deadmut

A Go linter that detects mutations to range loop value copies.

Name origin: "dead mutation" = mutation with no effect

Background

Go's range loop copies values, so modifications to the copy don't affect the original slice:

for _, user := range users {
    user.Name = "updated"  // Does NOT update the original slice!
}

This is:

• A common pitfall for beginners
• Not a compile error
• Not a runtime panic
• A silent bug

Comparison with Existing Linters

Linter	Detects	Range loop value mutation
staticcheck SA4005	Field assignment to value receiver	No
copyloopvar	i := i copy pattern	Different issue
ineffassign	Assignment to unused variable	No
loopclosure	Capture in goroutine/defer	Different issue
deadmut	Mutation to range loop value	Yes

Installation

go install github.com/mickamy/deadmut@latest

Usage

deadmut ./...

What It Detects

1. Field Assignment

for _, user := range users {
    user.Name = "updated"  // deadmut: mutation to range value copy has no effect
}

2. Pointer Receiver Method Call

for _, user := range users {
    user.SetName("updated")  // deadmut: pointer receiver method on range value copy has no effect
}

3. Taking Pointer of Value

for _, user := range users {
    updateUser(&user)  // deadmut: pointer to range value copy may not have intended effect
}

What It Ignores

Index Access

for i := range users {
    users[i].Name = "updated"  // OK - direct modification
}

Slice of Pointers

for _, user := range users {  // users is []*User
    user.Name = "updated"  // OK - modifying via pointer
}

Value Used After Mutation

for _, user := range users {
    user.Name = "updated"
    process(user)  // OK - intentionally using the copy
}

Collecting Copies

for _, user := range users {
    user.Name = "updated"
    results = append(results, user)  // OK - collecting modified copies
}

How to Fix

// Before (bug)
for _, user := range users {
    user.Name = "updated"
}

// After (fix 1: use index)
for i := range users {
    users[i].Name = "updated"
}

// After (fix 2: use pointer slice)
for _, user := range users {  // users is []*User
    user.Name = "updated"
}

License

MIT