {
-    let message = "Will set a to \(newValue)"
+    let _ = "Will set a to \(newValue)"
   }
   didSet {
-    let message = "Did set a with old value of \(oldValue)"
+    let _ = "Did set a with old value of \(oldValue)"
   }
   }
 }

validation-test/compiler_crashers_fixed/0040-std-function-func-swift-constraints-solution-computesubstitutions.swift

@@ -5,6 +5,6 @@
 // rdar://18175202
 
 func d<b: SequenceType, e where Optional<e> == b.Generator.Element>(c : b) -> e? {
-    for mx : e? in c {
+    for mx : e? in c { // expected-warning {{immutable value 'mx' was never used}}
     }
 }