Markertrait

Marker trait is a term used to describe a trait that provides no methods or associated items and exists solely to convey information about a type to the compiler. Marker traits are used in generic programming to express constraints without requiring any behavior from the type. By using a marker trait in a trait bound, code can be conditionally compiled or optimized based on properties of the type rather than its functionality.

In Rust, common examples include Copy, Send, Sync, and Sized. Copy indicates that values can be duplicated with a simple bitwise copy. Send and Sync describe thread-safety properties and determine whether a type can be transferred or shared between threads. Sized indicates that a type has a known, fixed size. Some marker traits are auto traits, meaning the compiler automatically implements them for a type when its components satisfy the necessary conditions; others may be implemented explicitly or derived.

Marker traits are especially useful for enforcing safety and correctness at compile time. They enable generic functions and data structures to constrain their type parameters in meaningful ways without requiring additional methods or data on the type.

Beyond Rust, the concept resembles marker interfaces in other languages, such as Java’s Serializable or Cloneable, which tag classes with metadata used by tooling or runtime checks. While related, marker traits focus on type-level information within a language’s trait system, and marker interfaces focus on interface-based tagging in class-based languages.

See also: trait bounds, auto traits, marker interfaces.