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HRTB on subtrait unsoundly provides HTRB on supertrait with weaker implied bounds #84591
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Just wanted to write a couple thoughts:
is kind of the wrong naming. Here Second, I wonder if this might just be solved to make this |
How did I miss this… One moment, let me fix this…… Edit: Done.
Why should it not be WF? The implementation could just |
Assigning |
Discussed in T-compiler triage meeting. Added relevant labels. Believed to be known long-standing issue, and something that the traits WG wants to address alongside improvements to trait infrastructure. But: Not a release blocker, so downgrading to P-high. |
Strongly related to #25860 |
here's an example where the trait hierarchy is a bit more problematic. trait Subtrait<T>: Supertrait {}
trait Supertrait {
fn action(self);
}
fn subs_to_soup<T, U>(x: T)
where
T: Subtrait<U>,
{
soup(x)
}
fn soup<T: Supertrait>(x: T) {
x.action();
}
impl<'a, 'b: 'a> Supertrait for (&'b str, &mut &'a str) {
fn action(self) {
*self.1 = self.0;
}
}
impl<'a, 'b> Subtrait<&'a &'b str> for (&'b str, &mut &'a str) {}
fn main() {
let mut d = "hi";
{
let x = "Hello World".to_string();
subs_to_soup((x.as_str(), &mut d));
}
println!("{}", d);
} we should require trait solving to prove wf for the types it creates while matching impls ✨ |
…unsound-issues, r=jackh726 Add `known-bug` tests for 11 unsound issues r? `@jackh726` Should tests for other issues be in separate PRs? Thanks. Edit: Partially addresses rust-lang#105107. This PR adds `known-bug` tests for 11 unsound issues: - rust-lang#25860 - rust-lang#49206 - rust-lang#57893 - rust-lang#84366 - rust-lang#84533 - rust-lang#84591 - rust-lang#85099 - rust-lang#98117 - rust-lang#100041 - rust-lang#100051 - rust-lang#104005
…unsound-issues, r=jackh726 Add `known-bug` tests for 11 unsound issues r? ``@jackh726`` Should tests for other issues be in separate PRs? Thanks. Edit: Partially addresses rust-lang#105107. This PR adds `known-bug` tests for 11 unsound issues: - rust-lang#25860 - rust-lang#49206 - rust-lang#57893 - rust-lang#84366 - rust-lang#84533 - rust-lang#84591 - rust-lang#85099 - rust-lang#98117 - rust-lang#100041 - rust-lang#100051 - rust-lang#104005
…unsound-issues, r=jackh726 Add `known-bug` tests for 11 unsound issues r? `@jackh726` Should tests for other issues be in separate PRs? Thanks. Edit: Partially addresses rust-lang#105107. This PR adds `known-bug` tests for 11 unsound issues: - rust-lang#25860 - rust-lang#49206 - rust-lang#57893 - rust-lang#84366 - rust-lang#84533 - rust-lang#84591 - rust-lang#85099 - rust-lang#98117 - rust-lang#100041 - rust-lang#100051 - rust-lang#104005
…unsound-issues, r=jackh726 Add `known-bug` tests for 11 unsound issues r? ``@jackh726`` Should tests for other issues be in separate PRs? Thanks. Edit: Partially addresses rust-lang#105107. This PR adds `known-bug` tests for 11 unsound issues: - rust-lang#25860 - rust-lang#49206 - rust-lang#57893 - rust-lang#84366 - rust-lang#84533 - rust-lang#84591 - rust-lang#85099 - rust-lang#98117 - rust-lang#100041 - rust-lang#100051 - rust-lang#104005
…unsound-issues, r=jackh726 Add `known-bug` tests for 11 unsound issues r? `@jackh726` Should tests for other issues be in separate PRs? Thanks. Edit: Partially addresses rust-lang#105107. This PR adds `known-bug` tests for 11 unsound issues: - rust-lang#25860 - rust-lang#49206 - rust-lang#57893 - rust-lang#84366 - rust-lang#84533 - rust-lang#84591 - rust-lang#85099 - rust-lang#98117 - rust-lang#100041 - rust-lang#100051 - rust-lang#104005
…unsound-issues, r=jackh726 Add `known-bug` tests for 11 unsound issues r? ``@jackh726`` Should tests for other issues be in separate PRs? Thanks. Edit: Partially addresses rust-lang#105107. This PR adds `known-bug` tests for 11 unsound issues: - rust-lang#25860 - rust-lang#49206 - rust-lang#57893 - rust-lang#84366 - rust-lang#84533 - rust-lang#84591 - rust-lang#85099 - rust-lang#98117 - rust-lang#100041 - rust-lang#100051 - rust-lang#104005
I’m giving an exploitation below at the end of this description. This is my interpretation of where exactly the unsoundness lies:
If I have a trait hierarchy
then a higher ranked trait bound (HRTB) like this
does/should only apply to such lifetimes
'a
,'b
that fulfill the outlives-relation'a: 'b
.(
'a: 'b
is needed for&'b &'a ()
to be a valid type.)However, the bound
appears to imply the bound
and in this one, the lifetimes
'a
and'b
are universally quantified without any implicit outlives-relation.This implication is demonstrated below:
(playground)
One could of course debate whether
for<'a, 'b> Subtrait<'a, 'b, &'b &'a ()>
should perhaps in-fact include afor<'a, 'b> Supertrait<'a, 'b>
bound, but that seems kind-of weird IMO, and also the following code demonstrates that you only needSupertrait<'a, 'b>
with'a: 'b
for a fully genericSubtrait<'a, 'b, &'b &'a ()>
implementation:(playground)
Finally, here’s how to turn this into actual UB:
(playground)
This demonstration compiles since Rust
1.7
.@rustbot modify labels: T-compiler, A-traits, A-lifetimes, A-typesystem
and someone please add “I-unsound 💥”.
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