Confusing compile error messages about algebraic data type

[fukatani]

I tried to compile this code:

async fn update_first_ac_of_problems<'a, I: Iterator<Item = &'a int>> (
    &self,
    first_ac_submissions: I,
) -> Result<()> {

I got compile error message,

error: future cannot be sent between threads safely
  --> sql-client/src/first_ac_submissions.rs:19:21
   |
19 |       ) -> Result<()> {
   |  _____________________^
20 | |         Ok(())
21 | |     }
   | |_____^ future created by async block is not `Send`
   |
note: captured value is not `Send`
  --> sql-client/src/first_ac_submissions.rs:18:9
   |
18 |         first_ac_submissions: I,
   |         ^^^^^^^^^^^^^^^^^^^^ has type `I` which is not `Send`
   = note: required for the cast to the object type `dyn futures::Future<Output = std::result::Result<(), anyhow::Error>> + std::marker::Send`
help: consider further restricting this bound
   |
16 |     async fn update_first_ac_of_problems<'a, I + std::marker::Send: Iterator<Item = &'a i32>> (
   |                                                ^^^^^^^^^^^^^^^^^^^

I expect 16 | async fn update_first_ac_of_problems<'a, I: Iterator<Item = &'a i32> + std::marker::Send> ( is right.

Meta

rustc --version --verbose:

rustc 1.51.0 (2fd73fabe 2021-03-23)

Backtrace

<backtrace>