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.