Go safety

Rewrite of samber/cc-skills-golang golang-safety @ 22c58a55. Not a vendor paste. Not an npx / marketplace pack. SKILL.md only — no evals/, no scripts/, no references/. Id: golang-safety.

security CLEAR (conditional): SKILL only. Safety is our bugs.

Iron law

No untested assumption about nil, capacity, or numeric range ships.

Did not initialize it, bound-check it, or copy it? Treat it as a latent panic or a silent wrap.

Scope

Do not merge golang-testing, golang-security and golang-safety into one body. Which Go skill loads, and what loads with it: language-router.

Always

Go 1.25+ reflection: reflect.TypeAssert[T](v) — not v.Interface().(T).

Ask first

Do not self-except. Record the decision.

Never

Traps (compressed)

Typed nil

An interface is nil only when type and value are both nil.

func handler(ok bool) http.Handler {
    if !ok {
        return nil // return plain nil; a nil *MyHandler in an interface is != nil
    }
    return &MyHandler{}
}

Nil containers

func (r *Reg) Put(k string, v int) {
    if r.items == nil {
        r.items = make(map[string]int)
    }
    r.items[k] = v
}

Append alias

// may share a's array — writes to out can mutate a
out := append(a, x)

// keep a stable: cap==len forces a new array
out := append(a[:len(a):len(a)], x)

Narrowing

if n < math.MinInt32 || n > math.MaxInt32 {
    return 0, fmt.Errorf("int32 overflow: %d", n)
}

Defer per item

for _, path := range paths {
    if err := one(path); err != nil {
        return err
    }
}

func one(path string) error {
    f, err := os.Open(path)
    if err != nil {
        return err
    }
    defer f.Close()
    return use(f)
}

Zero value

sync.Mutex and bytes.Buffer work at zero. A struct whose first write is m[k]=v on a nil map field does not. Lazy-init the map. If two goroutines can hit that init, sync.Once — do not invent a concurrency protocol in this skill.

Exported accessors that hand back []T or map[K]V return slices.Clone / maps.Clone, not the live field.

Roles

Workers do not bypass intake. A green gofmt is not a security clear.

Red flags

• "It's never nil in practice"
• "append always copies"
• "I'll close everything at function exit"
• "int conversion is fine, the number is small"
• "npx the samber golang pack"
• "Safety and security are the same skill"
• "Copy the evals so we can score it"

Stop. Initialize, bound-check, or copy. Or ask.

Upstream pin

Intent: samber/cc-skills-golang golang-safety @ 22c58a55a0a799b901aa251172923180bad9e010. Body is a rewrite. See SOURCES.md.