Unsized locals and async ICE

[DutchGhost]

The following ICE's:

#![feature(unsized_fn_params, unsized_locals)]

use std::future::Future;

async fn bug<T>(mut f: dyn Future<Output = T> + Unpin) -> T {
    (&mut f).await
}

Backtrace:

  Compiling playground v0.0.1 (/playground)
warning: function is never used: `bug`
 --> src/lib.rs:5:10
  |
5 | async fn bug<T>(mut f: dyn Future<Output = T> + Unpin) -> T {
  |          ^^^
  |
  = note: `#[warn(dead_code)]` on by default

thread 'rustc' panicked at 'assertion failed: !field.is_unsized()', src/librustc/ty/layout.rs:719:25
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:476
  12: std::panicking::begin_panic
  13: rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt>::layout_raw_uncached
  14: rustc::ty::layout::layout_raw
  15: rustc::ty::query::__query_compute::layout_raw
  16: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::layout_raw>::compute
  17: rustc::dep_graph::graph::DepGraph::with_task_impl
  18: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  19: <rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  20: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
  21: <core::iter::adapters::chain::Chain<A,B> as core::iter::traits::iterator::Iterator>::try_fold
  22: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  23: rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt>::layout_raw_uncached
  24: rustc::ty::layout::layout_raw
  25: rustc::ty::query::__query_compute::layout_raw
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::layout_raw>::compute
  27: rustc::dep_graph::graph::DepGraph::with_task_impl
  28: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  29: <rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  30: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  31: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  32: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  33: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  34: core::iter::adapters::process_results
  35: rustc::ty::layout::LayoutCx<rustc::ty::context::TyCtxt>::layout_raw_uncached
  36: rustc::ty::layout::layout_raw
  37: rustc::ty::query::__query_compute::layout_raw
  38: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::layout_raw>::compute
  39: rustc::dep_graph::graph::DepGraph::with_task_impl
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  41: <rustc::ty::layout::LayoutCx<rustc::ty::query::TyCtxtAt> as rustc_target::abi::LayoutOf>::layout_of
  42: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  43: rustc_mir::transform::run_passes
  44: rustc_mir::transform::run_optimization_passes
  45: rustc_mir::transform::optimized_mir
  46: rustc::ty::query::__query_compute::optimized_mir
  47: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  48: rustc::dep_graph::graph::DepGraph::with_task_impl
  49: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  50: rustc_metadata::rmeta::encoder::EncodeContext::encode_optimized_mir
  51: <rustc_metadata::rmeta::encoder::EncodeContext as rustc_hir::intravisit::Visitor>::visit_item
  52: rustc_hir::hir::Crate::visit_all_item_likes
  53: rustc_metadata::rmeta::encoder::EncodeContext::encode_crate_root
  54: rustc::ty::context::tls::with_context::{{closure}}
  55: rustc_metadata::rmeta::encoder::encode_metadata
  56: rustc_metadata::rmeta::decoder::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::creader::CStore>::encode_metadata
  57: rustc::ty::context::TyCtxt::encode_metadata
  58: rustc_interface::passes::QueryContext::enter
  59: rustc_interface::queries::Queries::ongoing_codegen
  60: rustc_interface::interface::run_compiler_in_existing_thread_pool
  61: scoped_tls::ScopedKey<T>::set
  62: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.42.0-nightly (c2d141df5 2020-01-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C opt-level=3 -C codegen-units=1 --crate-type lib

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

query stack during panic:
#0 [layout_raw] computing layout of `std::mem::MaybeUninit<dyn std::future::Future<Output = T> + std::marker::Unpin>`
#1 [layout_raw] computing layout of `[static generator@src/lib.rs:5:61: 7:2 __arg0:dyn std::future::Future<Output = T> + std::marker::Unpin for<'r, 's, 't0, 't1, 't2, 't3, 't4> {(dyn std::future::Future<Output = T> + std::marker::Unpin + 'r), (dyn std::future::Future<Output = T> + std::marker::Unpin + 's), (dyn std::future::Future<Output = T> + std::marker::Unpin + 't0), &'t1 mut (dyn std::future::Future<Output = T> + std::marker::Unpin + 't2), &'t3 mut (dyn std::future::Future<Output = T> + std::marker::Unpin + 't4), (), ()}]`
#2 [layout_raw] computing layout of `std::future::GenFuture<[static generator@src/lib.rs:5:61: 7:2 __arg0:dyn std::future::Future<Output = T> + std::marker::Unpin for<'r, 's, 't0, 't1, 't2, 't3, 't4> {(dyn std::future::Future<Output = T> + std::marker::Unpin + 'r), (dyn std::future::Future<Output = T> + std::marker::Unpin + 's), (dyn std::future::Future<Output = T> + std::marker::Unpin + 't0), &'t1 mut (dyn std::future::Future<Output = T> + std::marker::Unpin + 't2), &'t3 mut (dyn std::future::Future<Output = T> + std::marker::Unpin + 't4), (), ()}]>`
#3 [optimized_mir] processing `bug`
end of query stack
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[tmandry]

Visiting from async/await triage. Not making this a priority at the moment because it depends on an unstable feature.

[JohnTitor]

Triage: the feature gate requires unsized_fn_params as well now.

[DutchGhost]

Triage: the feature gate requires unsized_fn_params as well now.

No, it does not:

#![feature(unsized_locals)]

struct T {
    a: [(); {
        let y = async {
            let f: (dyn for<'a> std::future::Future<Output = ()> + Unpin);
            
            f.await
        };
        
        core::mem::forget(y);
        1
    }]
}

Also even though it has a #![feature(...)], the ICE also happens when compiling with stable

[DutchGhost]

The code in question now needs some extra (&mut f).await:

#![feature(const_async_blocks)]
#![feature(unsized_locals)]

struct T {
    a: [(); {
        let y = async {
            let f: (dyn for<'a> std::future::Future<Output = ()> + Unpin);
            
            (&mut f).await
        };
        
        core::mem::forget(y);
        1
    }]
}

[DutchGhost]

Same also occurs with generators and unsized locals acros yield points:

#![feature(generator_trait)]
#![feature(generators)]
#![feature(unsized_locals)]

use std::ops::Generator;

fn blah() -> impl Generator {
    move || {
        let b: [u8] = *(Box::new([]) as Box<[u8]>);
        
        yield;
        
        for elem in b.iter() {}
    }
}

fn main() {
    blah();
}