Add table.copy_with_metatables (#15754)

builtin/common/misc_helpers.lua

@@ -457,18 +457,37 @@ do
 	end
 end
 
---------------------------------------------------------------------------------
-function table.copy(t, seen)
-	local n = {}
-	seen = seen or {}
-	seen[t] = n
-	for k, v in pairs(t) do
-		n[(type(k) == "table" and (seen[k] or table.copy(k, seen))) or k] =
-			(type(v) == "table" and (seen[v] or table.copy(v, seen))) or v
+
+local function table_copy(value, preserve_metatables)
+	local seen = {}
+	local function copy(val)
+		if type(val) ~= "table" then
+			return val
+		end
+		local t = val
+		if seen[t] then
+			return seen[t]
+		end
+		local res = {}
+		seen[t] = res
+		for k, v in pairs(t) do
+			res[copy(k)] = copy(v)
+		end
+		if preserve_metatables then
+			setmetatable(res, getmetatable(t))
+		end
+		return res
 	end
-	return n
+	return copy(value)
 end
 
+function table.copy(value)
+	return table_copy(value, false)
+end
+
+function table.copy_with_metatables(value)
+	return table_copy(value, true)
+end
 
 function table.insert_all(t, other)
 	if table.move then -- LuaJIT

builtin/common/tests/misc_helpers_spec.lua

@@ -178,6 +178,35 @@ describe("table", function()
 		assert.equal(2, table.keyof({[2] = "foo", [3] = "bar"}, "foo"))
 		assert.equal(3, table.keyof({[1] = "foo", [3] = "bar"}, "bar"))
 	end)
+
+	describe("copy()", function()
+		it("strips metatables", function()
+			local v = vector.new(1, 2, 3)
+			local w = table.copy(v)
+			assert.are_not.equal(v, w)
+			assert.same(v, w)
+			assert.equal(nil, getmetatable(w))
+		end)
+		it("preserves referential structure", function()
+			local t = {{}, {}}
+			t[1][1] = t[2]
+			t[2][1] = t[1]
+			local copy = table.copy(t)
+			assert.same(t, copy)
+			assert.equal(copy[1][1], copy[2])
+			assert.equal(copy[2][1], copy[1])
+		end)
+	end)
+
+	describe("copy_with_metatables()", function()
+		it("preserves metatables", function()
+			local v = vector.new(1, 2, 3)
+			local w = table.copy_with_metatables(v)
+			assert.equal(getmetatable(v), getmetatable(w))
+			assert(vector.check(w))
+			assert.equal(v, w) -- vector overrides ==
+		end)
+	end)
 end)
 
 describe("formspec_escape", function()