Forbid freely casting lifetime bounds of dyn-types #136776
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…, r=<try> [WIP] Forbid object lifetime changing pointer casts Fixes rust-lang#136702 r? `@ghost`
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Most of these are on github; in terms of crates.io regressions all we have is:
Overall, 142 regressions are caused by EDIT: Ah, there's also |
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We discussed this in the lang triage call today. We wanted to think more about it, so we're leaving it nominated to discuss again. |
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@BoxyUwU Do you think it would be possible to implement this as an FCW? We talked about this in lang triage today and would prefer to start with that if we can. If it's not feasible, a hard error can also work (I would say though that we should upstream PRs to any crates we break). Another small thing I noticed is that the error message links to the Nomicon section on variance, but it would be ideal to link to a tracking issue or something describing this issue in particular. |
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To add on to what tmandry, said, in our discussions we did feel that the approach taken in this PR is generally the right way forward, and we're happy to see this progress so as to help clear the way for cc @rust-lang/lang |
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@tmandry I do expect it to be possible to FCW this. We can likely do something hacky around to fully emulate the fix (but as a lint), but if that doesn't work out all the regression we found were relatively "simple" cases that can probably be taken advantage of (if need be) to lint a subset of the actual cases we'd break with this PR edit: see compiler-errors' comment, I'm not so convinced this will be possible to FCW anymore and will likely investigate improving the diagnostics here. I've already filed PRs to the affected crates to migrate them over to a transmute to avoid the breakage if this lands |
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I was thinking earlier that it may be possible to implement a lint to detect, but it seems to me that MIR borrowck is not equipped to implement such a lint. Specifically, it seems near impossible to answer whether a region outlives constraint (like, To fix this would require some significant engineering effort to refactor how NLL processes its region graph to make it easier to clone and reprocess with new constraints. |
…uto_to_object-hard-error, r=oli-obk Make `ptr_cast_add_auto_to_object` lint into hard error In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter, and was part of the work needed to restabilize trait upcasting. We considered just making this a hard error, but opted against it at that time due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's see about making a hard error of this. r? ghost cc `@adetaylor` `@Darksonn` `@BoxyUwU` `@RalfJung` `@compiler-errors` `@oli-obk` `@WaffleLapkin` Related: - rust-lang#135881 - rust-lang#136702 - rust-lang#136776 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
…uto_to_object-hard-error, r=oli-obk Make `ptr_cast_add_auto_to_object` lint into hard error In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter, and was part of the work needed to restabilize trait upcasting. We considered just making this a hard error, but opted against it at that time due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's see about making a hard error of this. r? ghost cc ``@adetaylor`` ``@Darksonn`` ``@BoxyUwU`` ``@RalfJung`` ``@compiler-errors`` ``@oli-obk`` ``@WaffleLapkin`` Related: - rust-lang#135881 - rust-lang#136702 - rust-lang#136776 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
…uto_to_object-hard-error, r=oli-obk Make `ptr_cast_add_auto_to_object` lint into hard error In Rust 1.81, we added a FCW lint (including linting in dependencies) against pointer casts that add an auto trait to dyn bounds. This was part of work making casts of pointers involving trait objects stricter, and was part of the work needed to restabilize trait upcasting. We considered just making this a hard error, but opted against it at that time due to breakage found by crater. This breakage was mostly due to the `anymap` crate which has been a persistent problem for us. It's now a year later, and the fact that this is not yet a hard error is giving us pause about stabilizing arbitrary self types and `derive(CoercePointee)`. So let's see about making a hard error of this. r? ghost cc ```@adetaylor``` ```@Darksonn``` ```@BoxyUwU``` ```@RalfJung``` ```@compiler-errors``` ```@oli-obk``` ```@WaffleLapkin``` Related: - rust-lang#135881 - rust-lang#136702 - rust-lang#136776 Tracking: - rust-lang#127323 - rust-lang#44874 - rust-lang#123430
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cc @ehuss |
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@rfcbot reviewed |
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@rfcbot reviewed |
traviscross
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@rfcbot concern needs FCP proposal want to actually write up something for the types folks first before they have to look at this |
I will not have time to make such a PR myself and am not particularly interested in writing one either. it seems like the existing state of the reference does not really talk whatsoever about the restrictions on pointer casts let alone about subtle details of region constraints arising from each MIR operation. I am surprised that this PR would require a reference PR when previous PRs in this area did not, e.g. disallowing casting |
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If documentation is required to land this, then I am happy to write it. |
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@rfcbot concern can we do a FCW I want to take a second look at whether a FCW would be possible |
Thanks for that. |
The relevant change for that went in ahead of the PR, in rust-lang/reference#1622.
You'll notice I put up #136764 the day after merging the Reference PR. I don't recall the specifics, but it probably wasn't a coincidence. That said, I'm sure you can find cases where we let something through that should have had a Reference PR but did not. We're trying to be better about this. As you may have seen, e.g., in the PR for the release notes of Rust 1.89, in #144097 (comment), I spot-checked how we did about this for that release. |
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I don't see that documentation you linked anywhere in the reference. It's not in the pointer casts section and there's nothing under the big table in the type casts sections that talks about this. |
In rust-lang/reference#1732, it was moved to the footnote here: https://doc.rust-lang.org/1.88.0/reference/expressions/operator-expr.html#footnote-meta-compat Admittedly that's probably not the best place for it. (Partly it's a rendering issue; we'd prefer for footnotes on tables to appear directly below the table.) |
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Reference PR submitted: rust-lang/reference#1951 |
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@rfcbot resolve needs FCP proposal |
Fixes #136702
Background reading about VTable calls/dyn compatibility: https://hackmd.io/zUp-sgZ0RFuFgsNfD4JqYw
This PR causes us to start enforcing that lifetimes of dyn types are constrained through pointer casts. Currently on stable casting
*mut dyn Trait + 'ato*mut dyn Trait + 'bpasses with no requirements on'aor'b. Under this PR we now require'ato outlive'b.Even though the pointee of
*mutpointers is considered to be invariant, we still use subtyping rather than equality. This mirrors how we support coercing&mut dyn Trait + 'ato&mut dyn Trait + 'bwhile requiring only'a: 'b. I believe this coercion is sound as there is no way for safe code tomem::swaptwodyn Trait's, and the same is definitely true of raw pointers.See the changes to this test: https://github.com/rust-lang/rust/pull/136776/files#diff-5523f20a800287a89c9f3e92646c887f3f7599be006b29dd9315f734a2137764
We also do not enforce any constraints on the lifetime of the dyn types if there are multiple pointer indirections. For example
*mut *mut dyn Trait + 'ais allowed to be casted to*mut *mut dyn Trait + 'bwith no requirements on'aor 'b`. This case is just a normal thin pointer cast where we do not care about the pointee type as there is no VTable in play.Test: https://github.com/rust-lang/rust/pull/136776/files#diff-3b6c8da342bb6530524158d686455a545bb8fd6f59cf5ff50d1d991ce74c9649
Finally, this is about any cast where the pointee is unsized with dyn-type metadata, not just literally the pointee type being a dyn-type. E.g. casting
*mut Wrapper<dyn Trait + 'a>to*mut Wrapper<dyn Trait + 'b>requires'a: 'bunder this PR.Test: https://github.com/rust-lang/rust/pull/136776/files#diff-ca0c44df62ae1ad1be70f892f01a59714336c7baf78602a5887ac1cf81145c96
Breakage
This is a breaking change.
Crater Report Comment: #136776 (comment)
Generated Report: https://crater-reports.s3.amazonaws.com/pr-136776/index.html
The majority of the breakage is caused by the
metricscrate with 142 of the regressions, and themaycrate with 14 of the regressions. Themetricscrate has been fixed and has backported the fix to previous versions of the crate that were also affected. Themaycrate has also been fixed.PRs against affected crates have been opened and can be seen here:
There were three regressions I've not filed PRs against:
r? @ghost