go-fix

Update packages to use new APIs

$ go fix [packages]

go fix [packages]

For more control:
go tool fix [flags] [packages]

-diff
    
Show a diff of the changes that would be applied to the files, rather than modifying the files in place. This is useful for reviewing the proposed changes before committing them.

-r rule
    
Apply only the specific rewrite rule named. If rule is `all` (the default when `-r` is not specified), all applicable rules are applied. This allows for targeted application of fixes.

-v
    
Verbose output. Print the names of packages as they are processed, providing feedback on the command's progress.

-list
    
List all available fix rules. When this flag is used, no files are processed, and the command simply outputs a list of the built-in fix transformations that `go fix` can perform.

-cpuprofile file
    
Write a CPU profile to the specified file. This is primarily for debugging or performance analysis of the `go tool fix` command itself.

-memprofile file
    
Write a memory profile to the specified file. Similar to `-cpuprofile`, this is for profiling the `go tool fix` command's memory usage.

The go fix command is a powerful utility within the Go programming language toolchain, primarily used for automatically updating Go source code to adapt to changes in the language or standard library APIs. As the Go language evolves, certain constructs, functions, or package behaviors might be modified or deprecated. go fix applies a predefined set of rewrite rules (often referred to as 'fixups') to your code, bringing it in line with the expectations of a newer Go version.

While `go fix` itself is a simple wrapper, the underlying tool `go tool fix` provides the core functionality and more granular control. This command is invaluable for developers migrating older Go codebases to newer Go releases, significantly reducing the manual effort required for refactoring due to syntactic or API-level changes. It parses the Go source files, applies the relevant transformations, and overwrites the original files. It is important to note that `go fix` addresses only known syntactic patterns and API changes, not semantic or logical issues in your code.

While `go fix` automates many common code updates, it has limitations:

• It handles known syntactic and API changes but does not understand semantic or logical implications of code. Manual review after running `go fix` is strongly recommended.
• By default, it modifies files in place, so using version control (like Git) before running the command is crucial for easy rollback.
• It may not cover all possible deprecations or idiomatic changes; some manual refactoring might still be necessary.
• It only works on valid Go source code that can be parsed; syntax errors will prevent it from running effectively.

At its heart, `go fix` operates by applying a series of predefined 'rewrite rules'. Each rule targets a specific change, such as renaming a function (e.g., `os.Error` to `error`), updating a type signature (e.g., `http.ResponseWriter` changes), or modernizing syntax. Developers can inspect these rules using the `go tool fix -list` command.

`go fix` is designed to be idempotent. Running the command multiple times on the same codebase will typically produce the same result once all applicable fixes have been applied. It will not endlessly modify files, assuming no new fixes are introduced or code is changed externally.

The most common use case for `go fix` is after updating your Go toolchain to a newer major or minor version, especially if the release notes mention breaking changes that `go fix` can handle. You would typically navigate to your module root and run `go fix ./...` to process all packages in your module, or `go fix .` for the current directory.

The `fix` tool, which forms the core of `go fix`, was introduced early in the Go project's development, predating the stable Go 1.0 release. Its primary purpose was to facilitate the evolution of the Go language and its standard library by providing an automated way to update existing codebases to new APIs and syntax. This mechanism has been crucial for Go's philosophy of backward compatibility, allowing the language to evolve without imposing an unbearable burden of manual refactoring on its users. Initially, it was accessed more commonly via `go tool fix`, but the `go fix` subcommand later provided a more user-friendly interface for common operations.

go(1), go build(1), go mod(1), go vet(1), gofmt(1)