ICE with `#![feature(allocator_api)]`

[jfrimmel]

Hello,
I've tested the allocator_api feature and encountered an ICE:

Code

If tried this code (playground):

#![feature(allocator_api)]
use std::alloc::{self, Allocator, Global, AllocError};
use std::ptr;

struct InlineAlloc<A: Allocator = Global> {
    upstream: A,
    buffer: [u8; 32],
}
impl InlineAlloc<Global> {
    pub fn new() -> Self {
        Self {
            upstream: Global,
            buffer: [0; 32],
        }
    }
}
unsafe impl<A: Allocator> Allocator for InlineAlloc<A> {
    fn allocate(&self, _: alloc::Layout) -> Result<ptr::NonNull<[u8]>, AllocError> {
        todo!();
    }
    
    unsafe fn deallocate(&self, _: ptr::NonNull<u8>, _: alloc::Layout) {
        todo!();
    }
}

fn main() {
    let x = Box::new_in(0, InlineAlloc::new());
}

Meta

I've used the "current" playground nighty compiler, but I can reproduce locally.

rustc --version --verbose:

rustc 1.51.0-nightly (a4cbb44ae 2021-01-20)
binary: rustc
commit-hash: a4cbb44ae2c80545db957763b502dc7f6ea22085
commit-date: 2021-01-20
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

and also on

rustc 1.51.0-nightly (202720bf4 2021-01-21)
binary: rustc
commit-hash: 202720bf483088dbdb343f78c0aa77067fdd8156
commit-date: 2021-01-21
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

Error output

error: internal compiler error: /rustc/202720bf483088dbdb343f78c0aa77067fdd8156/compiler/rustc_codegen_ssa/src/mir/operand.rs:133:38: Deref of by-Ref operand OperandRef(Ref((%"std::boxed::Box<std::mem::MaybeUninit<i32>, InlineAlloc>"*:  %10 = alloca %"std::boxed::Box<std::mem::MaybeUninit<i32>, InlineAlloc>", align 8), None, Align { pow2: 3 }) @ TyAndLayout { ty: std::boxed::Box<std::mem::MaybeUninit<i32>, InlineAlloc>, layout: Layout { fields: Arbitrary { offsets: [Size { raw: 0 }, Size { raw: 8 }], 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: 40 } } })

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:958:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.51.0-nightly (202720bf4 2021-01-21) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
end of query stack
error: aborting due to previous error; 3 warnings emitted

error: could not compile `test2`

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

Backtrace

Standard Error

   Compiling playground v0.0.1 (/playground)
warning: unused variable: `x`
  --> src/main.rs:28:9
   |
28 |     let x = Box::new_in(0, InlineAlloc::new());
   |         ^ help: if this is intentional, prefix it with an underscore: `_x`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: field is never read: `upstream`
 --> src/main.rs:6:5
  |
6 |     upstream: A,
  |     ^^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: field is never read: `buffer`
 --> src/main.rs:7:5
  |
7 |     buffer: [u8; 32],
  |     ^^^^^^^^^^^^^^^^

error: internal compiler error: /rustc/202720bf483088dbdb343f78c0aa77067fdd8156/compiler/rustc_codegen_ssa/src/mir/operand.rs:133:38: Deref of by-Ref operand OperandRef(Ref((%"std::boxed::Box<std::mem::MaybeUninit<i32>, InlineAlloc>"*:  %10 = alloca %"std::boxed::Box<std::mem::MaybeUninit<i32>, InlineAlloc>", align 8), None, Align { pow2: 3 }) @ TyAndLayout { ty: std::boxed::Box<std::mem::MaybeUninit<i32>, InlineAlloc>, layout: Layout { fields: Arbitrary { offsets: [Size { raw: 0 }, Size { raw: 8 }], 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: 40 } } })

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:958:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::bug
   2: rustc_errors::Handler::bug
   3: rustc_middle::ty::context::tls::with_opt
   4: rustc_middle::util::bug::opt_span_bug_fmt
   5: rustc_middle::util::bug::bug_fmt
   6: rustc_codegen_ssa::mir::operand::OperandRef<V>::deref
   7: rustc_codegen_ssa::mir::place::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_place
   8: rustc_codegen_ssa::mir::rvalue::<impl rustc_codegen_ssa::mir::FunctionCx<Bx>>::codegen_rvalue_operand
   9: rustc_codegen_ssa::mir::codegen_mir
  10: rustc_codegen_ssa::base::codegen_instance
  11: <rustc_middle::mir::mono::MonoItem as rustc_codegen_ssa::mono_item::MonoItemExt>::define
  12: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  13: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  14: rustc_codegen_llvm::base::compile_codegen_unit
  15: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  16: rustc_session::utils::<impl rustc_session::session::Session>::time
  17: rustc_interface::passes::QueryContext::enter
  18: rustc_interface::queries::Queries::ongoing_codegen
  19: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  20: rustc_span::with_source_map
  21: rustc_interface::interface::create_compiler_and_run
  22: rustc_span::with_session_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.51.0-nightly (202720bf4 2021-01-21) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

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

query stack during panic:
end of query stack
error: aborting due to previous error; 3 warnings emitted

error: could not compile `playground`

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

[jfrimmel]

@rustbot label: +requires-nightly +A-allocators

[jfrimmel]

Slightly minimized:

#![feature(allocator_api)]
use std::alloc::{Layout, Allocator, Global, AllocError};
use std::ptr::NonNull;
use std::marker::PhantomData;

struct S<A>{ a: PhantomData<A>, b: [u8; 1] }
unsafe impl<A> Allocator for S<A> {
    fn allocate(&self, _: Layout) -> Result<NonNull<[u8]>, AllocError> {
        todo!();
    }
    unsafe fn deallocate(&self, _: NonNull<u8>, _: Layout) {
        todo!();
    }
}

fn main() {
    let x: Box<u8, S<u8>> = Box::new_in(0, S { a: PhantomData, b: [0; 1] });
}

Doesn't ICE anymore if you either remove the field b or if it is not an array.

[mbartlett21]

Duplicate of #78459?

[jfrimmel]

Duplicate of #78459?

No, not really a duplicate (but possibly related). This issue and the linked one have completely different backtraces.

[mbartlett21]

Just by the way, using inline storage in an allocator is actually unsafe unless the impl of the Allocator trait is using a reference.

See the Allocator safety notes:

cloning or moving the allocator must not invalidate memory blocks returned from this allocator. A cloned allocator must behave like the same allocator.

(Emphasis mine)

(link)