[Diagnostics] Improve warning suggestion for var in for loop

When iterator consists of tuple of variable and iteration only mutates
the tuple partially, improve the warning message from "changing to 'let"
to "changing the pattern to '(..., case let, ...)"

Author: pwongxy

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
+  }
+}