Values does not live long enough in constructor and setter in OOP Rust - oop

Question

I have the following code:
//! # Messages
/// Represents a simple text message.
pub struct SimpleMessage<'a> {
pub user: &'a str,
pub content: &'a str,
}
impl<'a> SimpleMessage<'a> {
/// Creates a new SimpleMessage.
fn new_msg(u: &'a str, c: &'a str) -> SimpleMessage<'a> {
SimpleMessage { user: u,
content: &c.to_string(), }
}
/// Sets a User in a Message.
pub fn set_user(&mut self, u: User<'a>){
self.user = &u;
}
}
But $ cargo run returns:
error[E0597]: borrowed value does not live long enough
--> src/messages.rs:34:35
|
34 | content: &c.to_string(), }
| ^^^^^^^^^^^^^ temporary value does not live long enough
35 | }
| - temporary value only lives until here
|
note: borrowed value must be valid for the lifetime 'a as defined on the impl at 28:1...
|
28 | impl<'a> SimpleMessage<'a> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
error[E0597]: `u` does not live long enough
|
54 | self.user = &u;
| ^ borrowed value does not live long enough
55 | }
| - borrowed value only lives until here
|
note: borrowed value must be valid for the lifetime 'a as defined on the impl at 28:1...
--> src/messages.rs:28:1
|
28 | impl<'a> SimpleMessage<'a> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
I've tried changing the borrowing format of the variables at the function signature and it's contents with no success, it doesn't seem as a borrowing issue, but I don't really understand it, as the lifetime <'a> defined at pub struct SimpleMessage<'a> clearly designates the longest lifetime to it's components, and the impl<'a> SimpleMessage<'a> uses the same lifetime.
What am I missing?
Similar Question:
“borrowed value does not live long enough” when using the builder pattern
Doesn't really help resolve this issue.

Answer 1

str.to_string() will create an owned String, which will not live longer than the new_msg method, so you will not be able to pass a slice of it anywhere. Instead, just use the &str argument, since it is valid for the lifetime 'a, which is what you need.
/// Creates a new SimpleMessage.
fn new_msg(u: &'a User, c: &'a str) -> SimpleMessage<'a> {
SimpleMessage { user: u, content: c, }
}
The other method also has a problem. You are trying to give an owned User, but the SimpleMessage struct requires a reference. It should look like this:
/// Sets a User in a Message.
pub fn set_user(&mut self, u: &'a User<'a>){
self.user = u;
}

Answer 2

You don't need to_string if you want to store a reference to a string. Also, set_user must also take a reference, not a value (because there's no field in the struct to store it):
pub struct User<'a> {
pub data: &'a u8,
}
/// Represents a simple text message.
pub struct SimpleMessage<'a> {
pub user: &'a User<'a>,
pub content: &'a str,
}
impl<'a> SimpleMessage<'a> {
fn new_msg(user: &'a User, content: &'a str) -> SimpleMessage<'a> {
SimpleMessage { user, content }
}
pub fn set_user(&mut self, user: &'a User<'a>) {
self.user = user;
}
}
fn main() {
let data1 = 1;
let data2 = 2;
let user1 = User { data: &data1 };
let user2 = User { data: &data2 };
let mut msg = SimpleMessage::new_msg(&user1, "test");
msg.set_user(&user2);
}
Playground
If you want to save strings that are created at runtime (for example, with format!() call) you may want to store a String:
pub struct SimpleMessage<'a> {
pub user: &'a User<'a>,
pub content: String,
}
. . .
fn new_msg(user: &'a User, content: String) -> SimpleMessage<'a> {
SimpleMessage { user, content }
}
. . .
let mut msg = SimpleMessage::new_msg(&user1, format!("created at {}", "runtime"));
Playground