//go:build vs. // build: What's the Difference and Why Should I Care?

What's the Difference Between //go:build and // build Directives?

While // build directives have been the standard for specifying build constraints, //go:build has emerged as the preferred directive in Go 1.18 and beyond. This new directive aims to replace the old syntax, offering several key advantages:

Go 1.18 and Beyond

Deprecation of // build: //go:build is now the preferred directive, and // build will be removed in future versions of Go. Modules targeting Go 1.18 or above will require dependencies that use //go:build exclusively, as // build lines will be automatically removed during the build process.

Go 1.17

Introduction of //go:build: //go:build was introduced in Go 1.17 as the next-generation build constraint directive. It offers the following improvements:

• Consistency: Aligns with other Go directives like //go:generate.
• Enhanced Syntax: Supports standard boolean expressions with intuitive operators like && and ||, making it easier to write complex constraint conditions.
• go fmt Support: go fmt will automatically fix incorrect placement of //go:build directives in source files.

Coexistence and Transition

To ensure a smooth transition, //go:build and // build directives will coexist for several Go releases. During this period:

• //go:build lines will be preferred for file selection in builds.
• If a file contains only // build lines, go fmt will add an equivalent //go:build line.
• If a file contains both //go:build and // build lines, go fmt will prioritize the //go:build directive and update the // build lines accordingly.

By embracing //go:build, developers can leverage an improved syntax for build constraints, enjoy automatic formatting by go fmt, and prepare for the future of Go development.

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