[netif] simplify ThreadNetif (#8882)

This commit simplifies `ThreadNetif` class by removing methods
like `IsOnMesh()` and `SendMessage()` which just call same method
on other modules (`NetworkData::Leader` and `MeshForwarder`). It
also moves the `RouteLookup()` method to `Ip6` class.

src/core/net/ip6.cpp

@@ -1192,7 +1192,7 @@ start:
                 forwardThread = true;
 #endif
             }
-            else if (Get<ThreadNetif>().RouteLookup(header.GetSource(), header.GetDestination()) == kErrorNone)
+            else if (RouteLookup(header.GetSource(), header.GetDestination()) == kErrorNone)
             {
                 forwardThread = true;
             }
@@ -1310,7 +1310,7 @@ start:
 #endif
 
         // `SendMessage()` takes custody of message in the success case
-        SuccessOrExit(error = Get<ThreadNetif>().SendMessage(aMessage));
+        SuccessOrExit(error = Get<MeshForwarder>().SendMessage(aMessage));
         shouldFreeMessage = false;
     }
 
@@ -1442,23 +1442,39 @@ exit:
 
 bool Ip6::IsOnLink(const Address &aAddress) const
 {
-    bool rval = false;
+    bool isOnLink = false;
 
-    if (Get<ThreadNetif>().IsOnMesh(aAddress))
+    if (Get<NetworkData::Leader>().IsOnMesh(aAddress))
     {
-        ExitNow(rval = true);
+        ExitNow(isOnLink = true);
     }
 
-    for (const Netif::UnicastAddress &cur : Get<ThreadNetif>().GetUnicastAddresses())
+    for (const Netif::UnicastAddress &unicastAddr : Get<ThreadNetif>().GetUnicastAddresses())
     {
-        if (cur.GetAddress().PrefixMatch(aAddress) >= cur.mPrefixLength)
+        if (unicastAddr.GetAddress().PrefixMatch(aAddress) >= unicastAddr.mPrefixLength)
         {
-            ExitNow(rval = true);
+            ExitNow(isOnLink = true);
         }
     }
 
 exit:
-    return rval;
+    return isOnLink;
+}
+
+Error Ip6::RouteLookup(const Address &aSource, const Address &aDestination) const
+{
+    Error    error;
+    uint16_t rloc;
+
+    SuccessOrExit(error = Get<NetworkData::Leader>().RouteLookup(aSource, aDestination, rloc));
+
+    if (rloc == Get<Mle::Mle>().GetRloc16())
+    {
+        error = kErrorNoRoute;
+    }
+
+exit:
+    return error;
 }
 
 #if OPENTHREAD_CONFIG_IP6_BR_COUNTERS_ENABLE

src/core/net/ip6.hpp

@@ -409,6 +409,7 @@ private:
                         uint8_t            aIpProto,
                         Message::Ownership aMessageOwnership);
     bool  IsOnLink(const Address &aAddress) const;
+    Error RouteLookup(const Address &aSource, const Address &aDestination) const;
 #if OPENTHREAD_CONFIG_IP6_BR_COUNTERS_ENABLE
     void UpdateBorderRoutingCounters(const Header &aHeader, uint16_t aMessageLength, bool aIsInbound);
 #endif

src/core/thread/child_supervision.cpp

@@ -83,7 +83,7 @@ void ChildSupervisor::SendMessage(Child &aChild)
     childIndex = Get<ChildTable>().GetChildIndex(aChild);
     SuccessOrExit(message->Append(childIndex));
 
-    SuccessOrExit(Get<ThreadNetif>().SendMessage(*message));
+    SuccessOrExit(Get<MeshForwarder>().SendMessage(*message));
     message = nullptr;
 
     LogInfo("Sending supervision message to child 0x%04x", aChild.GetRloc16());

src/core/thread/thread_netif.cpp

@@ -118,24 +118,4 @@ exit:
     return;
 }
 
-Error ThreadNetif::SendMessage(Message &aMessage) { return Get<MeshForwarder>().SendMessage(aMessage); }
-
-Error ThreadNetif::RouteLookup(const Ip6::Address &aSource, const Ip6::Address &aDestination)
-{
-    Error    error;
-    uint16_t rloc;
-
-    SuccessOrExit(error = Get<NetworkData::Leader>().RouteLookup(aSource, aDestination, rloc));
-
-    if (rloc == Get<Mle::MleRouter>().GetRloc16())
-    {
-        error = kErrorNoRoute;
-    }
-
-exit:
-    return error;
-}
-
-bool ThreadNetif::IsOnMesh(const Ip6::Address &aAddress) const { return Get<NetworkData::Leader>().IsOnMesh(aAddress); }
-
 } // namespace ot

src/core/thread/thread_netif.hpp

@@ -82,39 +82,6 @@ public:
      */
     bool IsUp(void) const { return mIsUp; }
 
-    /**
-     * This method submits a message to the network interface.
-     *
-     * @param[in]  aMessage  A reference to the message.
-     *
-     * @retval kErrorNone  Successfully submitted the message to the interface.
-     *
-     */
-    Error SendMessage(Message &aMessage);
-
-    /**
-     * This method performs a route lookup.
-     *
-     * @param[in]   aSource       A reference to the IPv6 source address.
-     * @param[in]   aDestination  A reference to the IPv6 destination address.
-     *
-     * @retval kErrorNone      Successfully found a route.
-     * @retval kErrorNoRoute   Could not find a valid route.
-     *
-     */
-    Error RouteLookup(const Ip6::Address &aSource, const Ip6::Address &aDestination);
-
-    /**
-     * This method indicates whether @p aAddress matches an on-mesh prefix.
-     *
-     * @param[in]  aAddress  The IPv6 address.
-     *
-     * @retval TRUE   If @p aAddress matches an on-mesh prefix.
-     * @retval FALSE  If @p aAddress does not match an on-mesh prefix.
-     *
-     */
-    bool IsOnMesh(const Ip6::Address &aAddress) const;
-
 private:
     bool mIsUp;
 };