Generic Traits

Traits can also be generic, just like types and functions. A trait's parameters get concrete types when it is used. For example the From<T> trait is used to define type conversions:

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
pub trait From<T>: Sized {
    fn from(value: T) -> Self;
}

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
#[derive(Debug)]
struct Foo(String);

impl From<u32> for Foo {
    fn from(from: u32) -> Foo {
        Foo(format!("Converted from integer: {from}"))
    }
}

impl From<bool> for Foo {
    fn from(from: bool) -> Foo {
        Foo(format!("Converted from bool: {from}"))
    }
}

fn main() {
    let from_int = Foo::from(123);
    let from_bool = Foo::from(true);
    dbg!(from_int);
    dbg!(from_bool);
}

• The From trait will be covered later in the course, but its definition in the std docs is simple, and copied here for reference.

• Implementations of the trait do not need to cover all possible type parameters. Here, Foo::from("hello") would not compile because there is no From<&str> implementation for Foo.

• Generic traits take types as "input", while associated types are a kind of "output" type. A trait can have multiple implementations for different input types.

• In fact, Rust requires that at most one implementation of a trait match for any type T. Unlike some other languages, Rust has no heuristic for choosing the "most specific" match. There is work on adding this support, called specialization.