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A struct literal in Go is a composite literal expression used to allocate and initialize a new instance of a struct type in a single operation. It constructs the struct value in memory and binds specified values to its fields during instantiation.

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Syntax and Mechanics

Given a defined struct type: 
type ServerConfig struct {
    Host string
    Port int
    TLS  bool
}
Go supports several syntactic forms for struct literals, each with specific initialization rules.

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1. Named Field Initialization

This is the idiomatic approach, where values are explicitly mapped to field names. 
config := ServerConfig{
    Host: "localhost",
    Port: 8080,
}
  • Order Independence: Fields can be initialized in any order.
  • Partial Initialization: Any field omitted from the literal is automatically initialized to its type’s respective zero value (e.g., TLS becomes false).

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2. Positional Initialization

Values are assigned based strictly on the order of the fields defined in the struct type declaration. 
config := ServerConfig{"localhost", 8080, true}
  • Strict Arity: You must provide a value for every field in the struct. You cannot omit values to rely on zero-value initialization.
  • Order Dependence: The sequence of values must exactly match the struct’s memory layout.

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3. Empty Literal (Zero-Value Instantiation)

An empty struct literal initializes all fields to their respective zero values. 
config := ServerConfig{}

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4. Pointer to Struct Literal

Prefixing the struct literal with the address-of operator (&) allocates the struct on the heap (if it escapes the local scope) or stack, initializes it, and returns a pointer to the newly allocated memory (*ServerConfig). 
configPtr := &ServerConfig{
    Host: "127.0.0.1",
}
This is syntactically equivalent to using the built-in new() function followed by field assignment, but it is executed as a single expression.

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5. Anonymous Struct Literal

A struct literal can be declared and instantiated simultaneously without binding the struct definition to a named type identifier. 
inlineConfig := struct {
    Timeout int
    Retries int
}{
    Timeout: 30,
    Retries: 3,
}

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Lexical Formatting Rules

Due to Go’s automatic semicolon insertion (ASI) rules, multi-line struct literals enforce strict comma placement. A trailing comma is mandatory after the final field assignment if the closing brace } is on a new line. Valid: 
config := ServerConfig{
    Host: "localhost",
    Port: 8080, // Trailing comma required
}
Valid: 
config := ServerConfig{Host: "localhost", Port: 8080} // No trailing comma needed
Compilation Error: 
config := ServerConfig{
    Host: "localhost",
    Port: 8080 // Syntax error: missing ',' before newline
}
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