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unsized_fn_params should not accept types that don't have a dynamically fixed size (such as extern types) #115709
Description
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
Activity
SNCPlay42 commentedon Sep 9, 2023
See also #79409.
rust-lang/rfcs#2984 and rust-lang/rfcs#3396 are open RFCs that attempt to address this.
RalfJung commentedon Sep 9, 2023
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)?
[-]unsized_fn_params should not accept types that don't have a dynamically fixed size[/-][+]unsized_fn_params should not accept types that don't have a dynamically fixed size (such as `extern` types)[/+]Auto merge of rust-lang#115748 - RalfJung:post-mono, r=<try>
Auto merge of rust-lang#115748 - RalfJung:post-mono, r=<try>
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 rust-lang#115748 - RalfJung:post-mono, r=oli-obk
Auto merge of #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 rust-lang#116115 - RalfJung:dyn-unsized-args, r=<try>
Auto merge of #115748 - RalfJung:post-mono, r=oli-obk
RalfJung commentedon Dec 2, 2023
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
Auto merge of #115748 - RalfJung:post-mono, r=oli-obk
Auto merge of #115748 - RalfJung:post-mono, r=oli-obk
Auto merge of #115748 - RalfJung:post-mono, r=oli-obk