lostfield

README

LostField

A Go linter that ensures converter functions use all fields from both input and output structs, preventing "leaky" conversions where fields are accidentally omitted.

What it does

lostfield analyzes converter functions (functions that transform one struct type to another) and reports when fields are missing from either:

• Input fields - fields that should be read but aren't
• Output fields - fields that should be set but aren't

This helps catch bugs where new fields are added to structs but converter functions aren't updated accordingly.

Installation

go install github.com/amberpixels/lostfield/cmd/lostfield@latest

Usage

Standalone

# With go vet
go vet -vettool=$(which lostfield) ./...

With golangci-lint

Add to your .golangci.yml:

version: "2"

linters:
  enable:
    - lostfield

linters-settings:
  lostfield:
    include-methods: false
    allow-getters: true
    exclude-fields: "CreatedAt,UpdatedAt"

See .golangci.example.yml for detailed configuration options.

Configuration

Command-line Flags

Flag	Type	Default	Description
-include-methods	bool	false	Check method receivers in addition to plain functions
-allow-getters	bool	true	Allow Get* methods as substitute for direct field access
-exclude-fields	string	""	Comma-separated regex patterns for field names to ignore
-min-similarity	float64	0.0	Type name similarity threshold (0.0-1.0, 0=substring matching)
-ignore-tags	string	""	Comma-separated struct tags that mark fields to ignore

Examples

# Check only plain functions (default)
lostfield ./...

# Also check methods
lostfield -include-methods=true ./...

# Ignore timestamp fields
lostfield -exclude-fields="CreatedAt,UpdatedAt,DeletedAt" ./...

# Strict mode: no getters, check methods
lostfield -include-methods=true -allow-getters=false ./...

Example

Given these types:

type User struct {
    ID        int64
    Username  string
    Email     string
    CreatedAt time.Time
}

type UserDTO struct {
    ID       int64
    Username string
    Email    string
}

This converter would trigger a warning:

func ConvertUserToDTO(user User) UserDTO {
    return UserDTO{
        ID:       user.ID,
        Username: user.Username,
        // Missing: Email
    }
}

Output:

 |
12 | func ConvertUserToDTO(user User) UserDTO {
 |      ^^^^^^^^^^^^^^^^^ converter function is leaking fields:
 |                        missing input fields: []
 |                        missing output fields: [Email]

Development Status

⚠️ Under active development - API and behavior may change.

See .claude/roadmap.md for development roadmap and future plans.

Contributing

Contributions are welcome! Please check the roadmap for planned features and areas that need work.

License

MIT