libutil: add shared_from_this support to ref

we don't need to unsafely cast a shared_from_this pointer to create a
ref (the pointer must be non-null to not invoke nasal demons anyway).

Change-Id: I133fe5f07b2cff8ec2c925f0528f4c5288827261

lix/libexpr/eval-cache.cc

@@ -352,7 +352,7 @@ Value * EvalCache::getRootValue(EvalState & state)
 
 ref<AttrCursor> EvalCache::getRoot()
 {
-    return make_ref<AttrCursor>(ref(shared_from_this()), std::nullopt);
+    return make_ref<AttrCursor>(*this, std::nullopt);
 }
 
 AttrCursor::AttrCursor(

lix/libstore/binary-cache-store.cc

@@ -530,7 +530,7 @@ try {
 
 ref<FSAccessor> BinaryCacheStore::getFSAccessor()
 {
-    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()), config().localNarCache);
+    return make_ref<RemoteFSAccessor>(*this, config().localNarCache);
 }
 
 kj::Promise<Result<void>>

lix/libstore/remote-store.cc

@@ -1022,7 +1022,7 @@ try {
 
 ref<FSAccessor> RemoteStore::getFSAccessor()
 {
-    return make_ref<RemoteFSAccessor>(ref<Store>(shared_from_this()));
+    return make_ref<RemoteFSAccessor>(*this);
 }
 
 static Logger::Fields readFields(Source & from)

lix/libutil/ref.hh

@@ -32,6 +32,11 @@ public:
             throw std::invalid_argument("null pointer cast to ref");
     }
 
+    template<std::derived_from<std::enable_shared_from_this<T>> T2>
+    ref(T2 & r) : p(r.shared_from_this())
+    {
+    }
+
     T* operator ->() const
     {
         return &*p;