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

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

In Go, build directives are used to specify conditions under which a file should be included in the package. Traditionally, the // build directive has been used for this purpose. However, with the introduction of Go 1.17, a new directive, //go:build, was introduced to replace // build.

Reasons for the Introduction of //go:build

1. Consistency: //go:build aligns with other Go directives and pragmas, such as //go:generate.
2. Enhanced Syntax: //go:build supports standard boolean expressions, making it more intuitive and less error-prone.
3. Code Formatting: Go fmt automatically formats //go:build directives, ensuring correct placement and avoiding common mistakes.

Coexistence and Transition

To ensure a smooth transition, //go:build and // build will coexist for a few Go releases. However, in Go 1.18 and beyond, //go:build will become the preferred directive. The toolchain will actively remove obsolete // build lines.

Key Differences

• Expression Syntax: //go:build uses standard boolean expressions, while // build uses a custom syntax with commas for AND and spaces for OR.
• Supported Expressions: //go:build supports complex expressions, including nested conditions and parentheses, while // build only supports simple expressions.

Example

In the provided example, the following snippet:

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

Is equivalent to the following, which uses //go:build:

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

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