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array repeat exprs do not check the array type is well formed #111091

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BoxyUwU opened this issue May 2, 2023 · 3 comments · Fixed by #111100
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array repeat exprs do not check the array type is well formed #111091

BoxyUwU opened this issue May 2, 2023 · 3 comments · Fixed by #111100
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@BoxyUwU
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C-bug Category: This is a bug. I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness T-types Relevant to the types team, which will review and decide on the PR/issue.

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@BoxyUwU
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BoxyUwU commented May 2, 2023

I tried this code:

trait Foo {
    const ASSOC: [u8];
}

fn bar<T: Foo>() {
    let a = [T::ASSOC; 2];
}

fn main() {}

I expected to see this happen: It should not compile as the local variable a has type [[u8]; 2] which is not well formed

Instead, this happened: it compiled 😬

Meta

playground version:

Nightly channel

Build using the Nightly version: 1.71.0-nightly

(2023-04-30 9ecda8de85ce893cc3fc)
Backtrace 

<backtrace>

@BoxyUwU BoxyUwU added the C-bug Category: This is a bug. label May 2, 2023
@BoxyUwU BoxyUwU self-assigned this May 2, 2023
@jyn514 jyn514 added I-unsound Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness T-types Relevant to the types team, which will review and decide on the PR/issue. labels May 2, 2023
@rustbot rustbot added the I-prioritize Issue: Indicates that prioritization has been requested for this issue. label May 2, 2023
@asquared31415
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I am pretty sure bar can't be called, since Foo can't be implemented by any type, since the const has a type that's not Sized (maybe some nightly feature enables this?). Is it an issue that a trait can have a const with a type that's not Sized?

@BoxyUwU BoxyUwU mentioned this issue May 2, 2023
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Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue May 3, 2023
@Dylan-DPC
Rollup merge of rust-lang#111100 - BoxyUwU:array_repeat_expr_wf, r=co…
1831409 
…mpiler-errors

check array type of repeat exprs is wf

Fixes rust-lang#111091

Also makes sure that we actually renumber regions in the length of repeat exprs which we previously weren't doing and would cause ICEs in `adt_const_params` + `generic_const_exprs` from attempting to prove the wf goals when the length was an unevaluated constant with `'erased` in the `ty` field of `Const`

The duplicate errors are caused by the fact that `const_arg_to_const`/`array_len_to_const` in `FnCtxt` adds a `WellFormed` goal for the created `Const` which is also checked by the added `WellFormed(array_ty)`. I don't want to change this to just emit a `T: Sized` goal for the element type since that would ignore `ConstArgHasType` wf requirements and generally uncomfortable with the idea of trying to sync up `wf::obligations` for arrays and the code in hir typeck for repeat exprs.

r? `@compiler-errors`
Dylan-DPC added a commit to Dylan-DPC/rust that referenced this issue May 3, 2023
@Dylan-DPC
Rollup merge of rust-lang#111100 - BoxyUwU:array_repeat_expr_wf, r=co…
8215170 
…mpiler-errors

check array type of repeat exprs is wf

Fixes rust-lang#111091

Also makes sure that we actually renumber regions in the length of repeat exprs which we previously weren't doing and would cause ICEs in `adt_const_params` + `generic_const_exprs` from attempting to prove the wf goals when the length was an unevaluated constant with `'erased` in the `ty` field of `Const`

The duplicate errors are caused by the fact that `const_arg_to_const`/`array_len_to_const` in `FnCtxt` adds a `WellFormed` goal for the created `Const` which is also checked by the added `WellFormed(array_ty)`. I don't want to change this to just emit a `T: Sized` goal for the element type since that would ignore `ConstArgHasType` wf requirements and generally uncomfortable with the idea of trying to sync up `wf::obligations` for arrays and the code in hir typeck for repeat exprs.

r? ``@compiler-errors``
@apiraino
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apiraino commented May 4, 2023 •
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@BoxyUwU I've tried bisecting this but I can't find a previous version where compiling with rustc returns an error. Is this unsoundness always been there or can we bisect to a previous version that it didn't?

@apiraino apiraino removed the I-prioritize Issue: Indicates that prioritization has been requested for this issue. label May 4, 2023
@bors bors closed this as completed in c0ca84b May 4, 2023
@BoxyUwU
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BoxyUwU commented May 4, 2023

I imagine its likely that this always was broken lol

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check array type of repeat exprs is wf BoxyUwU/rust
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@apiraino @BoxyUwU @jyn514 @asquared31415 @rustbot