[FixCode] Help users construct mutable pointer from a pointer

include/swift/AST/DiagnosticsSema.def

@@ -204,6 +204,8 @@ ERROR(cannot_call_with_params, none,
       "cannot invoke %select{|initializer for type }2'%0' with an argument list"
       " of type '%1'", (StringRef, StringRef, bool))
 
+NOTE(pointer_init_add_mutating,none,
+     "do you want to add 'mutating'", ())
 ERROR(expected_do_in_statement,none,
       "expected 'do' keyword to designate a block of statements", ())
 

lib/Sema/CSDiag.cpp

@@ -4824,6 +4824,25 @@ bool FailureDiagnosis::diagnoseNilLiteralComparison(
   return true;
 }
 
+static bool shouldAddMutating(ASTContext &Ctx, const Expr *Fn,
+                              const Expr* Arg) {
+  auto *TypeExp = dyn_cast<TypeExpr>(Fn);
+  auto *ParenExp = dyn_cast<ParenExpr>(Arg);
+  if (!TypeExp || !ParenExp)
+    return false;
+  auto InitType = TypeExp->getInstanceType();
+  auto ArgType = ParenExp->getSubExpr()->getType();
+  if (InitType.isNull() || ArgType.isNull())
+    return false;
+  if (auto *InitNom = InitType->getAnyNominal()) {
+    if (auto *ArgNom = ArgType->getAnyNominal()) {
+      return InitNom == Ctx.getUnsafeMutablePointerDecl() &&
+        ArgNom == Ctx.getUnsafePointerDecl();
+    }
+  }
+  return false;
+}
+
 bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
   // Type check the function subexpression to resolve a type for it if possible.
   auto fnExpr = typeCheckChildIndependently(callExpr->getFn());
@@ -5138,6 +5157,12 @@ bool FailureDiagnosis::visitApplyExpr(ApplyExpr *callExpr) {
     diagnose(fnExpr->getLoc(), diag::cannot_call_with_params,
              overloadName, argString, isInitializer);
   }
+
+
+  if (shouldAddMutating(CS->DC->getASTContext(), fnExpr, argExpr)) {
+    diagnose(fnExpr->getLoc(), diag::pointer_init_add_mutating).fixItInsert(
+      dyn_cast<ParenExpr>(argExpr)->getSubExpr()->getStartLoc(), "mutating: ");
+  }
 
   // Did the user intend on invoking a different overload?
   calleeInfo.suggestPotentialOverloads(fnExpr->getLoc());

test/Sema/diag_init.swift

@@ -0,0 +1,5 @@
+// RUN: %target-parse-verify-swift
+
+func foo(a :  UnsafePointer<Void>)->UnsafeMutablePointer<Void> {
+  return UnsafeMutablePointer(a) // expected-error {{cannot invoke initializer for type 'UnsafeMutablePointer<_>' with an argument list of type '(UnsafePointer<Void>)'}} // expected-note {{do you want to add 'mutating'}}{{31-31=mutating: }} // expected-note {{overloads for 'UnsafeMutablePointer<_>' exist with these partially matching parameter lists: (RawPointer), (OpaquePointer), (OpaquePointer?), (UnsafeMutablePointer<Pointee>), (UnsafeMutablePointer<Pointee>?)}}
+}