Large allocations in const eval can OOM crash the compiler

[oli-obk]

I tried this code:

const FOO: () = {
    let x = [0_u8; 1000*1000*1000];  
};

I expected to see this happen: An error about the constant evaluation using more memory than the compiler having available

Instead, this happened: The compiler triggered an abort

Meta

rustc --version --verbose:

1.48.0

Backtrace

• somewhere in the Vec machinery...

• rust/compiler/rustc_middle/src/mir/interpret/allocation.rs

  Line 110 in 0fa9d31

  bytes: vec![0; size.bytes_usize()],

• rust/compiler/rustc_mir/src/interpret/memory.rs

  Line 197 in 0fa9d31

  let alloc = Allocation::uninit(size, align);

I think we'll need to stop using Vec and move to manually using AllocRef::alloc (and maybe Box<[u8]>? in order to allow bubbling up allocation errors during const eval back to the user via error messages.

cc @rust-lang/wg-const-eval

related: #23600

[RalfJung]

I think we'll need to stop using Vec and move to manually using AllocRef::alloc (and maybe Box<[u8]>? in order to allow bubbling up allocation errors during const eval back to the user via error messages.

IIRC the plan of the allocator WG was to have a form of Vec::try_reserve that could be used in such situations?
Cc @TimDiekmann @Amanieu

EDIT: seems like such a method actually already exists.

[lcnr]

even if we use try_reserve here if the allocation is big enough but doesn't quite hit the limit by itself we will end up aborting somewhere else instead. It might make sense to add something like #[const_allocation_size_limit = 1_000_000] which restricts the total size of all allocations during const eval.