Enable warnings about never mutated weak var.

Author: nickolas-pohilets

lib/Sema/MiscDiagnostics.cpp

@@ -4000,7 +4000,7 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
 
     // If this variable has WeakStorageType, then it can be mutated in ways we
     // don't know.
-    if (var->getInterfaceType()->is<WeakStorageType>())
+    if (var->getInterfaceType()->is<WeakStorageType>() && !DC->getASTContext().LangOpts.hasFeature(Feature::WeakLet))
       access |= RK_Written;
 
     // Diagnose variables that were never used (other than their

test/Concurrency/weak_ref_sendability.swift

@@ -110,6 +110,7 @@ final class CheckSendability12: Sendable {
 
 
 func checkWeakCapture1(_ strongRef: S) -> @Sendable () -> Void {
+  // expected-new-warning@+1 {{variable 'weakRef' was never mutated; consider changing to 'let' constant}}
   weak var weakRef: S? = strongRef
   return {
     // expected-old-error@+2 {{reference to captured var 'weakRef' in concurrently-executing code}}
@@ -137,7 +138,7 @@ func checkWeakCapture3(_ strongRef: S) -> @Sendable () -> Void {
 }
 
 func checkWeakCapture4(_ strongRef: NS) -> @Sendable () -> Void {
-  // TODO: warning consider changing to 'let' constant
+  // expected-new-warning@+1 {{variable 'weakRef' was never mutated; consider changing to 'let' constant}}
   weak var weakRef: NS? = strongRef
   return {
     // expected-old-error@+4 {{capture of 'weakRef' with non-sendable type 'NS?' in a '@Sendable' closure}}