Caution
You’re reading a draft of the Ferrocene Language Specification. Some parts of this document might be missing, incomplete or incorrect. Our aim is to have the specification ready by the end of 2022.
11. Implementations¶
Syntax
Implementation::=InherentImplementation|TraitImplementationInherentImplementation::= implGenericParameterList?ImplementingTypeWhereClause? {InnerAttributeOrDoc*AssociatedItem* }TraitImplementation::= unsafe? implGenericParameterList? !?ImplementedTraitforImplementingTypeWhereClause? {InnerAttributeOrDoc*AssociatedItem* }ImplementingType::=TypeSpecificationImplementedTrait::=TypePath
Legality Rules
11:1 An implementation is an item that supplements an implementing type by extending its functionality.
11:2 An implementing type is the type that the associated items of an implementation are associated with.
11:3 An inherent implementation is an implementation that adds direct functionality.
11:4 An implementing type may have multiple inherent implementations. Inherent implementations of the same implementing type share a common scope.
11:5 Inherent implementations of the same implementing type shall be defined within the same crate.
11:6 A trait implementation is an implementation that adds functionality specified by a trait.
11:7
An unsafe trait implementation is a trait implementation subject to
keyword unsafe.
11:8 An implemented trait is a trait whose functionality has been implemented by an implementing type.
11:9 The type path of a trait implementation shall resolve to a trait.
11:10 A trait implementation shall be an unsafe trait implementation if and only if it implements an unsafe trait.
11:11 Trait implementations are subject to implementation coherence and implementation conformance.
11:12 Inherent implementations of the same implementing type shall not define more than one associated item with the same name in the same namespace.
Examples
trait Shape {
fn area(self) -> f64;
}
11:13
Circle is an implementing type.
struct Circle {
radius: f64
}
11:14 The following is an inherent implementation:
impl Circle {
fn set_radius(mut self, new_radius: f64) {
self.radius = new_radius;
}
}
11:15 The following is a trait implementation:
impl Shape for Circle {
fn area(self) -> f64 {
self.radius.powi(2) * std::f64::consts::PI
}
}
11.1. Implementation Coherence¶
Legality Rules
11.1:1 A trait implementation exhibits implementation coherence when it is valid and does not overlap with another trait implementation.
11.1:2 Two trait implementations of the same implemented trait overlap when the intersection of the implementing types is non-empty.
11.1:3
Given trait implementation impl<P1, P2, .., PN> Trait<T1, T2, .., TN> for
T0, the trait implementation is considered valid when
11.1:4
Traitis a local trait, or11.1:5 All of
11.1:6 At least one of types
T0, T1, .., TNis a local type.
11.1:7 No type parameter of
P1, P2, .., PNthat is not used in another type may appear in the non-local types ofT0, T1, .., TN.
11.1:8 A trait implementation shall be coherent.
11.2. Implementation Conformance¶
Legality Rules
11.2:1 A trait implementation exhibits implementation conformance when it satisfies the constraints of its implemented trait.
11.2:2 An associated trait constant is conformant with an associated constant of an implemented trait when
11.2:3 The names of both associated constants are the same, and
11.2:4 The types of both associated constants are the same.
11.2:5 An associated trait function is conformant with an associated function of an implemented trait when the function signatures of both functions are the same.
11.2:6 The function signature of the associated function of the implemented trait is a subtype of the function signature of the associated trait function, and
11.2:7 The bounds of the associated function of the implemented trait are more general that the bounds of the associated trait function.
11.2:8 An associated type of a trait implementation is conformant with an associated type of an implemented trait when:
11.2:10 The type specification of the associated type of the implemented trait conforms to the bounds of the associated type of the trait implementation.
11.2:11 A trait implementation is conformant with an implemented trait when:
11.2:12 The trait implementation has a conformant associated constant for each associated constant of the implemented trait, unless the associated constant of the implemented trait has a default value,
11.2:13 The trait implementation has a conformant associated function for each associated function of the implemented trait, unless the associated function of the implemented trait has a default implementation in the implemented trait,
11.2:14 The trait implementation has a conformant associated type for each associated type of the implemented trait.
11.2:15 A trait implementation shall be conformant.