`//go:build vs // build: Which Conditional Compilation Directive Should You Use in Go?`

//go:build vs // build: A New Era in Conditional Compilation

The Dilemma

Consider the following Go code snippet:

//go:build (386 || amd64 || amd64p32) && gccgo
// +build 386 amd64 amd64p32
// +build gccgo

package cpu

As a build tag, // build seemed to suffice. Why was //go:build explicitly specified? And why is it difficult to find documentation for //go:build while // build has extensive documentation?

Go 1.18: A New Path

The new directive //go:build is now the preferred conditional compilation approach, and the toolchain actively removes old // build directives.

Go 1.17: The Introduction of //go:build

//go:build was introduced in Go 1.17 to replace // build. It offers several advantages:

• Consistency with other Go directives (e.g., //go:generate)
• Explicit boolean expression support (e.g., //go:build foo && bar)
• Go fmt compatibility, which automatically corrects directive placement

Coexistence and Transition

Both directives coexist for a smooth transition.

Syntax Changes and Compatibility

• Go builds prioritize //go:build over // build
• //go:build alone is no longer a build failure
• Misplaced //go:build directives are detected and moved
• Go fmt corrects //go:build expressions
• Gofmt adds //go:build when only // build is present
• Gofmt updates // build to match //go:build
• Go vet checks for discrepancies between //go:build and // build

Moving Forward

//go:build is the recommended approach for conditional compilation in Go. Its improved syntax, Go fmt compatibility, and coexistence with // build ensure a seamless transition to the future of Go development.

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