Introduce FunctionRef to replace std::function when not storing

std::function is not necessary when we just want to pass a type
erased callback that does not need to own its target. FunctionRef
provides that functionality for a much lower compile time cost.

src/command_manager.cc

@@ -462,10 +462,9 @@ String expand(StringView str, const Context& context,
 
 String expand(StringView str, const Context& context,
               const ShellContext& shell_context,
-              const std::function<String (String)>& postprocess)
+              const FunctionRef<String (String)>& postprocess)
 {
-    return expand_impl(str, context, shell_context,
-                       [&](String s) { return postprocess(std::move(s)); });
+    return expand_impl(str, context, shell_context, postprocess);
 }
 
 struct command_not_found : runtime_error

src/command_manager.hh

@@ -168,7 +168,7 @@ String expand(StringView str, const Context& context,
 
 String expand(StringView str, const Context& context,
               const ShellContext& shell_context,
-              const std::function<String (String)>& postprocess);
+              const FunctionRef<String (String)>& postprocess);
 
 }
 

src/line_modification.cc

@@ -145,7 +145,7 @@ void LineRangeSet::update(ConstArrayView<LineModification> modifs)
     erase(std::remove_if(begin(), end(), [](auto& r) { return r.begin >= r.end; }), end());
 }
 
-void LineRangeSet::add_range(LineRange range, std::function<void (LineRange)> on_new_range)
+void LineRangeSet::add_range(LineRange range, FunctionRef<void (LineRange)> on_new_range)
 {
     auto it = std::lower_bound(begin(), end(), range.begin,
                                [](LineRange range, LineCount line) { return range.end < line; });

src/line_modification.hh

@@ -7,8 +7,6 @@
 #include "range.hh"
 #include "vector.hh"
 
-#include <functional>
-
 namespace Kakoune
 {
 
@@ -40,7 +38,7 @@ struct LineRangeSet : private Vector<LineRange, MemoryDomain::Highlight>
     void reset(LineRange range) { Base::operator=({range}); }
 
     void update(ConstArrayView<LineModification> modifs);
-    void add_range(LineRange range, std::function<void (LineRange)> on_new_range);
+    void add_range(LineRange range, FunctionRef<void (LineRange)> on_new_range);
     void remove_range(LineRange range);
 };
 

src/utils.hh

@@ -154,6 +154,38 @@ auto to_underlying(E value)
     return static_cast<std::underlying_type_t<E>>(value);
 }
 
+template<typename> class FunctionRef;
+
+template<typename Res, typename... Args>
+class FunctionRef<Res(Args...)>
+{
+public:
+    FunctionRef()
+      : m_target{nullptr},
+        m_invoker{[](void* target, Args... args) {
+            if constexpr (!std::is_same_v<Res, void>) return Res{};
+        }}
+    {}
+
+    template<typename Target>
+    FunctionRef(Target&& target)
+      : m_target{&target},
+        m_invoker{[](void* target, Args... args) {
+            return (*reinterpret_cast<Target*>(target))(static_cast<Args>(args)...);
+        }}
+    {}
+
+    Res operator()(Args... args) const
+    {
+        return m_invoker(m_target, static_cast<Args>(args)...);
+    }
+
+private:
+    using Invoker = Res (void*, Args...);
+    void* m_target;
+    Invoker* m_invoker;
+};
+
 }
 
 #endif // utils_hh_INCLUDED