Rust's trait system does not allow orphans: roughly, every impl must live
either in the crate that defines the trait or the implementing
type. Consequently, crates that define new types should eagerly implement all
applicable, common traits.
To see why, consider the following situation:
std defines trait Debug.url defines type Url, without implementing Debug.webapp imports from both std and url,There is no way for webapp to add Debug to url, since it defines neither.
(Note: the newtype pattern can provide an efficient, but inconvenient
workaround; see newtype for views)
The most important common traits to implement from std are:
Clone, Debug, Hash, Eq
Send and Share. [FIXME][FIXME]. This guideline is in flux while the "opt-in" nature of built-in traits is being decided. See https://github.com/rust-lang/rfcs/pull/127
Deriving saves implementation effort, makes correctness trivial, and automatically adapts to upstream changes.
Operators with built in syntax (*, |, and so on) can be provided for a type
by implementing the traits in core::ops. These operators come with strong
expectations: implement Mul only for an operation that bears some resemblance
to multiplication (and shares the expected properties, e.g. associativity), and
so on for the other traits.
Drop traitThe Drop trait is treated specially by the compiler as a way of
associating destructors with types. See
the section on destructors for
guidance.
Deref/DerefMut traitsDeref/DerefMut only for smart pointers. [FIXME: needs RFC]The Deref traits are used implicitly by the compiler in many circumstances,
and interact with method resolution. The relevant rules are designed
specifically to accommodate smart pointers, and so the traits should be used
only for that purpose.
Deref/DerefMut implementation. [FIXME: needs RFC]Because the Deref traits are invoked implicitly by the compiler in sometimes
subtle ways, failure during dereferencing can be extremely confusing. If a
dereference might not succeed, target the Deref trait as a Result or
Option type instead.
Deref/DerefMut [FIXME: needs RFC]The rules around method resolution and Deref are in flux, but inherent methods
on a type implementing Deref are likely to shadow any methods of the referent
with the same name.