[Diagnostics] Improve warning suggestion for var in for loop

include/swift/AST/DiagnosticsSema.def

@@ -7032,6 +7032,10 @@ WARNING(variable_never_mutated, none,
         "variable %0 was never mutated; "
         "consider %select{removing 'var' to make it|changing to 'let'}1 constant",
         (Identifier, bool))
+WARNING(variable_tuple_elt_never_mutated, none,
+        "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

@@ -4102,8 +4102,20 @@ VarDeclUsageChecker::~VarDeclUsageChecker() {
 
       // If this is a parameter explicitly marked 'var', remove it.
       if (FixItLoc.isInvalid()) {
-        Diags.diagnose(var->getLoc(), diag::variable_never_mutated,
-                       var->getName(), true);
+        bool suggestCaseLet = false;
+        if (auto *stmt = var->getRecursiveParentPatternStmt()) {
+          // Suggest 'var' -> 'case let' conversion
+          // in case of 'for' loop and invalid because it's
+          // tuple variable.
+          suggestCaseLet = isa<ForEachStmt>(stmt);
+        }
+        if (suggestCaseLet)
+          Diags.diagnose(var->getLoc(), diag::variable_tuple_elt_never_mutated,
+                         var->getName(), var->getNameStr());
+        else
+          Diags.diagnose(var->getLoc(), diag::variable_never_mutated,
+                         var->getName(), true);
+
       }
       else {
         bool suggestLet = true;

test/decl/var/usage.swift

@@ -563,3 +563,11 @@ func testUselessCastWithInvalidParam(foo: Any?) -> Int {
   if let bar = foo as? Foo { return 42 } // expected-warning {{value 'bar' was defined but never used; consider replacing with boolean test}} {{6-16=}} {{20-23=is}}
   else { return 54 }
 }
+
+// https://github.com/swiftlang/swift/issues/72811
+func testEnumeratedForLoop(a: [Int]) {
+  for var (b, c) in a.enumerated() {  // expected-warning {{variable 'b' was never mutated; consider changing the pattern to 'case (..., let b, ...)'}}
+    c = b
+    let _ = c
+  }
+}