ICE when using Box<T, A> with non-zero sized A

[exrook]

While working on #77438, I've run into the following ICE. It seems to be caused by the type layout code not expecting Box to have more than 3 fields, in this case the fat pointer containing two fields, plus the AllocRef instance. On further consideration, the issue also appears with Box<T, A> as long as A is non zero sized.

This ICE was likely not encountered during testing of #77187 because in the case of Box<T, Global>, Global is zero sized. In the example below, removing the borrow of Global will cause the compilation to succeed.

@eddyb could you take a look at this? :)

cc @TimDiekmann you may run into this as well on RawVec

Code

Old code

Playground link

#![feature(allocator_api)]

fn main() {
    Box::new_in(&[0, 1], &std::alloc::Global);
}

New, more minimal code

Playground link

#![feature(allocator_api)]

fn main() {
    Box::new_in((), &std::alloc::Global);
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (fd542592f 2020-10-26) running on x86_64-unknown-linux-gnu - playground
rustc 1.48.0-beta.3 (4708ac76e 2020-10-15) running on x86_64-unknown-linux-gnu - local dev environment

Error output

rust/compiler/rustc_middle/src/ty/layout.rs

Line 2056 in 07e968b

assert!(i < this.fields.count());

thread 'rustc' panicked at 'assertion failed: i < this.fields.count()', /rustc/fd542592f08ca0d1f7255600115c2eafdf6b5da7/compiler/rustc_middle/src/ty/layout.rs:2056:17

Backtrace

From playground

thread 'rustc' panicked at 'assertion failed: i < this.fields.count()', /rustc/fd542592f08ca0d1f7255600115c2eafdf6b5da7/compiler/rustc_middle/src/ty/layout.rs:2056:17
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_middle::ty::layout::<impl rustc_target::abi::TyAndLayoutMethods<C> for &rustc_middle::ty::TyS>::field
   2: <rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS> as rustc_codegen_llvm::type_of::LayoutLlvmExt>::scalar_pair_element_llvm_type
   3: <rustc_target::abi::TyAndLayout<&rustc_middle::ty::TyS> as rustc_codegen_llvm::type_of::LayoutLlvmExt>::scalar_pair_element_llvm_type
   4: <rustc_target::abi::call::FnAbi<&rustc_middle::ty::TyS> as rustc_codegen_llvm::abi::FnAbiLlvmExt>::llvm_type
   5: rustc_codegen_llvm::mono_item::<impl rustc_codegen_ssa::traits::declare::PreDefineMethods for rustc_codegen_llvm::context::CodegenCx>::predefine_fn
   6: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
   7: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
   8: rustc_codegen_llvm::base::compile_codegen_unit
   9: rustc_codegen_ssa::base::codegen_crate
  10: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  11: rustc_interface::passes::QueryContext::enter
  12: rustc_interface::queries::Queries::ongoing_codegen
  13: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  14: rustc_span::with_source_map
  15: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[rmja]

This ice is still showing up in nightly-2020-12-21. We are using it over in Drone OS in embedded programming to allocate from heaps located on externally mounted sdram.

[beepster4096]

Vec is not affected by this, and can even be used to get a Box.

#![feature(allocator_api)]

fn main(){
    let vec = Vec::new_in(&std::alloc::Global);

    let _: Box<[()], _> = vec.into_boxed_slice();
}

This code did not ICE on playground.

[exrook]

@drmeepster Interestingly, trying to print the Box created in that way gives the same ICE as #81270

#![feature(allocator_api)]

fn main(){
    let vec = Vec::new_in(&std::alloc::Global);
    let a: Box<[()], _> = vec.into_boxed_slice();
    println!("{:?}", a);
}

playground

error: internal compiler error: /rustc/23adf9fd843da7a3394c824b056f93151aaa40ad/compiler/rustc_codegen_ssa/src/mir/operand.rs:132:38: Deref of by-Ref operand OperandRef(Ref((%"std::boxed::Box<[()], &std::alloc::Global>"*:%"std::boxed::Box<[()], &std::alloc::Global>"* %0), None, Align { pow2: 3 }) @ TyAndLayout { ty: std::boxed::Box<[()], &std::alloc::Global>, layout: Layout { fields: Arbitrary { offsets: [Size { raw: 0 }, Size { raw: 16 }], memory_index: [0, 1] }, variants: Single { index: 0 }, abi: Aggregate { sized: true }, largest_niche: Some(Niche { offset: Size { raw: 0 }, scalar: Scalar { value: Pointer, valid_range: 1..=18446744073709551615 } }), align: AbiAndPrefAlign { abi: Align { pow2: 3 }, pref: Align { pow2: 3 } }, size: Size { raw: 24 } } })