Support ->LibX.fun_name in the interpreter

spec/compiler/interpreter/lib_spec.cr

@@ -91,4 +91,27 @@ describe Crystal::Repl::Interpreter do
         CRYSTAL
     end
   end
+
+  context "proc pointer" do
+    it "calls extern fun" do
+      interpret(<<-CRYSTAL).should eq 6
+        @[Link(ldflags: #{ldflags_with_backtick.inspect})]
+        lib LibSum
+          fun simple_sum_int(a : Int32, b : Int32) : Int32
+        end
+
+        class Foo
+          def initialize(@method : Proc(Int32, Int32, Int32))
+          end
+
+          def call(a, b)
+            @method.call(a, b)
+          end
+        end
+
+        foo = Foo.new(->LibSum.simple_sum_int)
+        foo.call(1, 5)
+        CRYSTAL
+    end
+  end
 end

src/compiler/crystal/interpreter/compiler.cr

@@ -2925,6 +2925,28 @@ class Crystal::Repl::Compiler < Crystal::Visitor
     false
   end
 
+  def visit(node : ProcPointer)
+    target_def = node.call.target_def
+
+    unless target_def.owner.is_a?(LibType)
+      # LLVM codegen supports more cases like closure data and obj/self
+      # target, but I can't trigger them — does LiteralExpander expand
+      # these cases into ProcLiteral?
+      raise "BUG: missing interpret for ProcPointer to non Lib fun"
+    end
+
+    # find or build a compiled_def
+    proc_type = node.type.as(ProcInstanceType)
+    symbol = @context.c_function(target_def.as(External).real_name)
+    compiled_def = @context.extern_proc_wrapper(proc_type, symbol)
+
+    # push compiled_def to stack + no closure data (null pointer)
+    put_i64 compiled_def.object_id.to_i64!, node: node
+    put_i64 0, node: node
+
+    false
+  end
+
   def visit(node : Break)
     exp = node.exp