Fix the soundness bug in the representation of extern types - #5021
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Looks like I need to skip these in tests, and the ZST flags |
tgross35
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March 16, 2026 22:42
RalfJung
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| // Representation based on the Nomicon: | ||
| // <https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs>. | ||
| // | ||
| // FIXME(1.0): the type is uninhabited so these traits are unreachable and could be |
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You're right; we should still remove the traits but the reason needs updating (will fix)
JohnTitor
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Since the very first import dafaca9 ("Initial import of liblibc"), `libc` has used uninhabited enums to represent C's incomplete/opaque types. While this is, as far as I know, techincally okay when working behind raw pointers, it means that using reference types like `&FILE` can lead to easy UB. Resolve this by changing the representation to a `!Sync + !Send + !Unpin` ZST, as recommended by the nomicon [1]. The loss of auto traits technically makes this user-visible, but it is unlikely that anybody who is doing sound things was relying on these. I also used this as an opportunity to add a forward-compatibility note about intended use that should allow us to switch to real extern types once those are available. [1]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs
The new extern type representation raises the lint prior to 1.72 because ZSTs behind pointers were not allowed.
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This was referenced Jun 24, 2026
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Since the very first import dafaca9 ("Initial import of liblibc"), `libc` has used uninhabited enums to represent C's incomplete/opaque types. While this is, as far as I know, techincally okay when working behind raw pointers, it means that using reference types like `&FILE` can lead to easy UB. Resolve this by changing the representation to a `!Sync + !Send + !Unpin` ZST, as recommended by the nomicon [1]. The loss of auto traits technically makes this user-visible, but it is unlikely that anybody who is doing sound things was relying on these. I also used this as an opportunity to add a forward-compatibility note about intended use that should allow us to switch to real extern types once those are available. [1]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs (backport <rust-lang#5021>) (cherry picked from commit 1cf49df)
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(backport <rust-lang#5021>) (cherry picked from commit fbaea87)
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The new extern type representation raises the lint prior to 1.72 because ZSTs behind pointers were not allowed. (backport <rust-lang#5021>) (cherry picked from commit 96a27a4)
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Since the very first import dafaca9 ("Initial import of liblibc"), `libc` has used uninhabited enums to represent C's incomplete/opaque types. While this is, as far as I know, techincally okay when working behind raw pointers, it means that using reference types like `&FILE` can lead to easy UB. Resolve this by changing the representation to a `!Sync + !Send + !Unpin` ZST, as recommended by the nomicon [1]. The loss of auto traits technically makes this user-visible, but it is unlikely that anybody who is doing sound things was relying on these. I also used this as an opportunity to add a forward-compatibility note about intended use that should allow us to switch to real extern types once those are available. [1]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs (backport <rust-lang#5021>) (cherry picked from commit 1cf49df)
tgross35
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(backport <rust-lang#5021>) (cherry picked from commit fbaea87)
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The new extern type representation raises the lint prior to 1.72 because ZSTs behind pointers were not allowed. (backport <rust-lang#5021>) (cherry picked from commit 96a27a4)
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Since the very first import dafaca9 ("Initial import of liblibc"),
libchas used uninhabited enums to represent C's incomplete/opaque types. While this is, as far as I know, technically okay when working behind raw pointers, it means that using reference types like&FILEcan lead to easy UB.Resolve this by changing the representation to a
!Sync + !Send + !UnpinZST, as recommended by the nomicon 1. The loss of auto traits technically makes this user-visible, but it is unlikely that anybody who is doing sound things was relying on these.I also used this as an opportunity to add a forward-compatibility note about intended use that should allow us to switch to real extern types once those are available.
Cc @RalfJung