rust-lang / rust-lang/rust

Adding explicit lifetimes causes compilation error

Open
#131,336 3 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

A-async-await A-lifetimes C-discussion
Dominant language
Rust
Stars
119k
Forks
16.1k
PR merge metrics
PR metrics pending

Description

Compiler version 1.81.0.

I am writing an asynchronous function that takes asynchronous callback, I managed to write it so that it compiles:

pub trait Callback<State> {
    type Output: Future<Output = ()>;

    fn call(&self, state: State) -> Self::Output;
}

impl<State, F, Fut> Callback<State> for F
where
    F: Fn(State) -> Fut,
    Fut: Future<Output = ()>,
{
    type Output = Fut;

    fn call(&self, state: State) -> Self::Output {
        self(state)
    }
}

pub async fn aaa<State, Cb>(state: &mut State, callback: &Cb)
where
    for<'state> Cb: Callback<&'state mut State>,
{
}

pub async fn bbb<State, Cb>(state: &mut State) {
    async fn callback<State>(state: &mut State) {}
    aaa(state, &callback).await;
}

However, if I try to add explicit lifetimes to the Callback trait, I get compilation error:

error[E0309]: the parameter type `State` may not live long enough
   --> src/storage.rs:252:33
    |
250 | impl<'state, State, F, Fut> Callback<'state, State> for F
    |      ------ the parameter type `State` must be valid for the lifetime `'state` as defined here...
251 | where
252 |     F: Fn(&'state mut State) -> Fut,
    |                                 ^^^ ...so that the reference type `&'state mut State` does not outlive the data it points at
    |
help: consider adding an explicit lifetime bound
    |
250 | impl<'state, State: 'state, F, Fut> Callback<'state, State> for F
    |                   ++++++++

for the following code:

pub trait Callback<'state, State> {
    type Output: Future<Output = ()>;

    fn call(&self, state: &'state mut State) -> Self::Output;
}

impl<'state, State, F, Fut> Callback<'state, State> for F
where
    F: Fn(&'state mut State) -> Fut,
    Fut: Future<Output = ()>,
{
    type Output = Fut;

    fn call(&self, state: &'state mut State) -> Self::Output {
        self(state)
    }
}

pub async fn aaa<State, Cb>(state: &mut State, callback: &Cb)
where
    for<'state> Cb: Callback<'state, State>,
{
}

pub async fn bbb<State, Cb>(state: &mut State) {
    async fn callback<State>(state: &mut State) {}
    aaa(state, &callback).await;
}

But if I fix it according to the suggestion, I get the following error:

error[E0310]: the parameter type `State` may not live long enough
   --> src/storage.rs:270:5
    |
270 |     aaa(state, &callback).await;
    |     ^^^^^^^^^^^^^^^^^^^^^
    |     |
    |     the parameter type `State` must be valid for the static lifetime...
    |     ...so that the type `State` will meet its required lifetime bounds
    |
help: consider adding an explicit lifetime bound
    |
268 | pub async fn bbb<State: 'static, Cb>(state: &mut State) {
    |                       +++++++++

error[E0310]: the parameter type `State` may not live long enough
   --> src/storage.rs:270:27
    |
270 |     aaa(state, &callback).await;
    |                           ^^^^^
    |                           |
    |                           the parameter type `State` must be valid for the static lifetime...
    |                           ...so that the type `State` will meet its required lifetime bounds
    |
help: consider adding an explicit lifetime bound
    |
268 | pub async fn bbb<State: 'static, Cb>(state: &mut State) {
    |                       +++++++++

for the code

pub trait Callback<'state, State> {
    type Output: Future<Output = ()>;

    fn call(&self, state: &'state mut State) -> Self::Output;
}

impl<'state, State: 'state, F, Fut> Callback<'state, State> for F
where
    F: Fn(&'state mut State) -> Fut,
    Fut: Future<Output = ()>,
{
    type Output = Fut;

    fn call(&self, state: &'state mut State) -> Self::Output {
        self(state)
    }
}

pub async fn aaa<State, Cb>(state: &mut State, callback: &Cb)
where
    for<'state> Cb: Callback<'state, State>,
{
}

pub async fn bbb<State, Cb>(state: &mut State) {
    async fn callback<State>(state: &mut State) {}
    aaa(state, &callback).await;
}

I thing something is not correct about treating explicit lifetimes by the compiler, or I added the lifetimes incorrectly.
Please help.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reducing the Rust 1.81.0 reproducer around the Callback trait, explicit lifetimes, and the aaa/bbb async functions, then compile both versions to compare diagnostics. Read the lifetime and async-trait handling involved; done means determining whether the errors are expected or producing a focused compiler fix or issue resolution with regression coverage.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.