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Trait object types aren’t enforcing equality constraints on associated types of super-traits #80800

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steffahn opened this issue Jan 8, 2021 · 4 comments · Fixed by #92285
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A-associated-items Area: Associated items (types, constants & functions) A-DSTs Area: Dynamically-sized types (DSTs) A-traits Area: Trait system A-typesystem Area: The type system C-bug Category: This is a bug. I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness P-high High priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@steffahn
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steffahn commented Jan 8, 2021

Found in #57893 (comment)

trait SuperTrait {
    type A;
    type B;
}

trait Trait: SuperTrait<A = <Self as SuperTrait>::B> {}

fn transmute<A, B>(x: A) -> B {
    // why does rustc not complain about
    // the type `dyn Trait<A = A, B = B>` ?!?
    foo::<A, B, dyn Trait<A = A, B = B>>(x)
}

fn foo<A, B, T: ?Sized>(x: T::A) -> B
where
    T: Trait<B = B>,
{
    x
}

static X: u8 = 0;
fn main() {
    let x = transmute::<&u8, &[u8; 1_000_000]>(&X);
    println!("{:?}", x[100_000]);
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 1.13s
     Running `target/debug/playground`
timeout: the monitored command dumped core
/playground/tools/entrypoint.sh: line 11:     7 Segmentation fault      timeout --signal=KILL ${timeout} "$@"

@rustbot modify labels: T-compiler, C-bug, A-traits, A-dst, A-associated-items, A-typesystem
and please add “I-unsound 💥”

@rustbot rustbot added A-associated-items Area: Associated items (types, constants & functions) A-DSTs Area: Dynamically-sized types (DSTs) A-traits Area: Trait system A-typesystem Area: The type system C-bug Category: This is a bug. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 8, 2021
@jonas-schievink jonas-schievink added the I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness label Jan 8, 2021
@rustbot rustbot added the I-prioritize Issue: Indicates that prioritization has been requested for this issue. label Jan 8, 2021
@Aaron1011
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cc @matthewjasper - this looks like it might be related to some of your work.

@matthewjasper matthewjasper self-assigned this Jan 10, 2021
@apiraino apiraino added P-high High priority and removed I-prioritize Issue: Indicates that prioritization has been requested for this issue. labels Jan 13, 2021
@apiraino
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Assigning P-high as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

@steffahn
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Without the dyn keyword, the transmute function type-checks since Rust 1.0.0. And a main function like e.g.

static X: u8 = 123;
fn main() {
    let x = vec![None::<&[u8]>];
    let r = transmute::<&Option<&[u8]>, &Option<&[u8]>>(&x[0]);
    drop(x);
    let _x = vec![&X as *const _ as usize, 1000000];
    println!("{:?}", r.unwrap());
}

leads to reading a bunch of memory and segfaulting on every Rust version, as far as I can tell (at least in debug builds).

@scalexm
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scalexm commented Jun 17, 2021

To me, this is the same root cause as #44454: we unconditionally say that any dyn Trait type is well-formed and implements Trait without checking the actual where clauses declared on the trait, so you can deduce contradictory things with the help of implied bounds (to be fair, #73905 already introduced some checks).

Except that I was wrong in my comment saying that an easy fix for our current limited form of implied bounds would be to just check transitive lifetime requirements. Obviously, @steffahn’s example shows that implied bounds for super traits play a role as well.

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Labels
A-associated-items Area: Associated items (types, constants & functions) A-DSTs Area: Dynamically-sized types (DSTs) A-traits Area: Trait system A-typesystem Area: The type system C-bug Category: This is a bug. I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness P-high High priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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