rustc panics when compiling code that uses tokio's LocalSet

[Alovchin91]

Compiler panics when compiling the following code that uses tokio's LocalSet:

#[tokio::main]
async fn main() {
    tokio::spawn(async move {
        let local = tokio::task::LocalSet::new();
        local.run_until(async { })
    }).await
}

--- or ---

#[tokio::main]
async fn main() {
    tokio::spawn({
        let local = tokio::task::LocalSet::new();
        local.run_until(async {})
    }).await
}

Panic message is as follows:

➜ RUST_BACKTRACE=full cargo build --verbose
       ...
       Fresh tokio v0.2.9
   Compiling rust-bug v0.1.0 (/tmp/rust-bug)
     Running `rustc --edition=2018 --crate-name rust_bug src/main.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=b5f969b2f9a6a1a5 -C extra-filename=-b5f969b2f9a6a1a5 --out-dir /tmp/rust-bug/target/debug/deps -C incremental=/tmp/rust-bug/target/debug/incremental -L dependency=/tmp/rust-bug/target/debug/deps --extern tokio=/tmp/rust-bug/target/debug/deps/libtokio-e472db2be190b401.rlib`
thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
stack backtrace:
   0:        0x104e46b75 - <unknown>
   1:        0x104e7dcc0 - <unknown>
             ...
 101:        0x104e59a6e - <unknown>
 102:     0x7fff6bb56e65 - <unknown>

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.40.0 (73528e339 2019-12-16) running on x86_64-apple-darwin

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [typeck_tables_of] processing `tokio::task::local::LocalSet::run_until`
#1 [typeck_tables_of] processing `tokio::task::local::LocalSet::run_until::{{closure}}#0`
#2 [typeck_tables_of] processing `main`
#3 [typeck_tables_of] processing `main::{{closure}}#0`
#4 [type_of] processing `main::{{closure}}#0`
#5 [collect_mod_item_types] collecting item types in top-level module
#6 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `rust-bug`.

Caused by:
  process didn't exit successfully: `rustc --edition=2018 --crate-name rust_bug src/main.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=b5f969b2f9a6a1a5 -C extra-filename=-b5f969b2f9a6a1a5 --out-dir /tmp/rust-bug/target/debug/deps -C incremental=/tmp/rust-bug/target/debug/incremental -L dependency=/tmp/rust-bug/target/debug/deps --extern tokio=/tmp/rust-bug/target/debug/deps/libtokio-e472db2be190b401.rlib` (exit code: 101)

Dependencies:

[dependencies]
tokio = { version = "0.2.9", features = ["full"] }

Sample:

rust-bug.zip

[hellow554]

Does not occur on beta or nightly, although the error message could be improved:

error[E0277]: `std::rc::Rc<tokio::task::local::Scheduler>` cannot be sent between threads safely
   --> src/main.rs:3:5
    |
3   |     tokio::spawn(async move {
    |     ^^^^^^^^^^^^ `std::rc::Rc<tokio::task::local::Scheduler>` cannot be sent between threads safely
    | 
   ::: /home/op/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.9/src/task/spawn.rs:124:20
    |
124 |         T::Output: Send + 'static,
    |                    ---- required by this bound in `tokio::task::spawn::spawn`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<tokio::task::local::Scheduler>`
    = note: required because it appears within the type `for<'r> {&'r tokio::task::local::LocalSet, impl std::future::Future, std::rc::Rc<tokio::task::local::Scheduler>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, ()}`
    = note: required because it appears within the type `[static generator@DefId(15:7135 ~ tokio[52aa]::task[0]::local[0]::{{impl}}[0]::run_until[0]::{{closure}}[0]) 0:&tokio::task::local::LocalSet, 1:impl std::future::Future for<'r> {&'r tokio::task::local::LocalSet, impl std::future::Future, std::rc::Rc<tokio::task::local::Scheduler>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, ()}]`
    = note: required because it appears within the type `std::future::GenFuture<[static generator@DefId(15:7135 ~ tokio[52aa]::task[0]::local[0]::{{impl}}[0]::run_until[0]::{{closure}}[0]) 0:&tokio::task::local::LocalSet, 1:impl std::future::Future for<'r> {&'r tokio::task::local::LocalSet, impl std::future::Future, std::rc::Rc<tokio::task::local::Scheduler>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`
    = note: required because it appears within the type `impl std::future::Future`

error[E0277]: `std::rc::Rc<tokio::task::local::Scheduler>` cannot be shared between threads safely
   --> src/main.rs:3:5
    |
3   |     tokio::spawn(async move {
    |     ^^^^^^^^^^^^ `std::rc::Rc<tokio::task::local::Scheduler>` cannot be shared between threads safely
    | 
   ::: /home/op/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.9/src/task/spawn.rs:124:20
    |
124 |         T::Output: Send + 'static,
    |                    ---- required by this bound in `tokio::task::spawn::spawn`
    |
    = help: within `tokio::task::local::LocalSet`, the trait `std::marker::Sync` is not implemented for `std::rc::Rc<tokio::task::local::Scheduler>`
    = note: required because it appears within the type `tokio::task::local::LocalSet`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&tokio::task::local::LocalSet`
    = note: required because it appears within the type `[static generator@DefId(15:7135 ~ tokio[52aa]::task[0]::local[0]::{{impl}}[0]::run_until[0]::{{closure}}[0]) 0:&tokio::task::local::LocalSet, 1:impl std::future::Future for<'r> {&'r tokio::task::local::LocalSet, impl std::future::Future, std::rc::Rc<tokio::task::local::Scheduler>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, ()}]`
    = note: required because it appears within the type `std::future::GenFuture<[static generator@DefId(15:7135 ~ tokio[52aa]::task[0]::local[0]::{{impl}}[0]::run_until[0]::{{closure}}[0]) 0:&tokio::task::local::LocalSet, 1:impl std::future::Future for<'r> {&'r tokio::task::local::LocalSet, impl std::future::Future, std::rc::Rc<tokio::task::local::Scheduler>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, tokio::task::local::LocalFuture<impl std::future::Future>, ()}]>`
    = note: required because it appears within the type `impl std::future::Future`
    = note: required because it appears within the type `impl std::future::Future`

error[E0308]: mismatched types
 --> src/main.rs:1:1
  |
1 | #[tokio::main]
  | ^^^^^^^^^^^^^^- help: try adding a semicolon: `;`
  | |
  | expected `()`, found enum `std::result::Result`
  |
  = note: expected unit type `()`
                  found enum `std::result::Result<impl std::future::Future, tokio::task::error::JoinError>`

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `rust-bug`.

[hellow554]

Has been fixed in 41601a8 (maybe this was fixed by #67071?)

[nikomatsakis]

This could use a regression test, but it's hard because we'd have to isolate it out from the tokio dependency.

[gilescope]

@rustbot claim

[pnkfelix]

T-compiler triage: P-high, removing nomination label.

[dtolnay]

@rustbot release-assignment

[pnkfelix]

@rustbot claim

[pnkfelix]

I attempted to recreate the problem atop 1.40.0 stable.

I was not able to recreate the ICE.

Unfortunately the .zip file attached to the issue did not include a Cargo.lock file in it, so I cannot be totally sure that all the upstream dependencies that I am grabbing match that of the issue authro.

even more unfortunately, the raw stack trace has no symbols, and the assertion is just an "unwrap of None" assert. So the only contextual info we have is the query stack, and that isn't enough for me to identify what the problem might have been at that time.

(e.g. could it have been an incremental compilation bug? Its impossible to tell at this point.)

So, I'm sadly forced to close this as non-actionable.