# Common traits

### Eagerly implement common traits. [FIXME: needs RFC]

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:

- Crate `std` defines trait `Debug`.
- Crate `url` defines type `Url`, without implementing `Debug`.
- Crate `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

#### When safe, derive or otherwise implement `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

### Prefer to derive, rather than implement. [FIXME: needs RFC]

Deriving saves implementation effort, makes correctness trivial, and automatically adapts to upstream changes.

### Do not overload operators in surprising ways. [FIXME: needs RFC]

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.

### The `Drop` trait

The `Drop` trait is treated specially by the compiler as a way of associating destructors with types. See the section on destructors for guidance.

### The `Deref`/`DerefMut` traits

#### Use `Deref`/`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.

#### Do not fail within a `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.

#### Avoid inherent methods when implementing `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.