ICE when passing unsized trait object closure to function

[xldenis]

Playing around with unsized_fn_params, @jhjourdan and I encountered an ICE caused by move closures. Removing the marked move allows the code to compile successfully.

Playground

Code

#![feature(unsized_fn_params)]

pub enum Tree {
    Node(Box<Tree>, Box<Tree>),
    Leaf,
}

pub type Cont<'a> = dyn Fn(u64) -> u64 + 'a;

pub fn height_cps<'a>(x : &'a Tree, k : Cont<'a>) -> u64 {
    match x {
        Tree::Leaf => k(0),
        Tree::Node(l, r) => {
            //                                          v-- causes crash
            height_cps(l, {let k : Box<Cont> = Box::new(move |lh| {
                height_cps(r, {let k : Box<Cont> = Box::new(|lr| { k(1 + lh.max(lr)) }); *k})
            }); *k})
        }
    }
}

fn main () {
    height_cps(&Tree::Leaf, *(Box::new(|i| {i}) as Box<Cont>));
}

Meta

rustc --version --verbose:

rustc 1.60.0-nightly (ad46af247 2022-01-14)
binary: rustc
commit-hash: ad46af24713115e7b9b258346e66b9b2d14eacfc
commit-date: 2022-01-14
host: aarch64-apple-darwin
release: 1.60.0-nightly
LLVM version: 13.0.0

Error output

error: internal compiler error: /rustc/ad46af24713115e7b9b258346e66b9b2d14eacfc/compiler/rustc_const_eval/src/interpret/eval_context.rs:689:18: size_and_align_of::<[closure@cps.rs:14:57: 16:14]> not supported
  --> cps.rs:11:11
   |
11 |     match x {
   |           ^

thread 'rustc' panicked at 'Box<dyn Any>', /rustc/ad46af24713115e7b9b258346e66b9b2d14eacfc/compiler/rustc_errors/src/lib.rs:1115:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrac

Backtrace

stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner>::span_bug::<rustc_span::span_encoding::Span>
   3: <rustc_errors::Handler>::span_bug::<rustc_span::span_encoding::Span>
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, ()>
   5: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>
   6: rustc_middle::util::bug::span_bug_fmt::<rustc_span::span_encoding::Span>
   7: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_mir_transform::const_prop::ConstPropMachine>>::size_and_align_of
   8: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_mir_transform::const_prop::ConstPropMachine>>::force_allocation_maybe_sized
   9: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_mir_transform::const_prop::ConstPropMachine>>::place_field
  10: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_mir_transform::const_prop::ConstPropMachine>>::eval_place
  11: <rustc_const_eval::interpret::eval_context::InterpCx<rustc_mir_transform::const_prop::ConstPropMachine>>::eval_rvalue_into_place
  12: <rustc_mir_transform::const_prop::ConstPropagator as rustc_middle::mir::visit::MutVisitor>::visit_statement
  13: <rustc_mir_transform::const_prop::ConstPropagator as rustc_middle::mir::visit::MutVisitor>::visit_body
  14: <rustc_mir_transform::const_prop::ConstProp as rustc_middle::mir::MirPass>::run_pass
  15: rustc_mir_transform::pass_manager::run_passes
  16: rustc_mir_transform::optimized_mir
  17: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, &rustc_middle::mir::Body>>
  18: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::optimized_mir, rustc_query_impl::plumbing::QueryCtxt>
  19: <rustc_middle::ty::context::TyCtxt>::instance_mir
  20: rustc_monomorphize::collector::collect_neighbours
  21: rustc_monomorphize::collector::collect_items_rec
  22: rustc_monomorphize::collector::collect_items_rec
  23: <rustc_session::session::Session>::time::<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}>
  24: rustc_monomorphize::collector::collect_crate_mono_items
  25: rustc_monomorphize::partitioning::collect_and_partition_mono_items
  26: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), (&std::collections::hash::set::HashSet<rustc_span::def_id::DefId, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>, &[rustc_middle::mir::mono::CodegenUnit])>>
  27: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::collect_and_partition_mono_items, rustc_query_impl::plumbing::QueryCtxt>
  28: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend>
  29: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  30: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  31: <rustc_interface::queries::Queries>::ongoing_codegen
  32: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorReported>>
  33: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
  34: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>
  35: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>

[xldenis]

The following code also gives the same error but at a different location in the code:

#![feature(unsized_fn_params)]

pub enum Tree {
    Node(Box<Tree>, Box<Tree>),
    Leaf,
}

pub type Cont<'a> = dyn FnOnce(u64) -> u64 + 'a;

pub fn height_cps<'a>(x : &'a Tree, f : Cont<'a>) -> u64 {
    match x {
        Tree::Leaf => f(0),
        Tree::Node(l, r) => {
            height_cps(l, {let k : Box<Cont> = Box::new(move |lh| {
                height_cps(r, {let k : Box<Cont> = Box::new(move |lr| { let lh = lh; f(1 + lh.max(lr)) }); *k})
            }); *k})
        }
    }
}

[compiler-errors]

The issue here is moving an unsized fn param into a box by capture.

Minimized:

#![feature(unsized_fn_params)]

pub fn f(k: dyn std::fmt::Display) {
    let k2 = Box::new(move || { let x = k.to_string(); });
}

[compiler-errors]

A similar ICE:

#![feature(unsized_fn_params)]

pub fn f(k: dyn std::fmt::Display) {
    let k2 = move || {
        k.to_string();
    };
}

Backtrace

thread 'rustc' panicked at 'fat_pointer_kind() - Encountered unexpected `pointee_ty`: [closure@src/lib.rs:4:14: 4:21]', compiler/rustc_codegen_llvm/src/debuginfo/utils.rs:102:13
stack backtrace:
   0:     0x7fd74e146ebc - std::backtrace_rs::backtrace::libunwind::trace::hddd5a7e459ce4327
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fd74e146ebc - std::backtrace_rs::backtrace::trace_unsynchronized::h69db3e09cfd8bf11
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fd74e146ebc - std::sys_common::backtrace::_print_fmt::h83b461b46804fdab
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7fd74e146ebc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h2f6e5cd8872f3d74
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7fd74e1a82dc - core::fmt::write::hd64a00f3787c90bd
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/core/src/fmt/mod.rs:1190:17
   5:     0x7fd74e136383 - std::io::Write::write_fmt::h6347402c88d40325
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/io/mod.rs:1653:15
   6:     0x7fd74e14b181 - std::sys_common::backtrace::_print::h989064c3e0b39e3c
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7fd74e14b181 - std::sys_common::backtrace::print::h4af8c55ce731a045
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7fd74e14b181 - std::panicking::default_hook::{{closure}}::h41ef6f6a9fac6b53
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:295:22
   9:     0x7fd74e14ae3f - std::panicking::default_hook::hbaf11e031e0e4a5f
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:314:9
  10:     0x7fd74e928541 - rustc_driver[7b59b981d4c23d31]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7fd74e14ba5b - std::panicking::rust_panic_with_hook::h4788f0bd625b1005
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:702:17
  12:     0x7fd74e14b717 - std::panicking::begin_panic_handler::{{closure}}::h3fc193fd0625168a
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:588:13
  13:     0x7fd74e147364 - std::sys_common::backtrace::__rust_end_short_backtrace::h379714bd69912a88
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys_common/backtrace.rs:138:18
  14:     0x7fd74e14b429 - rust_begin_unwind
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/panicking.rs:584:5
  15:     0x7fd74e112993 - core::panicking::panic_fmt::hc13e2c29da5b89d3
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/core/src/panicking.rs:135:14
  16:     0x7fd74fb63631 - rustc_codegen_llvm[b175ffb6fe16318c]::debuginfo::utils::fat_pointer_kind
  17:     0x7fd74fb65fb3 - rustc_codegen_llvm[b175ffb6fe16318c]::debuginfo::metadata::pointer_or_reference_metadata
  18:     0x7fd74fb66d04 - rustc_codegen_llvm[b175ffb6fe16318c]::debuginfo::metadata::type_metadata
  19:     0x7fd74fb80e5b - <rustc_codegen_llvm[b175ffb6fe16318c]::context::CodegenCx as rustc_codegen_ssa[caeff511e0898096]::traits::debuginfo::DebugInfoMethods>::dbg_scope_fn
  20:     0x7fd74fbab083 - rustc_codegen_ssa[caeff511e0898096]::mir::codegen_mir::<rustc_codegen_llvm[b175ffb6fe16318c]::builder::Builder>
  21:     0x7fd74fb74e62 - rustc_codegen_llvm[b175ffb6fe16318c]::base::compile_codegen_unit::module_codegen
  22:     0x7fd750802896 - <rustc_query_system[59fab566a51c2c24]::dep_graph::graph::DepGraph<rustc_middle[e664fb30aec2b69e]::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle[e664fb30aec2b69e]::ty::context::TyCtxt, rustc_span[26d1c5a8904e1196]::symbol::Symbol, rustc_codegen_ssa[caeff511e0898096]::ModuleCodegen<rustc_codegen_llvm[b175ffb6fe16318c]::ModuleLlvm>>
  23:     0x7fd7507fb1ae - rustc_codegen_llvm[b175ffb6fe16318c]::base::compile_codegen_unit
  24:     0x7fd75082b1b2 - <rustc_codegen_llvm[b175ffb6fe16318c]::LlvmCodegenBackend as rustc_codegen_ssa[caeff511e0898096]::traits::backend::CodegenBackend>::codegen_crate
  25:     0x7fd7507d9337 - <rustc_session[bcbf86d149cb5e94]::session::Session>::time::<alloc[53f90fd9233e64f9]::boxed::Box<dyn core[90c82cfbe27f5d33]::any::Any>, rustc_interface[be440f50dcda62f9]::passes::start_codegen::{closure#0}>
  26:     0x7fd7507c1007 - <rustc_interface[be440f50dcda62f9]::passes::QueryContext>::enter::<<rustc_interface[be440f50dcda62f9]::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core[90c82cfbe27f5d33]::result::Result<alloc[53f90fd9233e64f9]::boxed::Box<dyn core[90c82cfbe27f5d33]::any::Any>, rustc_errors[b77837aed2b0d3e7]::ErrorReported>>
  27:     0x7fd7507b6c9f - <rustc_interface[be440f50dcda62f9]::queries::Queries>::ongoing_codegen
  28:     0x7fd75079684d - <rustc_interface[be440f50dcda62f9]::interface::Compiler>::enter::<rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}::{closure#2}, core[90c82cfbe27f5d33]::result::Result<core[90c82cfbe27f5d33]::option::Option<rustc_interface[be440f50dcda62f9]::queries::Linker>, rustc_errors[b77837aed2b0d3e7]::ErrorReported>>
  29:     0x7fd750799876 - rustc_span[26d1c5a8904e1196]::with_source_map::<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_interface[be440f50dcda62f9]::interface::create_compiler_and_run<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>::{closure#1}>
  30:     0x7fd7507961fe - rustc_interface[be440f50dcda62f9]::interface::create_compiler_and_run::<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>
  31:     0x7fd75077c302 - std[daefdbcf0dca51d0]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[be440f50dcda62f9]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[be440f50dcda62f9]::interface::run_compiler<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>
  32:     0x7fd750779a19 - <<std[daefdbcf0dca51d0]::thread::Builder>::spawn_unchecked_<rustc_interface[be440f50dcda62f9]::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface[be440f50dcda62f9]::interface::run_compiler<core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>, rustc_driver[7b59b981d4c23d31]::run_compiler::{closure#1}>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>::{closure#0}, core[90c82cfbe27f5d33]::result::Result<(), rustc_errors[b77837aed2b0d3e7]::ErrorReported>>::{closure#1} as core[90c82cfbe27f5d33]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x7fd74e157803 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hed49a6c6e767ba83
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/alloc/src/boxed.rs:1854:9
  34:     0x7fd74e157803 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h14238373d01dc9bc
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/alloc/src/boxed.rs:1854:9
  35:     0x7fd74e157803 - std::sys::unix::thread::Thread::new::thread_start::hc6dbf73cd24f68cc
                               at /rustc/4e8fb743ccbec27344b2dd42de7057f41d4ebfdd/library/std/src/sys/unix/thread.rs:108:17
  36:     0x7fd74e08b609 - start_thread
  37:     0x7fd74dfa5293 - clone
  38:                0x0 - <unknown>

[compiler-errors]

To be fair, unsized_fn_params probably should be like unsized_locals and marked with incomplete_features...

My guess would be a lot of the behaviors that make the latter an incomplete compiler feature also apply to unsized_fn_params.. especially considering expressions that move the unsized params..