orphan check incorrectly handles projections

We consider projections to be foreign types during orphan check

rust/compiler/rustc_trait_selection/src/traits/coherence.rs

Lines 780 to 782 in 7ed044c

    
           | ty::Alias(ty::Projection | ty::Inherent | ty::Weak, ..) => { 
        
               self.found_non_local_ty(ty) 
        
           }

but do not consider them to be parameters when checking for uncovered types

rust/compiler/rustc_trait_selection/src/traits/coherence.rs

Line 621 in ceeb5ad

} else if let ty::Param(_) = input_ty.kind() {

This is more obvious after #99552

this means that the following impl passes the orphan check even though it shouldn't

// crate a
pub trait Foreign<T, U> {
    type Assoc;
}

// crate b
use a::Foreign;

trait Id {
    type Assoc;
}

impl<T> Id for T {
    type Assoc = T;
}

pub struct B;
impl<T> Foreign<B, T> for <T as Id>::Assoc {
    type Assoc = usize;
}

The impl in b overlaps with an impl impl<U> Foreign<T, LocalTy> for LocalTy in another crate c which passes the orphan check.

While I wasn't able to cause runtime UB with this, I was able to get an ICE during codegen_fulfill_obligation: https://github.com/lcnr/orphan-check-ub

cc @rust-lang/types

after #99552 has landed, we should be able to fix this by:

rename found_param_ty to found_uncovered_ty
also use that function for ty::Projection
add tests + update the error message to also make sense for projections
run crater

@rustbot

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

@rustbot

@rustbot claim

Auto merge of rust-lang#117164 - fmease:orphan-norm, r=<try>

Auto merge of rust-lang#117164 - fmease:orphan-norm, r=lcnr

Reopening because the soundness issue still exists. However, since #117164 we now emit a future-incompat dont-report-in-deps warn-by-default lint for it. Tracked separately in #124559.

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	\| ty::Alias(ty::Projection \| ty::Inherent \| ty::Weak, ..) => {
	self.found_non_local_ty(ty)
	}

orphan check incorrectly handles projections #99554

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lcnr commented on Jul 21, 2022

apiraino commented on Jul 21, 2022

atsuzaki commented on Aug 3, 2022

4 remaining items

fmease commented on Apr 30, 2024

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