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An identifier pattern matches any value and binds that evaluated value to a variable or constant name. It is the most fundamental pattern in Swift’s pattern matching engine, acting as an unconditional match that captures the assigned value into a newly declared identifier in the current scope. In Swift’s grammar, an identifier pattern is simply a valid identifier name. It is almost always utilized as a sub-pattern within a value-binding pattern (using let or var), which dictates the mutability of the bound identifier. 
// Syntax
// <value-binding-keyword> <identifier-pattern> = <expression>

// The identifier pattern 'x' matches the value 42 and binds it.
let x = 42

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Mechanics

  1. Unconditional Matching: The identifier pattern does not evaluate the structure, type, or state of the value for equality. It inherently succeeds and accepts whatever value is passed to it.
  2. Binding: Upon a successful match, the Swift compiler allocates the identifier in the local scope and binds the matched value to it.
  3. Composition: Identifier patterns are frequently nested within structural patterns (such as tuple patterns or enumeration case patterns) to extract and bind specific sub-values.
// 'a' and 'b' are identifier patterns nested within a tuple pattern.
let (a, b) = (10, 20)

// 'value' is an identifier pattern nested within an enumeration case pattern.
let response: Result<String, Error> = .success("OK")
if case .success(let value) = response {
    print(value)
}

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Grammar and Resolution Context

The Swift compiler resolves an identifier based on its surrounding lexical context. An identifier only functions as an identifier pattern when it is explicitly introducing a new binding. If an identifier is used within a pattern-matching context (like a switch statement’s case) without a value-binding keyword, the compiler resolves it as an expression pattern (evaluating against an existing variable in scope) rather than an identifier pattern. 
let existingValue = 5

switch 5 {
// Evaluated as an EXPRESSION pattern matching the existing variable.
case existingValue:
    break
    
// Evaluated as an IDENTIFIER pattern binding the matched value to a new identifier.
case let newValue:
    print(newValue)
}
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