implied bounds from projections in function signatures can be unsound #129005
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A-associated-items
Area: Associated items (types, constants & functions)
I-unsound
Issue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness
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High priority
T-types
Relevant to the types team, which will review and decide on the PR/issue.
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… r=lcnr Check WF of source type's signature on fn pointer cast This PR patches the implied bounds holes slightly for rust-lang#129005, rust-lang#25860. Like most implied bounds related unsoundness fixes, this isn't complete w.r.t. higher-ranked function signatures, but I believe it implements a pretty good heuristic for now. ### What does this do? This PR makes a partial patch for a soundness hole in a `FnDef` -> `FnPtr` "reifying" pointer cast where we were never checking that the signature we are casting *from* is actually well-formed. Because of this, and because `FnDef` doesn't require its signature to be well-formed (just its predicates must hold), we are essentially allowed to "cast away" implied bounds that are assumed within the body of the `FnDef`: ``` fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v } fn bad<'short, T>(x: &'short T) -> &'static T { let f: fn(_, &'short T) -> &'static T = foo; f(&&(), x) } ``` In this example, subtyping ends up casting the `_` type (which should be `&'static &'short ()`) to some other type that no longer serves as a "witness" to the lifetime relationship `'short: 'static` which would otherwise be required for this call to be WF. This happens regardless of if `foo`'s lifetimes are early- or late-bound. This PR implements two checks: 1. We check that the signature of the `FnDef` is well-formed *before* casting it. This ensures that there is at least one point in the MIR where we ensure that the `FnDef`'s implied bounds are actually satisfied by the caller. 2. Implements a special case where if we're casting from a higher-ranked `FnDef` to a non-higher-ranked, we instantiate the binder of the `FnDef` with *infer vars* and ensure that it is a supertype of the target of the cast. The (2.) is necessary to validate that these pointer casts are valid for higher-ranked `FnDef`. Otherwise, the example above would still pass even if `help`'s `'a` lifetime were late-bound. ### Further work The WF checks for function calls are scattered all over the MIR. We check the WF of args in call terminators, we check the WF of `FnDef` when we create a `const` operand referencing it, and we check the WF of the return type in rust-lang#115538, to name a few. One way to make this a bit cleaner is to simply extend rust-lang#115538 to always check that the signature is WF for `FnDef` types. I may do this as a follow-up, but I wanted to keep this simple since this leads to some pretty bad NLL diagnostics regressions, and AFAICT this solution is *complete enough*. ### Crater triage Done here: rust-lang#129021 (comment) r? lcnr
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… r=lcnr Check WF of source type's signature on fn pointer cast This PR patches the implied bounds holes slightly for rust-lang#129005, rust-lang#25860. Like most implied bounds related unsoundness fixes, this isn't complete w.r.t. higher-ranked function signatures, but I believe it implements a pretty good heuristic for now. ### What does this do? This PR makes a partial patch for a soundness hole in a `FnDef` -> `FnPtr` "reifying" pointer cast where we were never checking that the signature we are casting *from* is actually well-formed. Because of this, and because `FnDef` doesn't require its signature to be well-formed (just its predicates must hold), we are essentially allowed to "cast away" implied bounds that are assumed within the body of the `FnDef`: ``` fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v } fn bad<'short, T>(x: &'short T) -> &'static T { let f: fn(_, &'short T) -> &'static T = foo; f(&&(), x) } ``` In this example, subtyping ends up casting the `_` type (which should be `&'static &'short ()`) to some other type that no longer serves as a "witness" to the lifetime relationship `'short: 'static` which would otherwise be required for this call to be WF. This happens regardless of if `foo`'s lifetimes are early- or late-bound. This PR implements two checks: 1. We check that the signature of the `FnDef` is well-formed *before* casting it. This ensures that there is at least one point in the MIR where we ensure that the `FnDef`'s implied bounds are actually satisfied by the caller. 2. Implements a special case where if we're casting from a higher-ranked `FnDef` to a non-higher-ranked, we instantiate the binder of the `FnDef` with *infer vars* and ensure that it is a supertype of the target of the cast. The (2.) is necessary to validate that these pointer casts are valid for higher-ranked `FnDef`. Otherwise, the example above would still pass even if `help`'s `'a` lifetime were late-bound. ### Further work The WF checks for function calls are scattered all over the MIR. We check the WF of args in call terminators, we check the WF of `FnDef` when we create a `const` operand referencing it, and we check the WF of the return type in rust-lang#115538, to name a few. One way to make this a bit cleaner is to simply extend rust-lang#115538 to always check that the signature is WF for `FnDef` types. I may do this as a follow-up, but I wanted to keep this simple since this leads to some pretty bad NLL diagnostics regressions, and AFAICT this solution is *complete enough*. ### Crater triage Done here: rust-lang#129021 (comment) r? lcnr
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… r=lcnr Check WF of source type's signature on fn pointer cast This PR patches the implied bounds holes slightly for rust-lang#129005, rust-lang#25860. Like most implied bounds related unsoundness fixes, this isn't complete w.r.t. higher-ranked function signatures, but I believe it implements a pretty good heuristic for now. ### What does this do? This PR makes a partial patch for a soundness hole in a `FnDef` -> `FnPtr` "reifying" pointer cast where we were never checking that the signature we are casting *from* is actually well-formed. Because of this, and because `FnDef` doesn't require its signature to be well-formed (just its predicates must hold), we are essentially allowed to "cast away" implied bounds that are assumed within the body of the `FnDef`: ``` fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v } fn bad<'short, T>(x: &'short T) -> &'static T { let f: fn(_, &'short T) -> &'static T = foo; f(&&(), x) } ``` In this example, subtyping ends up casting the `_` type (which should be `&'static &'short ()`) to some other type that no longer serves as a "witness" to the lifetime relationship `'short: 'static` which would otherwise be required for this call to be WF. This happens regardless of if `foo`'s lifetimes are early- or late-bound. This PR implements two checks: 1. We check that the signature of the `FnDef` is well-formed *before* casting it. This ensures that there is at least one point in the MIR where we ensure that the `FnDef`'s implied bounds are actually satisfied by the caller. 2. Implements a special case where if we're casting from a higher-ranked `FnDef` to a non-higher-ranked, we instantiate the binder of the `FnDef` with *infer vars* and ensure that it is a supertype of the target of the cast. The (2.) is necessary to validate that these pointer casts are valid for higher-ranked `FnDef`. Otherwise, the example above would still pass even if `help`'s `'a` lifetime were late-bound. ### Further work The WF checks for function calls are scattered all over the MIR. We check the WF of args in call terminators, we check the WF of `FnDef` when we create a `const` operand referencing it, and we check the WF of the return type in rust-lang#115538, to name a few. One way to make this a bit cleaner is to simply extend rust-lang#115538 to always check that the signature is WF for `FnDef` types. I may do this as a follow-up, but I wanted to keep this simple since this leads to some pretty bad NLL diagnostics regressions, and AFAICT this solution is *complete enough*. ### Crater triage Done here: rust-lang#129021 (comment) r? lcnr
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Rollup merge of rust-lang#129021 - compiler-errors:ptr-cast-outlives, r=lcnr Check WF of source type's signature on fn pointer cast This PR patches the implied bounds holes slightly for rust-lang#129005, rust-lang#25860. Like most implied bounds related unsoundness fixes, this isn't complete w.r.t. higher-ranked function signatures, but I believe it implements a pretty good heuristic for now. ### What does this do? This PR makes a partial patch for a soundness hole in a `FnDef` -> `FnPtr` "reifying" pointer cast where we were never checking that the signature we are casting *from* is actually well-formed. Because of this, and because `FnDef` doesn't require its signature to be well-formed (just its predicates must hold), we are essentially allowed to "cast away" implied bounds that are assumed within the body of the `FnDef`: ``` fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v } fn bad<'short, T>(x: &'short T) -> &'static T { let f: fn(_, &'short T) -> &'static T = foo; f(&&(), x) } ``` In this example, subtyping ends up casting the `_` type (which should be `&'static &'short ()`) to some other type that no longer serves as a "witness" to the lifetime relationship `'short: 'static` which would otherwise be required for this call to be WF. This happens regardless of if `foo`'s lifetimes are early- or late-bound. This PR implements two checks: 1. We check that the signature of the `FnDef` is well-formed *before* casting it. This ensures that there is at least one point in the MIR where we ensure that the `FnDef`'s implied bounds are actually satisfied by the caller. 2. Implements a special case where if we're casting from a higher-ranked `FnDef` to a non-higher-ranked, we instantiate the binder of the `FnDef` with *infer vars* and ensure that it is a supertype of the target of the cast. The (2.) is necessary to validate that these pointer casts are valid for higher-ranked `FnDef`. Otherwise, the example above would still pass even if `help`'s `'a` lifetime were late-bound. ### Further work The WF checks for function calls are scattered all over the MIR. We check the WF of args in call terminators, we check the WF of `FnDef` when we create a `const` operand referencing it, and we check the WF of the return type in rust-lang#115538, to name a few. One way to make this a bit cleaner is to simply extend rust-lang#115538 to always check that the signature is WF for `FnDef` types. I may do this as a follow-up, but I wanted to keep this simple since this leads to some pretty bad NLL diagnostics regressions, and AFAICT this solution is *complete enough*. ### Crater triage Done here: rust-lang#129021 (comment) r? lcnr
saethlin
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@saethlin , Is this issue still open? Can you assign it to me? |
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While this issue is currently not assigned to anyone and still exists, it is really not a beginner issue. Fixing it requires deep knowledge and insight of how the type system is implemented in rustc. If you want to start with beginner issues, I recommend looking at issues labelled
E-easy
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I fixed this in #129021. I don't think it needs another test? |
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Related to #100051, but the underlying reason is slightly different. The full fix will be the same however and is blocked on the new trait solver.
Introduced by #99217. The idea of that PR was that we check that the unnormalized function signature is well-formed when calling a function. We previously only checked the normalized signature, causing another unsoundness #98543 by having an associated type which we're able to normalize in the definition, but not the use. Checking that the unnormalized signature is well-formed, without assuming that it is well-formed, resulted in breaking changes. Because of this, we changed it to both check, and assume, that the unnormalized function signature is well-formed. This would be sound, except that we can first cast the function to a function pointer, discarding its unnormalized signature, and then call it.
We may be able to partially fix this by checking that the unnormalized signature is well-formed when casting function definitions to function pointers. This still leaves us with the general set of unsound higher-ranked implied bounds, cc #100051 #84591 #25860, which can only really be fixed by supporting implications in the type systems. cc @rust-lang/types
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