Correctly deny late-bound lifetimes from parent in anon consts and TAITs #115486
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Correctly deny late-bound lifetimes from parent in anon consts and TAITs
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,23 +1,13 @@ | ||
| // known-bug: unknown | ||
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| // If we want this to compile, then we'd need to do something like RPITs do, | ||
| // where nested associated constants have early-bound versions of their captured | ||
| // late-bound vars inserted into their generics. This gives us substitutable | ||
| // lifetimes to actually use when borrow-checking the associated const, which is | ||
| // lowered as a totally separate body from its parent. Since this doesn't exist, | ||
| // so we should just error rather than resolving this late-bound var with no | ||
| // binder to actually attach it to, or worse, as a free region that can't even be | ||
| // substituted correctly, and ICEing. - @compiler-errors | ||
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There was a problem hiding this comment. The purpose of this comment is to ramble a bit about a more general solution than what would be needed to fix just this test and its related test ( But, for the record, #79298 never really correctly supported referencing late-bound regions in anon consts that show up in general positions (i.e. other positions where the const wasn't just referencing a free region in the scope of a body), e.g.: // ICEs even on 1.51 after #79298
#![feature(const_generics)] // `generic_const_exprs` after it was renamed.
#![allow(incomplete_features)]
const fn inner<'a: 'a>() -> usize {
3
}
fn test<'a>() -> [u8; { inner::<'a>() }] {
[1, 2, 3]
}
fn main() {
test();
}or in other binders, like: trait Trait {
type Assoc;
}
fn test() {
let x: dyn for<'a> Trait<Assoc = [u8; { inner::<'a>() }]>;
}This isn't necessarily the only solution to fix this -- I guess we could (ab)use free regions to correctly borrowck consts that show up in the binders above, but it seems pretty obvious to me that the clearest and easiest way to do this is to just represent late-bound vars captured by nested consts as early-bound regions owned by those consts. |
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| #![feature(generic_const_exprs)] | ||
| #![allow(incomplete_features)] | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,10 +1,20 @@ | ||
| error: cannot capture late-bound lifetime in constant | ||
| --> $DIR/in_closure.rs:24:29 | ||
| | | ||
| LL | fn test<'a>() { | ||
| | -- lifetime defined here | ||
| LL | let _ = || { | ||
| LL | let _: [u8; inner::<'a>()]; | ||
| | ^^ | ||
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| error: cannot capture late-bound lifetime in constant | ||
| --> $DIR/in_closure.rs:25:29 | ||
| | | ||
| LL | fn test<'a>() { | ||
| | -- lifetime defined here | ||
| ... | ||
| LL | let _ = [0; inner::<'a>()]; | ||
| | ^^ | ||
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| error: aborting due to 2 previous errors | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,12 +1,19 @@ | ||
| error: cannot capture late-bound lifetime in constant | ||
| --> $DIR/simple.rs:20:25 | ||
| | | ||
| LL | fn test<'a>() { | ||
| | -- lifetime defined here | ||
| LL | let _: [u8; inner::<'a>()]; | ||
| | ^^ | ||
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| error: cannot capture late-bound lifetime in constant | ||
| --> $DIR/simple.rs:21:25 | ||
| | | ||
| LL | fn test<'a>() { | ||
| | -- lifetime defined here | ||
| LL | let _: [u8; inner::<'a>()]; | ||
| LL | let _ = [0; inner::<'a>()]; | ||
| | ^^ | ||
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| error: aborting due to 2 previous errors | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,13 @@ | ||
| #![feature(generic_const_exprs)] | ||
| //~^ WARN the feature `generic_const_exprs` is incomplete | ||
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| fn test<'a>( | ||
| _: &'a (), | ||
| ) -> [(); { | ||
| let x: &'a (); | ||
| //~^ ERROR cannot capture late-bound lifetime in constant | ||
| 1 | ||
| }] { | ||
| } | ||
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| fn main() {} |
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Both this branch and the next (
if let Some(body_id) = outermost_body) should only be executed for late-bound variables. Should they both be in anif let ResolvedArg::LateBound(_, _, param_def_id) = defto make it clearer?For my curiosity, what would happen if we made lifetimes in anon-consts
ResolvedArg::Free?There was a problem hiding this comment.
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See my comment here #115486 (comment), I kinda went into it there -- this idea would, in fact, fix at least tests like
const-generics/late-bound-vars/in_closure.rsandconst-generics/late-bound-vars/simple.rs, and was how #79298 worked, afaict.It doesn't seem like a general solution, though, since when it comes to, e.g., a binder on a where clause like
where for<'a> [(); { /* const that uses 'a */ }]: ..., it doesn't really make sense to useReFreefor that usage of'a, since the late-bound region does not originate from the function's generics (and therefore thefn_sig's binder).This also doesn't lead us towards a correct solution for handling things like
for<const C: usize> [(); C]: ..., since there's no equivalent notion of a "free" type or const var (nor does it make sense, since they must be substitutable).