Coroutine upvars are not required to be live in borrowck if witness has no drop requirements

I tried this code:

struct NeedsDrop<'a>(&'a Vec<i32>);

impl Drop for NeedsDrop<'_> {
    fn drop(&mut self) {
        println!("dropping {:#?}", self.0);
    }
}

async fn await_point() {}

fn main() {
    let v = vec![1, 2, 3];
    let x = NeedsDrop(&v);
    let c = async {
        std::future::ready(()).await;
        drop(x);
    };
    drop(v);
}

I expected it to fail, since dropping the coroutine c requires that NeedsDrop's 'a lifetime is live, since dropping the coroutine also performs a drop of NeedsDrop (as an upvar).

Instead, it compiled. This is UB.

This fails on rustc as of nightly 2025-07-18. ~~Not sure if this has either always been incorrect, or if~~ this was introduced due to the fix in #117134.

Bisection would be helpful if someone wants to do this; specifically, I want to know if this code went from fail->pass sometime in the past.

For me this has always failed since 1.36 until #117134 (or say, until 1.74 as I haven't bisected the 1.73 to 1.74 nightlies)

nightly-2023-11-02

error[E0505]: cannot move out of `v` because it is borrowed
  --> /tmp/crash.rs:18:10
   |
12 |     let v = vec![1, 2, 3];
   |         - binding `v` declared here
13 |     let x = NeedsDrop(&v);
   |                       -- borrow of `v` occurs here
...
18 |     drop(v);
   |          ^ move out of `v` occurs here
19 | }
   | - borrow might be used here, when `c` is dropped and runs the destructor for coroutine

error: aborting due to previous error; 1 warning emitted

nightly-2023-11-03
pass

prs:

commit[0] 2023-11-01: Auto merge of #117029 - rmehri01:mir_opt_filecheck_inline_tests, r=cjgillot
commit[1] 2023-11-01: Auto merge of #117498 - matthiaskrgr:rollup-z7mg4ck, r=matthiaskrgr
commit[2] 2023-11-02: Auto merge of #117506 - massivebird:ord-typo, r=ChrisDenton
commit[3] 2023-11-02: Auto merge of #117386 - roblabla:fix-switch-stdio-win7, r=ChrisDenton
commit[4] 2023-11-02: Auto merge of #117204 - nnethercote:rustc_ast_passes, r=compiler-errors
commit[5] 2023-11-02: Auto merge of #117466 - compiler-errors:alias-bound, r=aliemjay
commit[6] 2023-11-02: Auto merge of #117513 - matthiaskrgr:rollup-jvl6y84, r=matthiaskrgr
commit[7] 2023-11-02: Auto merge of #117521 - GuillaumeGomez:impl-on-foreign-order, r=notriddle
commit[8] 2023-11-02: Auto merge of #117471 - onur-ozkan:tmp-fix, r=clubby789
commit[9] 2023-11-02: Auto merge of #117134 - lcnr:dropck_outlives-coroutine, r=compiler-errors

commit[9] 2023-11-02: Auto merge of #117134 - lcnr:dropck_outlives-coroutine, r=compiler-errors

this one ^

Rollup merge of rust-lang#144156 - compiler-errors:dtorck-upvars, r=lcnr

Unrolled build for #144156

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Coroutine upvars are not required to be live in borrowck if witness has no drop requirements #144155

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Coroutine upvars are not required to be live in borrowck if witness has no drop requirements #144155

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compiler-errors commented on Jul 18, 2025

lqd commented on Jul 18, 2025

matthiaskrgr commented on Jul 18, 2025

compiler-errors commented on Jul 18, 2025

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