Why does this `impl Executor` does not live long enough?

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1

REPRODUCTION: undefined | Rust Explorer

I apologize in advance if the code seems confusing or complex for no apparent reason: this is just a minimal example compared to the real code (which is much more complex and bigger).

I would like to understand why this error:

error: `impl Executor` does not live long enough
   --> src/main.rs:136:5
    |
136 | /     async move {
137 | |         dbg!("I'm:", user);
138 | |
139 | |         let mut store = store.unwrap_or_default();
...   |
169 | |     }
170 | |     .boxed()
    | |____________^

The reason I'm using the trait Executor is because I need to use both DB connections and transactions in my services layer (represented here by team_list).

I can change everything in this code, the trait too.

Can you help me?

3

Ignoring where the 'static constraint comes from, but perhaps addressing the problem...

I see you already doing this elsewhere:

impl<E: 'static + Executor> TeamTrait for Repo<E> {

The corresponding bound where you're getting an error would be:

    db: &'a mut (impl Executor + 'static),

If it's reasonable that Executor never needs to hold on to something borrowed, you could

-pub trait Executor: Send + Sync {
+pub trait Executor: Send + Sync + 'static {

And then it will be implied everywhere, and you can use

impl<E: Executor> TeamTrait for Repo<E> {
// ...
    db: &'a mut impl Executor,
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4

Wow. This seems to work! But now I'm terrified of what might happen in the future with this change.

5

If you remove Send from the Boxing the future is fine with lifetimes. Not that you would find that useful in full program.

6

Why does an error prompt when there is no 'static added? Thank you for your reply

7

I have tested and found that the db.as_executor() passed in when calling fetch_all caused the lifetime check to fail.
Is it because of the lifetime 'this defined in the associated type, or because of the lifetime restriction in sqlx::executor::Executor<'c> ? Thank you for your reply

8

I believe it's some form of this bug.

For example.

1 Like
9

Thank you very much for your reply. I have been thinking about this issue for a long time.

10

More directly, probably.

Based on comments in that bug, removing the where Self: 'this bound on the GAT may also work around the issue.

pub trait Executor: Send + Sync {
    type Executor<'this>: Send + Sync; // + sqlx::PgExecutor<'this>

    fn _disable_lint(e: Self::Executor<'_>) -> Self::Executor<'_> { e }
2 Likes