AK+Kernel+LibELF: Remove the need for IteratorDecision::Continue

By constraining two implementations, the compiler will select the best
fitting one. All this will require is duplicating the implementation and
simplifying for the `void` case.

This constraining also informs both the caller and compiler by passing
the callback parameter types as part of the constraint
(e.g.: `IterationFunction<int>`).

Some `for_each` functions in LibELF only take functions which return
`void`. This is a minimal correctness check, as it removes one way for a
function to incompletely do something.

There seems to be a possible idiom where inside a lambda, a `return;` is
the same as `continue;` in a for-loop.

Kernel/VM/AnonymousVMObject.cpp

@@ -23,13 +23,11 @@ RefPtr<VMObject> AnonymousVMObject::clone()
     // so that the parent is still guaranteed to be able to have all
     // non-volatile memory available.
     size_t need_cow_pages = 0;
-    {
-        // We definitely need to commit non-volatile areas
-        for_each_nonvolatile_range([&](const VolatilePageRange& nonvolatile_range) {
-            need_cow_pages += nonvolatile_range.count;
-            return IterationDecision::Continue;
-        });
-    }
+
+    // We definitely need to commit non-volatile areas
+    for_each_nonvolatile_range([&](const VolatilePageRange& nonvolatile_range) {
+        need_cow_pages += nonvolatile_range.count;
+    });
 
     dbgln_if(COMMIT_DEBUG, "Cloning {:p}, need {} committed cow pages", this, need_cow_pages);
 
@@ -220,7 +218,6 @@ int AnonymousVMObject::purge_impl()
                 }
             });
         }
-        return IterationDecision::Continue;
     });
     return purged_page_count;
 }
@@ -284,7 +281,6 @@ void AnonymousVMObject::update_volatile_cache()
     m_volatile_ranges_cache.clear();
     for_each_nonvolatile_range([&](const VolatilePageRange& range) {
         m_volatile_ranges_cache.add_unchecked(range);
-        return IterationDecision::Continue;
     });
 
     m_volatile_ranges_cache_dirty = false;

Kernel/VM/AnonymousVMObject.h

@@ -40,7 +40,7 @@ public:
 
     bool is_any_volatile() const;
 
-    template<typename F>
+    template<IteratorFunction<const VolatilePageRange&> F>
     IterationDecision for_each_volatile_range(F f) const
     {
         VERIFY(m_lock.is_locked());
@@ -78,24 +78,42 @@ public:
         return IterationDecision::Continue;
     }
 
-    template<typename F>
+    template<IteratorFunction<const VolatilePageRange&> F>
     IterationDecision for_each_nonvolatile_range(F f) const
     {
         size_t base = 0;
         for_each_volatile_range([&](const VolatilePageRange& volatile_range) {
             if (volatile_range.base == base)
                 return IterationDecision::Continue;
-            IterationDecision decision = f({ base, volatile_range.base - base });
+            IterationDecision decision = f(VolatilePageRange { base, volatile_range.base - base });
             if (decision != IterationDecision::Continue)
                 return decision;
             base = volatile_range.base + volatile_range.count;
             return IterationDecision::Continue;
         });
         if (base < page_count())
-            return f({ base, page_count() - base });
+            return f(VolatilePageRange { base, page_count() - base });
         return IterationDecision::Continue;
     }
 
+    template<VoidFunction<const VolatilePageRange&> F>
+    IterationDecision for_each_volatile_range(F f) const
+    {
+        return for_each_volatile_range([&](auto& range) {
+            f(range);
+            return IterationDecision::Continue;
+        });
+    }
+
+    template<VoidFunction<const VolatilePageRange&> F>
+    IterationDecision for_each_nonvolatile_range(F f) const
+    {
+        return for_each_nonvolatile_range([&](auto range) {
+            f(move(range));
+            return IterationDecision::Continue;
+        });
+    }
+
 private:
     explicit AnonymousVMObject(size_t, AllocationStrategy);
     explicit AnonymousVMObject(PhysicalAddress, size_t);

Kernel/VM/MemoryManager.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/Concepts.h>
 #include <AK/HashTable.h>
 #include <AK/NonnullRefPtrVector.h>
 #include <AK/String.h>
@@ -157,7 +158,7 @@ public:
     unsigned super_physical_pages() const { return m_super_physical_pages; }
     unsigned super_physical_pages_used() const { return m_super_physical_pages_used; }
 
-    template<typename Callback>
+    template<IteratorFunction<VMObject&> Callback>
     static void for_each_vmobject(Callback callback)
     {
         for (auto& vmobject : MM.m_vmobjects) {
@@ -166,6 +167,13 @@ public:
         }
     }
 
+    template<VoidFunction<VMObject&> Callback>
+    static void for_each_vmobject(Callback callback)
+    {
+        for (auto& vmobject : MM.m_vmobjects)
+            callback(vmobject);
+    }
+
     static Region* find_region_from_vaddr(Space&, VirtualAddress);
     static Region* find_user_region_from_vaddr(Space&, VirtualAddress);