Created separated warning group for warnings about unnecessary mutable weak variables

Author: nickolas-pohilets

include/swift/AST/DiagnosticGroups.def

@@ -72,6 +72,7 @@ GROUP(TemporaryPointers, "temporary-pointers")
 GROUP(TrailingClosureMatching, "trailing-closure-matching")
 GROUP(UnknownWarningGroup, "unknown-warning-group")
 GROUP(CompilationCaching, "compilation-caching")
+GROUP(WeakMutability, "weak-mutability")
 
 #define UNDEFINE_DIAGNOSTIC_GROUPS_MACROS
 #include "swift/AST/DefineDiagnosticGroupsMacros.h"

include/swift/AST/DiagnosticsSema.def

@@ -7414,6 +7414,14 @@ WARNING(variable_tuple_elt_never_mutated, none,
         "variable %0 was never mutated; "
         "consider changing the pattern to 'case (..., let %1, ...)'",
         (Identifier, StringRef))
+GROUPED_WARNING(weak_variable_never_mutated, WeakMutability, none,
+                "weak variable %0 was never mutated; "
+                "consider %select{removing 'var' to make it|changing to 'let'}1 constant",
+                (Identifier, bool))
+GROUPED_WARNING(weak_variable_tuple_elt_never_mutated, WeakMutability, none,
+                "weak variable %0 was never mutated; "
+                "consider changing the pattern to 'case (..., let %1, ...)'",
+                (Identifier, StringRef))
 WARNING(variable_never_read, none,
         "variable %0 was written to, but never read",
         (Identifier))

lib/Sema/MiscDiagnostics.cpp

@@ -4283,6 +4283,8 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
       if (isUsedInInactive(var))
         continue;
 
+      bool isWeak = var->getInterfaceType()->is<WeakStorageType>();
+
       // If this is a parameter explicitly marked 'var', remove it.
       if (FixItLoc.isInvalid()) {
         bool suggestCaseLet = false;
@@ -4293,10 +4295,14 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
           suggestCaseLet = isa<ForEachStmt>(stmt);
         }
         if (suggestCaseLet)
-          Diags.diagnose(var->getLoc(), diag::variable_tuple_elt_never_mutated,
+          Diags.diagnose(var->getLoc(),
+                         isWeak ? diag::weak_variable_tuple_elt_never_mutated
+                                : diag::variable_tuple_elt_never_mutated,
                          var->getName(), var->getNameStr());
         else
-          Diags.diagnose(var->getLoc(), diag::variable_never_mutated,
+          Diags.diagnose(var->getLoc(),
+                         isWeak ? diag::weak_variable_never_mutated
+                                : diag::variable_never_mutated,
                          var->getName(), true);
 
       }
@@ -4309,7 +4315,9 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
           suggestLet = !isa<ForEachStmt>(stmt);
         }
 
-        auto diag = Diags.diagnose(var->getLoc(), diag::variable_never_mutated,
+        auto diag = Diags.diagnose(var->getLoc(),
+                                   isWeak ? diag::weak_variable_never_mutated
+                                          : diag::variable_never_mutated,
                                    var->getName(), suggestLet);
 
         if (suggestLet)

test/Concurrency/weak_ref_sendability.swift

@@ -98,7 +98,7 @@ final class CheckSendability12: Sendable {
 
 
 func checkWeakCapture1(_ strongRef: S) -> @Sendable () -> Void {
-  // expected-warning@+1 {{variable 'weakRef' was never mutated; consider changing to 'let' constant}}
+  // expected-warning@+1 {{weak variable 'weakRef' was never mutated; consider changing to 'let' constant}}
   weak var weakRef: S? = strongRef
   return {
     // expected-error@+1 {{reference to captured var 'weakRef' in concurrently-executing code}}
@@ -124,7 +124,7 @@ func checkWeakCapture3(_ strongRef: S) -> @Sendable () -> Void {
 }
 
 func checkWeakCapture4(_ strongRef: NS) -> @Sendable () -> Void {
-  // expected-warning@+1 {{variable 'weakRef' was never mutated; consider changing to 'let' constant}}
+  // expected-warning@+1 {{weak variable 'weakRef' was never mutated; consider changing to 'let' constant}}
   weak var weakRef: NS? = strongRef
   return {
     // expected-error@+2 {{capture of 'weakRef' with non-Sendable type 'NS?' in a '@Sendable' closure}}

test/SILOptimizer/definite_init_diagnostics.swift

@@ -103,7 +103,7 @@ func test2() {
 
 
   // Weak
-  // expected-warning@+1 {{variable 'w1' was never mutated; consider changing to 'let' constant}} {{8-11=let}}
+  // expected-warning@+1 {{weak variable 'w1' was never mutated; consider changing to 'let' constant}} {{8-11=let}}
   weak var w1 : SomeClass?
   _ = w1                // ok: default-initialized