Boxes with custom allocators break several underlying assumptions in Miri (and MIR more broadly) #95453
Description
This error got turned into a general design issue for Box as a language primitive vs Box having an 'allocator' field. See original bug report below. Also see a further testcase at rust-lang/miri#2072.
This is minimized from an error I ran into while implementing the Allocator API in my crate, bump-into, and trying to test the implementation with Miri:
Code
#![feature(allocator_api)] extern crate alloc; use alloc::alloc::{AllocError, Allocator}; use core::alloc::Layout; use core::cell::Cell; use core::mem::MaybeUninit; use core::ptr::{self, NonNull}; struct OnceAlloc<'a> { space: Cell<&'a mut [MaybeUninit<u8>]>, } unsafe impl<'shared, 'a: 'shared> Allocator for &'shared OnceAlloc<'a> { fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> { let space = self.space.replace(&mut []); let (ptr, len) = (space.as_mut_ptr(), space.len()); if ptr.align_offset(layout.align()) != 0 || len < layout.size() { return Err(AllocError); } let slice_ptr = ptr::slice_from_raw_parts_mut(ptr as *mut u8, len); unsafe { Ok(NonNull::new_unchecked(slice_ptr)) } } unsafe fn deallocate(&self, _ptr: NonNull<u8>, _layout: Layout) {} } fn main() { let mut space = vec![MaybeUninit::new(0); 1]; let once_alloc = OnceAlloc { space: Cell::new(&mut space[..]), }; let _boxed = Box::new_in([0u8; 1], &once_alloc); }Meta
rustc --version --verbose:
rustc 1.61.0-nightly (1d9c262ee 2022-03-26) binary: rustc commit-hash: 1d9c262eea411ec5230f8a4c9ba50b3647064da4 commit-date: 2022-03-26 host: x86_64-unknown-linux-gnu release: 1.61.0-nightly LLVM version: 14.0.0 Error output
$ env RUST_BACKTRACE=full cargo miri run Finished dev [unoptimized + debuginfo] target(s) in 0.00s Running `/home/autumn/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri target/miri/x86_64-unknown-linux-gnu/debug/miri-ice` error: internal compiler error: /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/compiler/rustc_const_eval/src/interpret/place.rs:777:26: write_immediate_to_mplace: invalid ScalarPair layout: TyAndLayout { ty: *mut [u8; 1], layout: Layout { fields: Primitive, variants: Single { index: 0, }, abi: Scalar( Scalar { value: Pointer, valid_range: 0..=18446744073709551615, }, ), largest_niche: None, align: AbiAndPrefAlign { abi: Align { pow2: 3, }, pref: Align { pow2: 3, }, }, size: Size { raw: 8, }, }, } --> /home/autumn/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:359:13 | 359 | boxed.as_mut_ptr().write(x); | ^^^^^^^^^^^^^^^^^^ Backtrace
thread 'rustc' panicked at 'Box<dyn Any>', /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/compiler/rustc_errors/src/lib.rs:1222:9 stack backtrace: 0: 0x7f4c0669d55d - std::backtrace_rs::backtrace::libunwind::trace::hb8664f5c920dcd17 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5 1: 0x7f4c0669d55d - std::backtrace_rs::backtrace::trace_unsynchronized::h4718639a025d9e7f at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5 2: 0x7f4c0669d55d - std::sys_common::backtrace::_print_fmt::h147f7c369d4f1e8e at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/sys_common/backtrace.rs:66:5 3: 0x7f4c0669d55d - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hee801905e555ec30 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/sys_common/backtrace.rs:45:22 4: 0x7f4c066f755c - core::fmt::write::h4bb35aa347c570f4 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/core/src/fmt/mod.rs:1190:17 5: 0x7f4c0668eab1 - std::io::Write::write_fmt::h6ed268cd0958975b at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/io/mod.rs:1655:15 6: 0x7f4c066a0645 - std::sys_common::backtrace::_print::h3f974cf4bd76b112 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/sys_common/backtrace.rs:48:5 7: 0x7f4c066a0645 - std::sys_common::backtrace::print::h56cc71e242d02ce1 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/sys_common/backtrace.rs:35:9 8: 0x7f4c066a0645 - std::panicking::default_hook::{{closure}}::hca67c27c011873e3 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/panicking.rs:295:22 9: 0x7f4c066a02f9 - std::panicking::default_hook::ha53803c35aaf5526 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/panicking.rs:314:9 10: 0x7f4c06ece6c1 - rustc_driver[1cb8e757554a7e43]::DEFAULT_HOOK::{closure#0}::{closure#0} 11: 0x7f4c066a0d90 - std::panicking::rust_panic_with_hook::hc072423965447af2 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/panicking.rs:702:17 12: 0x5584173e5793 - std[907b070526960113]::panicking::begin_panic::<rustc_errors[f8e1677070e2f6d9]::ExplicitBug>::{closure#0} 13: 0x5584173e5356 - std[907b070526960113]::sys_common::backtrace::__rust_end_short_backtrace::<std[907b070526960113]::panicking::begin_panic<rustc_errors[f8e1677070e2f6d9]::ExplicitBug>::{closure#0}, !> 14: 0x55841725f996 - std[907b070526960113]::panicking::begin_panic::<rustc_errors[f8e1677070e2f6d9]::ExplicitBug> 15: 0x558417309876 - std[907b070526960113]::panic::panic_any::<rustc_errors[f8e1677070e2f6d9]::ExplicitBug> 16: 0x558417307f2a - <rustc_errors[f8e1677070e2f6d9]::HandlerInner>::span_bug::<rustc_span[c0544ac70c34a1b7]::span_encoding::Span> 17: 0x558417307d70 - <rustc_errors[f8e1677070e2f6d9]::Handler>::span_bug::<rustc_span[c0544ac70c34a1b7]::span_encoding::Span> 18: 0x558417312215 - rustc_middle[4fcaad924d73098a]::ty::context::tls::with_opt::<rustc_middle[4fcaad924d73098a]::util::bug::opt_span_bug_fmt<rustc_span[c0544ac70c34a1b7]::span_encoding::Span>::{closure#0}, ()> 19: 0x558417311f69 - rustc_middle[4fcaad924d73098a]::util::bug::opt_span_bug_fmt::<rustc_span[c0544ac70c34a1b7]::span_encoding::Span> 20: 0x55841725fa63 - rustc_middle[4fcaad924d73098a]::util::bug::span_bug_fmt::<rustc_span[c0544ac70c34a1b7]::span_encoding::Span> 21: 0x558417350c5d - <rustc_const_eval[3ebf8ee4e08cef50]::interpret::eval_context::InterpCx<miri[17c863d3c0e6d9d6]::machine::Evaluator>>::write_immediate_to_mplace_no_validate 22: 0x55841734ff04 - <rustc_const_eval[3ebf8ee4e08cef50]::interpret::eval_context::InterpCx<miri[17c863d3c0e6d9d6]::machine::Evaluator>>::write_immediate_no_validate 23: 0x55841734f9c4 - <rustc_const_eval[3ebf8ee4e08cef50]::interpret::eval_context::InterpCx<miri[17c863d3c0e6d9d6]::machine::Evaluator>>::copy_op_no_validate 24: 0x55841734d4ba - <rustc_const_eval[3ebf8ee4e08cef50]::interpret::eval_context::InterpCx<miri[17c863d3c0e6d9d6]::machine::Evaluator>>::copy_op_transmute 25: 0x558417355324 - <rustc_const_eval[3ebf8ee4e08cef50]::interpret::eval_context::InterpCx<miri[17c863d3c0e6d9d6]::machine::Evaluator>>::pop_stack_frame 26: 0x558417342ddf - <rustc_const_eval[3ebf8ee4e08cef50]::interpret::eval_context::InterpCx<miri[17c863d3c0e6d9d6]::machine::Evaluator>>::terminator 27: 0x5584172f42f0 - miri[17c863d3c0e6d9d6]::eval::eval_entry 28: 0x558417265aec - <rustc_interface[38cec0579f964c4b]::passes::QueryContext>::enter::<<miri[f35c181d28723cbf]::MiriCompilerCalls as rustc_driver[1cb8e757554a7e43]::Callbacks>::after_analysis::{closure#0}, ()> 29: 0x5584172637e5 - <miri[f35c181d28723cbf]::MiriCompilerCalls as rustc_driver[1cb8e757554a7e43]::Callbacks>::after_analysis 30: 0x7f4c08d3e4b5 - <rustc_interface[38cec0579f964c4b]::interface::Compiler>::enter::<rustc_driver[1cb8e757554a7e43]::run_compiler::{closure#1}::{closure#2}, core[69ec914610f0cf77]::result::Result<core[69ec914610f0cf77]::option::Option<rustc_interface[38cec0579f964c4b]::queries::Linker>, rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>> 31: 0x7f4c08d514bf - rustc_span[c0544ac70c34a1b7]::with_source_map::<core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>, rustc_interface[38cec0579f964c4b]::interface::create_compiler_and_run<core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>, rustc_driver[1cb8e757554a7e43]::run_compiler::{closure#1}>::{closure#1}> 32: 0x7f4c08d3f114 - rustc_interface[38cec0579f964c4b]::interface::create_compiler_and_run::<core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>, rustc_driver[1cb8e757554a7e43]::run_compiler::{closure#1}> 33: 0x7f4c08d3be92 - <scoped_tls[9467090f342ff13]::ScopedKey<rustc_span[c0544ac70c34a1b7]::SessionGlobals>>::set::<rustc_interface[38cec0579f964c4b]::interface::run_compiler<core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>, rustc_driver[1cb8e757554a7e43]::run_compiler::{closure#1}>::{closure#0}, core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>> 34: 0x7f4c08d3a1ff - std[907b070526960113]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[38cec0579f964c4b]::util::run_in_thread_pool_with_globals<rustc_interface[38cec0579f964c4b]::interface::run_compiler<core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>, rustc_driver[1cb8e757554a7e43]::run_compiler::{closure#1}>::{closure#0}, core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>>::{closure#0}, core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>> 35: 0x7f4c08d52452 - <<std[907b070526960113]::thread::Builder>::spawn_unchecked_<rustc_interface[38cec0579f964c4b]::util::run_in_thread_pool_with_globals<rustc_interface[38cec0579f964c4b]::interface::run_compiler<core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>, rustc_driver[1cb8e757554a7e43]::run_compiler::{closure#1}>::{closure#0}, core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>>::{closure#0}, core[69ec914610f0cf77]::result::Result<(), rustc_errors[f8e1677070e2f6d9]::ErrorGuaranteed>>::{closure#1} as core[69ec914610f0cf77]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} 36: 0x7f4c066aaf83 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hf0cca23978c89840 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/alloc/src/boxed.rs:1861:9 37: 0x7f4c066aaf83 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h9da765b1ac504456 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/alloc/src/boxed.rs:1861:9 38: 0x7f4c066aaf83 - std::sys::unix::thread::Thread::new::thread_start::hb3629e14c6b629e3 at /rustc/1d9c262eea411ec5230f8a4c9ba50b3647064da4/library/std/src/sys/unix/thread.rs:108:17 39: 0x7f4c064835c2 - start_thread 40: 0x7f4c06508584 - __clone 41: 0x0 - <unknown>