deprecate mutating assignment to ModuleInfo

- It's not possible to deprecate default construction
  as `deprecate this()` is not allowed.

- Deprecating copy-construction `deprecate this(this)`
  results in a lot of false deprecation warnings
  due to compiler generated functions.

src/object.di

@@ -280,6 +280,17 @@ struct ModuleInfo
     uint _flags;
     uint _index;
 
+    version (all)
+    {
+        deprecated("ModuleInfo cannot be copy-assigned because it is a variable-sized struct.")
+        void opAssign(in ModuleInfo m) { _flags = m._flags; _index = m._index; }
+    }
+    else
+    {
+        @disable this();
+        @disable this(this) const;
+    }
+
 const:
     @property uint index() nothrow pure;
     @property uint flags() nothrow pure;

src/object_.d

@@ -1592,6 +1592,17 @@ struct ModuleInfo
     uint _flags;
     uint _index; // index into _moduleinfo_array[]
 
+    version (all)
+    {
+        deprecated("ModuleInfo cannot be copy-assigned because it is a variable-sized struct.")
+        void opAssign(in ModuleInfo m) { _flags = m._flags; _index = m._index; }
+    }
+    else
+    {
+        @disable this();
+        @disable this(this) const;
+    }
+
 const:
     private void* addrOf(int flag) nothrow pure
     in
@@ -1734,16 +1745,6 @@ const:
     }
 }
 
-unittest
-{
-    ModuleInfo mi;
-    foreach (m; ModuleInfo)
-    {
-        mi = *m;
-        break;
-    }
-}
-
 unittest
 {
     ModuleInfo* m1;

src/rt/minfo.d

@@ -392,6 +392,11 @@ unittest
 
     static struct UTModuleInfo
     {
+        this(uint flags)
+        {
+            mi._flags = flags;
+        }
+
         void setImports(immutable(ModuleInfo)*[] imports...)
         {
             import core.bitop;
@@ -413,7 +418,7 @@ unittest
 
     static UTModuleInfo mockMI(uint flags)
     {
-        auto mi = UTModuleInfo(ModuleInfo(flags | MIimportedModules));
+        auto mi = UTModuleInfo(flags | MIimportedModules);
         auto p = cast(void function()*)&mi.pad;
         if (flags & MItlsctor) *p++ = &stub;
         if (flags & MItlsdtor) *p++ = &stub;