Create a struct pointing to another - rust

Question

How can I create the Item instance pointing to the database received as parameter?
struct Something {}
struct Database<'a> {
something: &'a Something,
}
struct Item<'a> {
database: &'a mut Database<'a>,
}
impl<'a> Item<'a> {
fn new(database: &'a mut Database) -> Self {
let mut obj = Self { database };
obj
}
}
This produces the error:
error[E0308]: mismatched types
--> src/main.rs:16:13
|
16 | database
| ^^^^^^^^ lifetime mismatch
|
= note: expected type `&'a mut Database<'a>`
found type `&'a mut Database<'_>`
note: the lifetime 'a as defined on the impl at 13:1...
--> src/main.rs:13:1
|
13 | / impl<'a> Item<'a> {
14 | | fn new(database: &'a mut Database) -> Self {
15 | | let mut obj = Self {
16 | | database
... |
20 | | }
21 | | }
| |_^
note: ...does not necessarily outlive the anonymous lifetime #1 defined on the method body at 14:5
--> src/main.rs:14:5
|
14 | / fn new(database: &'a mut Database) -> Self {
15 | | let mut obj = Self {
16 | | database
17 | | };
18 | |
19 | | obj
20 | | }
| |_____^
I do not understand if the error is in the return type (Self) or another thing.

Answer 1

You defined Database to require a lifetime parameter. That means you also have to give it one. The error message is really helpful here and your problem is fixed by simply doing what it says:
expected type `&'a mut Database<'a>`
found type `&'a mut Database<'_>`
It suffices to simply add <'a> to Database in the signature of new.
N.B.: Always read error messages from top to bottom. Usually by resolving the first error all the successive ones disappear.
struct Something {}
struct Database<'a> {
something: &'a Something
}
struct Item<'a> {
database: &'a mut Database<'a>
}
impl<'a> Item<'a> {
fn new(database: &'a mut Database<'a>) -> Self {
let mut obj = Self {
database
};
obj
}
}
fn main() {
let something = &Something {};
let mut database = Database { something };
let item = Item::new(&mut database);
}
Playground