dump[2]: avoid misleading rounding of numbers

builtin/common/misc_helpers.lua

@@ -7,7 +7,15 @@ local math = math
 local function basic_dump(o)
 	local tp = type(o)
 	if tp == "number" then
-		return tostring(o)
+		local s = tostring(o)
+		if tonumber(s) == o then
+			return s
+		end
+		-- Prefer an exact representation over a compact representation.
+		-- e.g. basic_dump(0.3) == "0.3",
+		-- but basic_dump(0.1 + 0.2) == "0.30000000000000004"
+		-- so the user can see that 0.1 + 0.2 ~= 0.3
+		return string.format("%.17g", o)
 	elseif tp == "string" then
 		return string.format("%q", o)
 	elseif tp == "boolean" then

builtin/common/tests/misc_helpers_spec.lua

@@ -230,3 +230,10 @@ describe("math", function()
 		assert.equal(0, math.round(-0.49999999999999994))
 	end)
 end)
+
+describe("dump", function()
+	it("avoids misleading rounding of floating point numbers", function()
+		assert.equal("0.3", dump(0.3))
+		assert.equal("0.30000000000000004", dump(0.1 + 0.2))
+	end)
+end)