Single-line Comment

A single-line comment in Go is a lexical construct initiated by a double forward slash (//). During lexical analysis, the Go scanner reads the // marker and all subsequent characters up to the next newline character (\n) or the End-Of-File (EOF), classifying the entire sequence as a token.COMMENT.

Unlike languages where comments are stripped out entirely during early compilation phases, the Go toolchain retains them. The parser attaches these comment tokens to the Abstract Syntax Tree (AST) as distinct nodes. The Go toolchain and compiler actively evaluate specific single-line comments to enforce compiler instructions, build constraints, and generate documentation.

Lexical Rules and Behavior

• Automatic Semicolon Insertion (ASI): According to the Go specification, a line comment acts lexically like a newline character, not generic whitespace. If a single-line comment appears at the end of a line following a valid token, it triggers Go’s Automatic Semicolon Insertion for the preceding statement.
• Toolchain and Compiler Directives: Single-line comments starting exactly with //go: (with no space between the slashes and the text) are structurally significant. The Go compiler reads specific directives (e.g., //go:noinline), while the broader go command toolchain processes others. For example, //go:build is evaluated by the build toolchain to filter which files are passed to the compiler, and //go:generate is processed exclusively by the go generate command-line tool. Compiler directives must appear immediately before the declaration they apply to, with no intervening blank lines or doc comments.
• Doc Comments: Single-line comments placed immediately preceding a package, function, type, or variable declaration—without intervening blank lines—are structurally bound to that declaration in the AST. These are extracted by the toolchain (such as go doc) to generate source documentation.
• String Literals: If the // sequence appears within a raw or interpreted string literal, the scanner treats it as standard character data rather than a comment initiator.
• Nesting and Evaluation: Because the scanner consumes all text after the initial // until a newline or EOF, subsequent // sequences or multi-line comment initiators (/*) on the same line are parsed as plain text within the comment body.

// This is a standard single-line comment.
var count int = 10 // Acts as a newline, triggering ASI for the variable declaration.

// Doc comment structurally attached to the function declaration below in the AST.
//go:noinline
func compute() {
    var _ string = "https://example.com" // The // inside the string is literal character data.
    // /* This is still a single-line comment; the /* has no structural effect.
}

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