Trouble with Rust Lifetime in Generic function [duplicate] - rust

Question

This question already has answers here:
How to fix lifetime error when function returns a serde Deserialize type?
(2 answers)
Closed 3 years ago.
I have a simple function that I want to make generic, in rust. I am getting a lifetime error. I am still getting the hang of the lifetime side of rust.
The function simply converts 1 struct into another using serde's serialization.
Here is a a rust playground with the full simple scenario.
Code:
pub fn convert<'de: 'a, 'a, T>(from: &'a Left, print: bool) -> (T, &'a Left)
where
T: Deserialize<'de> + std::fmt::Debug {
let serialized = serde_json::to_string(&from);
let serialized = serialized.unwrap();
let deserialized: T;
{
let deserialized_raw = serde_json::from_str(&serialized);
deserialized = deserialized_raw.unwrap();
}
if print {
println!("-------------A-----------------------------------");
println!("serialized = {}", &serialized);
println!("--------------B----------------------------------");
println!("deserialized = {:?}", deserialized);
println!("--------------C----------------------------------");
};
(deserialized, from)
}
Error:
error[E0597]: `serialized` does not live long enough
--> src/main.rs:38:49
|
30 | pub fn convert<'de: 'a, 'a, T>(from: &'a Left, print: bool) -> (T, &'a Left)
| --- lifetime `'de` defined here
...
38 | let deserialized_raw = serde_json::from_str(&serialized);
| ---------------------^^^^^^^^^^^-
| | |
| | borrowed value does not live long enough
| argument requires that `serialized` is borrowed for `'de`
...
49 | }
| - `serialized` dropped here while still borrowed
I tried this a few ways with and without lifetimes. I tried adding blocks to see if that changes things with no luck.
Any thoughts on what I am doing wrong?
Edited:
- Added the full compiler error output

Answer 1

To the compiler, lifetime parameters always represent lifetimes that live strictly longer than the function call. However, here you're trying to use a local variable and claim it has lifetime 'de, which is impossible because it's a local variable, thus it has a lifetime shorter than the function call.
In order to mix lifetime parameters in traits with local variables, we must use higher-rank trait bounds. We want T to implement Deserialize<'de> for every lifetime 'de (not just one specific lifetime chosen by the caller). This is written like this (note that we can now elide the 'a lifetime):
pub fn convert<T>(from: &Left, print: bool) -> (T, &Left)
where
T: for<'de> Deserialize<'de> + std::fmt::Debug
{
// no changes here
}