feat: type key-values only

[rjfonseca]

Allowing for arbitrary lists of key-values was causing unintentional panicking when the developer mistakes the type of a value. E.g. using an error code with type string instead of errors.Code.

By Allowing only typed values to be used we eliminate one potential panic.

[Copilot]

Pull Request Overview

This PR restricts the errors.With API to accept only typed key-value arguments, preventing panics from incorrect varargs use.

• Changed With signature to only accept KeyValuer types and removed varargs parsing helpers.
• Updated tests to use the new KeyValuer-only interface.
• Revised README and example code to reflect the new usage model.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
with.go	Updated With signature, removed argsToKeyValues and cutKV.
with_test.go	Replaced generic varargs tests with KeyValuer-only tests.
op_test.go	Adjusted test to pass Op directly instead of splitting key/value.
README.md	Expanded usage section and examples to show KeyValuer calls.

Comments suppressed due to low confidence (4)

with_test.go:18

• [nitpick] Consider renaming this to TestWithNilErrorReturnsNil or TestWith_NilErr_ReturnsNil for clearer intent about testing the nil-err case.

func TestWithNoError(t *testing.T) {

with_test.go:25

• A test for the case where a non-nil error is passed with zero KeyValuer arguments (i.e. errors.With(rootErr)) would ensure the function still returns the original error.

func TestWith(t *testing.T) {

with.go:14

• [nitpick] Consider making this panic message more descriptive, e.g. include the key's type or change it to panic("key type is not comparable") to aid debugging.

panic("key is not comparable")

README.md:10

• The example uses fmt.Errorf but the fmt package isn't imported in the snippet. Add "fmt" to the import block for the example to compile as shown.

import (