Go 1.17's `//go:build`: How Does it Differ from `// build` for Conditional Compilation?

The Difference Between //go:build and // build Directives

The //go:build directive is the new conditional compilation directive introduced in Go 1.17, intended to replace the // build directive. While both directives allow you to specify the conditions under which a file should be included in the package, //go:build offers several key improvements:

• Consistency: The //go:build syntax is consistent with other existing Go directives and pragmas, such as //go:generate.
• Enhanced Expressions: //go:build supports standard boolean expressions (e.g., //go:build foo && bar), while // build used less intuitive syntax for AND (commas) and OR (spaces).
• Code Formatting: The //go:build directive is automatically formatted by gofmt. This avoids common mistakes, such as not leaving a blank line between the directive and the package statement.

Go 1.18 raised //go:build as the preferred directive, with plans to gradually remove // build lines. Transitioning between the two directives is handled as follows:

• In Go 1.18 and onward, //go:build is the primary build constraint directive.
• If a file lacks a //go:build directive, any // build directives will still apply.
• If a file contains both //go:build and // build directives, //go:build will be considered the source of truth, and gofmt will update // build to match.

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