Mismatched types shows 2 equal types

[marcoow]

I'm trying to add an async function argument to a function and run into a situation where it get a mismatched types error although the types I'm presented in the error message are actually both identical:

   = note: expected trait `for<'r> <for<'r> fn(&'r str, std::string::String) -> impl for<'r> Future<Output = u16> {request_sendgrid} as FnOnce<(&'r str, std::string::String)>>`
              found trait `for<'r> <for<'r> fn(&'r str, std::string::String) -> impl for<'r> Future<Output = u16> {request_sendgrid} as FnOnce<(&'r str, std::string::String)>>`

I'm aware I'm almost 100% certainly doing sth. wrong, still the note there seems probably wrong as well?

I tried this code:

pub async fn send_message<Fut>(payload: Payload, api_key: &str, sendgrid: impl FnOnce(&str, String) -> Fut) -> Result<Response>
where
Fut: Future<Output = u16>, {
    let message = payload.message.trim();
    let message = if !message.is_empty() { message } else { "–" };

    let data = json!({
        "personalizations": [{
            "to": [
                { "email": "contact@domain.tld", "name": "My Domain" }
            ]}
        ],
        "from": { "email": "no-reply@domain.tld", "name": format!("{} via domain.tld", payload.name) },
        "reply_to": { "email": payload.email, "name": payload.name },
        "subject": "Inquiry",
        "content": [{
            "type": "text/plain",
            "value": message
        }]
    });

    let client = reqwest::Client::new();
    let result = client
        .post("https://api.sendgrid.com/v3/mail/send")
        .header("Authorization", format!("Bearer {}", api_key))
        .header("Content-Type", "application/json")
        .body(data.to_string())
        .send()
        .await;

    match result {
        Ok(response) => match response.status() {
            reqwest::StatusCode::ACCEPTED => Response::ok(""),
            _ => Response::error("Bad Gateway", 502),
        },
        Err(_) => Response::error("Internal Server Error", 500),
    }
}

async fn request_sendgrid(api_key: &str, data: String) -> u16 {
    let client = reqwest::Client::new();
    let result = client
        .post("https://api.sendgrid.com/v3/mail/send")
        .header("Authorization", format!("Bearer {}", api_key))
        .header("Content-Type", "application/json")
        .body(data)
        .send()
        .await;

    match result {
        Ok(response) => response.status().as_u16(),
        Err(_) => 500u16,
    }
}

I expected to see this happen: 2 different types in the error message

Instead, this happened: 2 equal types in the error message

Meta

rustc --version --verbose:

rustc 1.64.0 (a55dd71d5 2022-09-19)
binary: rustc
commit-hash: a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52
commit-date: 2022-09-19
host: aarch64-apple-darwin
release: 1.64.0
LLVM version: 14.0.6

Backtrace

   Compiling mainmatter-website-mailer v0.0.0 (/Users/marcoow/Code/mainmatter-website-mailer)
error[E0308]: mismatched types
  --> src/lib.rs:43:32
   |
43 |                 Ok(payload) => send_message(payload, &api_key, &request_sendgrid).await,
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected trait `for<'r> <for<'r> fn(&'r str, std::string::String) -> impl for<'r> Future<Output = u16> {request_sendgrid} as FnOnce<(&'r str, std::string::String)>>`
              found trait `for<'r> <for<'r> fn(&'r str, std::string::String) -> impl for<'r> Future<Output = u16> {request_sendgrid} as FnOnce<(&'r str, std::string::String)>>`
note: the lifetime requirement is introduced here
  --> src/lib.rs:53:104
   |
53 | pub async fn send_message<Fut>(payload: Payload, api_key: &str, sendgrid: impl FnOnce(&str, String) -> Fut) -> Result<Response>
   |                                                                                                        ^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `mainmatter-website-mailer` due to previous error

[Noratrieb]

Thanks, that does look like a bug in the error message. Could you create a more minimal example for easier reproduction, or at least share the full code needed for reproducing the error (if your code is public, you could create a branch for this isse with the broken state)?

[lukas-code]

This is the same error message as #102400

[marcoow]

nvm, closing 👍