A trait bound constrains a generic type parameter to types implementing a specified trait. It lets generic code rely on shared behavior without naming a concrete type: a function that calls `t.describe()` needs a bound promising that `T` provides `describe`. section 2.2.4 section 2.3
fn describe_type<T>(t: &T) -> String
where
T: SelfDescribing,
{
t.describe()
}
The `where T: SelfDescribing` clause makes `t.describe()` valid. For a short bound, `fn describe_type<T: SelfDescribing>(t: &T) -> String` expresses the same requirement inline; `where` can be easier to read when bounds grow. section 2.2.4
Without the bound, the compiler reports that `describe` is unavailable on `&T`. Adding the bound fixes the function definition, but callers must still supply a type implementing `SelfDescribing`; otherwise compilation fails at the call. section 2.2.4
#[derive(Clone)]
struct ListItem<T>
where
T: Clone + Debug,
{
data: Box<T>,
next: Option<Box<ListItem<T>>>,
}
Bounds also apply to generic structs. Here `T: Clone + Debug` requires both traits for `ListItem<T>`. The `Clone` bound accompanies the struct’s derived `Clone` implementation. section 2.1.6
Choose a method in generic code
Suppose two traits implemented for one type both define `name`. Calling `container.name()` is ambiguous—even giving the methods different argument lists does not resolve it. Separate functions bounded by `T: First` and `T: Second` can call the intended trait method in each context. A generic `get_name<T>` with no bound cannot call `t.name()` at all. section 5.2.4
impl<T: Display> ToString for T {
// ...
}
Bounds can govern implementations as well as functions. This blanket implementation provides `ToString` for any `T` implementing `Display`, rather than requiring a separate implementation for every qualifying type. section 7.4
Use a bound when generic code needs a trait’s behavior or when an implementation should apply only to qualifying types. A bound is a requirement, not an implementation: the required trait must still be implemented for each concrete type, unless an applicable blanket implementation supplies it. section 2.2.4 section 7.4
