unsized_fn_params should not accept types that don't have a dynamically fixed size (such as extern types)

The following code should not be accepted:

#![feature(extern_types)]
#![feature(unsized_fn_params)]

extern {
    type E;
}

fn test(e: E) {}

fn calltest(e: Box<E>) {
    test(*e)
}

In terms of MIR semantics I think we want to view a function call as making copies of its arguments. This requires at least dynamically determining the size of the argument, and therefore we shouldn't accept extern type arguments.

I was extremely surprised earlier today when I learned that this code is getting accepted.

There is no generic bound to distinguish extern types from other unsized types, so the options we have are

require the unsized type to be concrete enough that we can determine its tail, and ensure it is not extern
raise the error during monomorphization / codegen

Trying to actually build any such code will lead to ICEs such as

error: internal compiler error: compiler/rustc_codegen_llvm/src/type_of.rs:306:13: TyAndLayout::scalar_pair_element_llty(TyAndLayout { ty: *mut E, layout: Layout { size: Size(8 bytes), align: AbiAndPrefAlign { abi: Align(8 bytes), pref: Align(8 bytes) }, abi: Scalar(Initialized { value: Pointer(AddressSpace(0)), valid_range: 0..=18446744073709551615 }), fields: Primitive, largest_niche: None, variants: Single { index: 0 }, max_repr_align: None, unadjusted_abi_align: Align(8 bytes) } }): not applicable

or

thread 'rustc' panicked at compiler/rustc_codegen_llvm/src/builder.rs:491:9:
assertion `left == right` failed
  left: false
 right: true

See also #79409.

There is no generic bound to distinguish extern types from other unsized types

rust-lang/rfcs#2984 and rust-lang/rfcs#3396 are open RFCs that attempt to address this.

Unfortunately just rejecting locals/arguments that are not known to be dyn-sized as a MIR pass does not work: the forget_unsized function in the standard library would stop compiling.

So currently something post-monomorphization is the only option. Is there even some place that check could be put without being duplicated across all codegen backends (and Miri)?

Auto merge of rust-lang#115748 - RalfJung:post-mono, r=<try>

Auto merge of rust-lang#115748 - RalfJung:post-mono, r=oli-obk

Auto merge of #115748 - RalfJung:post-mono, r=oli-obk

Auto merge of rust-lang#116115 - RalfJung:dyn-unsized-args, r=<try>

Auto merge of #115748 - RalfJung:post-mono, r=oli-obk

In #116115 we agreed that this is not worth a post-mono check; this needs proper design to find a way that makes this work without a post-mono check. So for now we are living with the ICEs.

Rollup merge of rust-lang#118551 - RalfJung:extern-types-bugs, r=comp…

Unrolled build for rust-lang#118551

Rollup merge of #118551 - RalfJung:extern-types-bugs, r=compiler-errors

Auto merge of #115748 - RalfJung:post-mono, r=oli-obk

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unsized_fn_params should not accept types that don't have a dynamically fixed size (such as `extern` types) #115709

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unsized_fn_params should not accept types that don't have a dynamically fixed size (such as extern types) #115709

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SNCPlay42 commented on Sep 9, 2023

RalfJung commented on Sep 9, 2023

RalfJung commented on Dec 2, 2023

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unsized_fn_params should not accept types that don't have a dynamically fixed size (such as `extern` types) #115709