missing rust_2021_incompatible_closure_captures with async block boxed future thingy

[ehuss]

Given the following code:

#![allow(unused)]
// #![warn(rust_2021_incompatible_closure_captures)]
#![feature(capture_disjoint_fields)]
use futures::future::BoxFuture;
use futures::FutureExt;

struct Connect {
    host: String,
    insecure: bool,
}

pub struct GremlinContext;

pub enum Command {
    Print(Option<String>),
    Exec(Box<dyn FnOnce(&GremlinContext) -> BoxFuture<'static, Vec<Command>> + Send>),
}

struct ConnectionOptions<'a> {
    host: &'a str,
    insecure: bool,
}

fn handle(args: Vec<String>) -> Vec<Command> {
    let connect = Connect {
        host: "".to_string(),
        insecure: false,
    };
    let task = |_ctx: &GremlinContext| {
        let future = async move {
            let options = ConnectionOptions {
                host: connect.host.as_str(),
                insecure: connect.insecure,
            };
            vec![Command::Print(Some("Connected!".into()))]
        };
        future.boxed()
    };

    vec![Command::Exec(Box::new(task))]
}
fn main() {}

With feature disabled, and the warning enabled, there is no warning issued. With the feature enabled, this fails with the error:

error[E0597]: `connect.insecure` does not live long enough
  --> src/main.rs:33:27
   |
29 |     let task = |_ctx: &GremlinContext| {
   |                ----------------------- value captured here
...
33 |                 insecure: connect.insecure,
   |                           ^^^^^^^^^^^^^^^^ borrowed value does not live long enough
...
40 |     vec![Command::Exec(Box::new(task))]
   |                        -------------- cast requires that `connect.insecure` is borrowed for `'static`
41 | }
   | - `connect.insecure` dropped here while still borrowed

I think it is missing a suggestion for let _ = &connect in the closure.

Found in the 2021 crater run for https://crater-reports.s3.amazonaws.com/pr-87190-3/try%23a7a572ce3edd6d476191fbfe92c9c1986e009b34/reg/gremlin-cli-0.1.0/log.txt

rustc 1.56.0-nightly (ac50a5335 2021-08-27)
binary: rustc
commit-hash: ac50a53359328a5d7f2f558833e63d59d372e4f7
commit-date: 2021-08-27
host: x86_64-apple-darwin
release: 1.56.0-nightly
LLVM version: 13.0.0

cc @rust-lang/wg-rfc-2229

[arora-aman]

@rustbot claim

So this is a bug in the capture analysis. The inner closure captures the field insecure by move, but where we propogate captures from inner to outer closers, we treat move of copy types as an ImmBorrow

[arora-aman]

So upon further thought the capture analysis doing the correct thing. We'd have seen similar behavior on Rust 2018.

I tried the following code that only uses a boolean within the closure:

#![allow(unused)]

use futures::future::BoxFuture;
use futures::FutureExt;

pub struct GremlinContext;

pub enum Command {
    Print(Option<String>),
    Exec(Box<dyn FnOnce(&GremlinContext) -> BoxFuture<'static, Vec<Command>> + Send>),
}

fn handle(args: Vec<String>) -> Vec<Command> {
    let f = false;

    let task = |_ctx: &GremlinContext| {
        let future = async move {
            let options = f;
            vec![Command::Print(Some("Connected!".into()))]
        };
        future.boxed()
    };

    vec![Command::Exec(Box::new(task))]
}
fn main() {}

I tried a rust build from early 2020 and it failed

❯ cargo bisect-rustc --test-dir=foo --start=2020-01-01

installing nightly-2020-01-01
rustc for x86_64-unknown-linux-gnu: 102.98 KB / 57.99 MB  0.17 % 1019.28 KB/s 5cargo for x86_64-unknown-linux-gnu: 4.80 MB / 4.80 MB [===] 100.00 % 4.47 MB/s testing...
RESULT: nightly-2020-01-01, ===> Yes
uninstalling nightly-2020-01-01

ERROR: the start of the range (nightly-2020-01-01) must not reproduce the regression

This is another variant of #88114. It would've been nice if this suggested that the outer clouser should be a move a closure.

[nikomatsakis]

I agree this is a duplicate of #88114