How do you create a generic function in Rust with a trait requiring a lifetime?

Question

I am trying to write a trait which works with a database and represents something which can be stored. To do this, the trait inherits from others, which includes the serde::Deserialize trait.

trait Storable<'de>: Serialize + Deserialize<'de> {
    fn global_id() -> &'static [u8];
    fn instance_id(&self) -> Vec<u8>;
}

struct Example {
    a: u8,
    b: u8
}

impl<'de> Storable<'de> for Example {
    fn global_id() -> &'static [u8] { b"p" }
    fn instance_id(&self) -> Vec<u8> { vec![self.a, self.b] }
}

Next, I am trying to write this data using a generic function:

pub fn put<'de, S: Storable>(&mut self, obj: &'de S) -> Result<(), String> {
    ...
    let value = bincode::serialize(obj, bincode::Infinite);
    ...
    db.put(key, value).map_err(|e| e.to_string())
}

However, I am getting the following error:

error[E0106]: missing lifetime specifier
--> src/database.rs:180:24
    |
180 |     pub fn put<'de, S: Storable>(&mut self, obj: &'de S) -> Result<(), String> {
    |                        ^^^^^^^^ expected lifetime parameter

Minimal example on the playground.

How would I resolve this, possibly avoid it altogether?

Answer 1

You have defined Storable with a generic parameter, in this case a lifetime. That means that the generic parameter has to be propagated throughout the entire application:

fn put<'de, S: Storable<'de>>(obj: &'de S) -> Result<(), String> { /* ... */ }

You can also decide to make the generic specific. That can be done with a concrete type or lifetime (e.g. 'static), or by putting it behind a trait object.

Serde also has a comprehensive page about deserializer lifetimes. It mentions that you can choose to use DeserializeOwned as well.

trait Storable: Serialize + DeserializeOwned { /* ... */ }

You can use the same concept as DeserializeOwned for your own trait as well:

trait StorableOwned: for<'de> Storable<'de> { }

fn put<'de, S: StorableOwned>(obj: &'de S) -> Result<(), String> {

Answer 2

You have the 'de lifetime in the wrong place -- you need it to specify the argument to Storable, not the lifetime of the reference obj.

Instead of

fn to_json<'de, S: Storable>(obj: &'de S) -> String {

use

fn to_json<'de, S: Storable<'de>>(obj: &S) -> String {

Playground.

The lifetime of obj doesn't actually matter here, because you're not returning any values derived from it. All you need to prove is that S implements Storable<'de> for some lifetime 'de.

If you want to eliminate the 'de altogether, you should use DeserializeOwned, as the other answer describes.