When to use protocols in SwiftSyntax.md

When to use protocols in SwiftSyntax

Learn when to use protocols value types like ExprSyntax over protocols like ExprSyntaxProtocol.

SwiftSyntax tries to minimize the use of existentials (aka. protocols spelled with any or protocols spelled without some) wherever possible. This is because when the stored value is more than 3 words (a word is the size of a pointer) large, these existentials store their data on the heap. The data stored inside RawSyntax is larger than 3 words and thus every time you pass a value around as a e.g. an ExprSyntaxProtocol, a new heap allocation will be made and that data needs to be reference-counted, which causes a very noticeable performance overhead.

swift-syntax offers two alternatives:

• When passing a single node around, use some ExprSyntaxProtocol. This allows the concrete expression node (e.g. an IntegerLiteralExprSyntax) to be passed directly without the need to wrap it in an existential and thus avoid the performance overhead.
• When multiple expression nodes need to be represented that might be of different types, eg. in an array of expressions, use the ExprSyntax type. ExprSyntax is a struct and can thus be allocated on the stack. The downside is that specific expression nodes need to explicitly be upcast to ExprSyntax (ExprSyntax(integerLiteral)) and downcast to more specific types using a custom as method (expr.as(IntegerLiteralExprSyntax.self)).