Trait Constraints

Traits are interface-like declarations that constrain generic types to implement specific methods or contain specific properties.

Declaration

<trait_declaration> ::=
    ["pub"] "trait" <ident> [<type_parameters>] [<trait_constraints>] "{"
    (
        | <type_declaration>
        | <type_assignment>
        | <constant_declaration>
        | <constant_assignment>
        | <function_declaration>
        | <function_assignment>
    )*
    "}" ;

Dependencies:

• <ident>
• <type_parameters>
• <type_declaration>
• <type_assignment>
• <constant_declaration>
• <constant_assignment>
• <function_declaration>
• <function_assignment>

The <trait_declaration> is a declaration of a set of associated types, constants, and functions that may itself take type parameters and may be constrained to a super type. Semantics of the declaration are listed under trait solving rules.

Constraints

<trait_constraints> ::= ":" <ident> ("&" <ident>)* ;

Dependencies:

• <ident>

The <trait_constraints> contains a colon followed by an ampersand separated list of identifiers of implemented traits. The ampersand is meant to indicate that all of the trait identifiers are implemented for the type.

Semantics

Traits can be defined with associated types, constants, and functions. The trait declaration itself allows for optional assignment for each item as a default. Any declarations in the trait that are not assigned in the trait declaration must be assigned in the implementation of the trait for the data type. Additionally, any assignments in the trait declaration can be overridden in the trait implementation.

While types can depend on trait constraints, traits can also depend on other trait constraints. These assert that types that implement a given trait also implement its "super traits".

Solving

todo