Modern Go

modern-go

Modernize Go source code by applying version-appropriate idioms, APIs, and language features. Works like go fix plus additional transformations curated from the Go team's modernize analysis passes and community best practices.

Usage

Invoke this skill when the user asks to modernize Go code. By default, modernize the entire project; the user may specify a file or directory instead.

When invoked:

1. Detect the project's Go version from go.mod (the go directive).
2. Find all .go files in the target scope (excluding vendor/, .git/, testdata/).
3. For each file, apply all transformations for versions ≤ the project's Go version, starting from the oldest to the newest.
4. After all transformations, print a summary of what was changed and what was skipped.

If the user specifies a file or directory, limit the scope to that path.

Transformation Catalog

Each transformation includes a Go version gate—only apply when the project's go.mod version ≥ that version. Never apply a transformation that requires a version higher than the project declares.

Go 1.0+ — time.Since

// before
elapsed := time.Now().Sub(start)
// after
elapsed := time.Since(start)

Go 1.8+ — time.Until

// before
remaining := deadline.Sub(time.Now())
// after
remaining := time.Until(deadline)

Go 1.10+ — strings.Builder (loop concatenation)

// before
s := ""
for _, item := range items {
    s += item
}
// after
var b strings.Builder
for _, item := range items {
    b.WriteString(item)
}
s := b.String()

Only when += concatenation happens inside a loop.

Go 1.13+ — errors.Is

// before
if err == io.EOF {
    return
}
// after
if errors.Is(err, io.EOF) {
    return
}

Go 1.17+ — //go:build constraints (plusbuild)

// before
//go:build linux && amd64
// +build linux,amd64

package foo
// after
//go:build linux && amd64

package foo

The plusbuild modernizer removes obsolete // +build constraint lines once the equivalent //go:build line is present (the //go:build syntax landed in Go 1.17). Only strip the old line when a matching //go:build already exists — never drop the sole constraint.

Go 1.17+ — unsafe.Add / unsafe.Slice (unsafefuncs)

// before — pointer arithmetic via uintptr
p2 := unsafe.Pointer(uintptr(ptr) + uintptr(offset))
// after
p2 := unsafe.Add(ptr, offset)

// before — building a slice from a base pointer
s := (*[1 << 30]byte)(unsafe.Pointer(p))[:n:n]
// after
s := unsafe.Slice(p, n)

The unsafefuncs modernizer (gopls v0.22.0) rewrites error-prone uintptr pointer math into unsafe.Add / unsafe.Slice, which the compiler and go vet understand as GC-safe.

Go 1.18+ — any

// before
func decode(v interface{}) error { ... }
// after
func decode(v any) error { ... }

Go 1.18+ — strings.Cut

// before
if i := strings.Index(s, "="); i >= 0 {
    key, val := s[:i], s[i+1:]
}
// after
if key, val, found := strings.Cut(s, "="); found {
    ...
}

Go 1.18+ — bytes.Cut

// before
if i := bytes.Index(b, sep); i >= 0 {
    before, after := b[:i], b[i+len(sep):]
}
// after
before, after, found := bytes.Cut(b, sep)

Go 1.19+ — fmt.Appendf

// before
buf = append(buf, fmt.Sprintf("x=%d", x)...)
// after
buf = fmt.Appendf(buf, "x=%d", x)

Go 1.19+ — Type-safe atomics (atomictypes)

// before
var ready int32
atomic.StoreInt32(&ready, 1)
if atomic.LoadInt32(&ready) == 1 { ... }

// after
var ready atomic.Int32
ready.Store(1)
if ready.Load() == 1 { ... }

// before
var cache atomic.Value
cache.Store(&Config{})
cfg := cache.Load().(*Config)

// after
var cache atomic.Pointer[Config]
cache.Store(&Config{})
cfg := cache.Load()

The atomictypes modernizer (gopls v0.22.0, AtomicTypesAnalyzer) rewrites both the variable declaration and every call site. Typed wrappers (atomic.Int32/Int64/Uint32/Uint64/Bool/Pointer[T]) have identical performance but prevent accidental non-atomic access and fix 64-bit alignment crashes on 32-bit architectures.

Go 1.20+ — strings.Clone

// before
s2 := string([]byte(s)) // force copy
// after
s2 := strings.Clone(s)

Go 1.20+ — bytes.Clone

// before
dst := make([]byte, len(src))
copy(dst, src)
// after
dst := bytes.Clone(src)

Go 1.20+ — strings.CutPrefix / strings.CutSuffix

// before
if strings.HasPrefix(s, "pre_") {
    s = s[len("pre_"):]
}
// after
if rest, ok := strings.CutPrefix(s, "pre_"); ok {
    s = rest
}

// before
if strings.HasSuffix(s, ".txt") {
    s = s[:len(s)-len(".txt")]
}
// after
if rest, ok := strings.CutSuffix(s, ".txt"); ok {
    s = rest
}

Go 1.20+ — errors.Join

// before
return fmt.Errorf("load config: %w: %w", err1, err2)
// after
return errors.Join(fmt.Errorf("load config"), err1, err2)

Go 1.20+ — context.WithCancelCause

// before
ctx, cancel := context.WithCancel(parent)
// ... somewhere ...
cancel()

// after
ctx, cancel := context.WithCancelCause(parent)
cancel(ErrShutdown)
// caller: context.Cause(ctx) → ErrShutdown

Go 1.21+ — min / max

// before
lo := a
if b < lo {
    lo = b
}
// after
lo := min(a, b)

// before
if x < 0 {
    x = 0
}
if x > 100 {
    x = 100
}
// after
x = min(max(x, 0), 100)

Go 1.21+ — clear

// before
for k := range m {
    delete(m, k)
}
// after
clear(m)

// before
for i := range s {
    s[i] = 0
}
// after
clear(s)

Go 1.21+ — slices package