[Diagnostics] Warn when the result of a Void-returning function is ignored (by assigning into '_') (swiftlang#29576)

Author: theblixguy

include/swift/AST/DiagnosticsSema.def

@@ -3193,6 +3193,9 @@ ERROR(object_literal_broken_proto,none,
 ERROR(discard_expr_outside_of_assignment,none,
       "'_' can only appear in a pattern or on the left side of an assignment",
       ())
+WARNING(discard_expr_void_result_redundant, none,
+        "using '_' to ignore the result of a Void-returning function "
+        "is redundant", ())
 ERROR(collection_literal_heterogeneous,none,
       "heterogeneous collection literal could only be inferred to %0; add"
       " explicit type annotation if this is intentional", (Type))

lib/Sema/MiscDiagnostics.cpp

@@ -221,8 +221,24 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
       }
 
       // If we have an assignment expression, scout ahead for acceptable _'s.
-      if (auto *AE = dyn_cast<AssignExpr>(E))
-        markAcceptableDiscardExprs(AE->getDest());
+      if (auto *AE = dyn_cast<AssignExpr>(E)) {
+        auto destExpr = AE->getDest();
+        markAcceptableDiscardExprs(destExpr);
+        // If the user is assigning the result of a function that returns
+        // Void to _ then warn, because that is redundant.
+        if (auto DAE = dyn_cast<DiscardAssignmentExpr>(destExpr)) {
+          if (auto CE = dyn_cast<CallExpr>(AE->getSrc())) {
+            if (CE->getCalledValue() && isa<FuncDecl>(CE->getCalledValue()) &&
+                CE->getType()->isVoid()) {
+              Ctx.Diags
+                  .diagnose(DAE->getLoc(),
+                            diag::discard_expr_void_result_redundant)
+                  .fixItRemoveChars(DAE->getStartLoc(),
+                                    AE->getSrc()->getStartLoc());
+            }
+          }
+        }
+      }
 
       /// Diagnose a '_' that isn't on the immediate LHS of an assignment.
       if (auto *DAE = dyn_cast<DiscardAssignmentExpr>(E)) {
@@ -409,7 +425,7 @@ static void diagSyntacticUseRestrictions(const Expr *E, const DeclContext *DC,
     /// in simple pattern-like expressions, so we reject anything complex here.
     void markAcceptableDiscardExprs(Expr *E) {
       if (!E) return;
-      
+
       if (auto *PE = dyn_cast<ParenExpr>(E))
         return markAcceptableDiscardExprs(PE->getSubExpr());
       if (auto *TE = dyn_cast<TupleExpr>(E)) {

test/ClangImporter/MixedSource/submodule.swift

@@ -4,6 +4,6 @@
 @_exported import Mixed.Submodule
 
 func test() {
-  _ = topLevel()
-  _ = fromSubmodule()
+  topLevel()
+  fromSubmodule()
 }

test/ClangImporter/private_frameworks.swift

@@ -39,8 +39,8 @@
 import SomeKit
 
 func testWidget(widget: SKWidget) {
-  _ = widget.someObjCMethod()
-  _ = widget.someObjCExtensionMethod()
+  widget.someObjCMethod()
+  widget.someObjCExtensionMethod()
 
   let ext = widget.extensionMethod()
   ext.foo()

test/Compatibility/special_func_name.swift

@@ -34,16 +34,16 @@ struct S3 {
 }
 
 _ = S11(0) // expected-error {{argument passed to call that takes no arguments}}
-_ = S11.init(0)
-_ = S11.`init`(0)
+S11.init(0)
+S11.`init`(0)
 
 _ = S12(0)
 _ = S12.init(0)
 _ = S12.`init`(0) // expected-error {{type 'S12' has no member 'init'}}
 
 _ = S21(0) // expected-error {{argument passed to call that takes no arguments}}
-_ = S21.init(0)
-_ = S21.`init`(0)
+S21.init(0)
+S21.`init`(0)
 
 _ = S22(0)
 _ = S22.init(0)

test/Constraints/assignment.swift

@@ -72,3 +72,8 @@ func f23798944() {
 }
 
 .sr_3506 = 0 // expected-error {{type 'Int' has no member 'sr_3506'}}
+
+// SR-1553
+
+func returnsVoid() {}
+_ = returnsVoid() // expected-warning {{using '_' to ignore the result of a Void-returning function is redundant}}{{1-5=}}

test/Constraints/dynamic_lookup.swift

@@ -224,7 +224,7 @@ let anyValue: Any = X()
 _ = anyValue.bar() // expected-error {{value of type 'Any' has no member 'bar'}}
 // expected-note@-1 {{cast 'Any' to 'AnyObject' or use 'as!' to force downcast to a more specific type to access members}}{{5-5=(}}{{13-13= as AnyObject)}}
 _ = (anyValue as AnyObject).bar()
-_ = (anyValue as! X).bar()
+(anyValue as! X).bar()
 
 var anyDict: [String : Any] = Dictionary<String, Any>()
 anyDict["test"] = anyValue

test/Constraints/optional.swift

@@ -391,8 +391,8 @@ func sr8411() {
 
   _ = S(&foo)      // Ok
   _ = S.init(&foo) // Ok
-  _ = S.foo(&foo)  // Ok
-  _ = S.bar(&foo, 42) // Ok
+  S.foo(&foo)  // Ok
+  S.bar(&foo, 42) // Ok
 }
 
 // SR-11104 - Slightly misleading diagnostics for contextual failures with multiple fixes

test/Constraints/override.swift

@@ -9,8 +9,8 @@ class Sub: Base {
 }
 
 func removeOverrides<SomeSub: Sub>(concrete: Sub, generic: SomeSub) {
-  _ = concrete.foo()
-  _ = generic.foo()
+  concrete.foo()
+  generic.foo()
 }
 
 class Base1 {

test/Constraints/rdar39931475.swift

@@ -5,5 +5,5 @@ protocol P {
 }
 
 func foo<T: P>(_ bar: T) {
-  _ = bar.b { a in Double((a, a += 1).0) }
+  bar.b { a in Double((a, a += 1).0) }
 }