Reject context-dependent enum discriminants - #3631
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We should be able to support this by scanning and normalizing any |
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Did you miss #3631 (comment)? Make sure to check top-level comments too.
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Authored by an AI agent acting on Josh Liebow-Feeser's behalf.
I did miss the top-level comment on the earlier pass. I have now fetched the complete top-level conversation as well as inline threads, investigated the proposed normalization, and replied with the language-level counterexample here: #3631 (comment)
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Authored by an AI agent acting on Josh Liebow-Feeser's behalf. I investigated the scope-aware rewrite. We can mechanically distinguish outer A concrete accepted counterexample is: #![feature(core_intrinsics)]
#![allow(internal_features)]
#[repr(u8)]
enum Packet {
Flag(bool) = Self::tag(),
Raw(u8),
}
impl Packet {
const fn tag() -> u8 {
core::intrinsics::is_val_statically_known(0) as u8
}
}The official intrinsic contract says this safe const fn nondeterministically returns The nested-item case has another independent problem: copying the block creates fresh nominal and anonymous types. Their I could not find a stable way around the second evaluation for data-bearing enums. The Reference specifies that For that reason, I think the current positive grammar is the necessary conservative boundary for this derive/helper architecture. We can add individual forms only when their repeatability is proved. Full support would require numeric discriminant reflection from the compiler or a source-owning macro/API that defines the source and validation tags from one canonical evaluation; a derive cannot retroactively rewrite the source enum. A separate fieldless-enum path could use casts of the original variants, but it would not solve the data-bearing case in #3619. |
Derives that validate enum tags copy explicit discriminants into a helper enum. Context-dependent expressions can evaluate differently in that helper, allowing the generated validator to accept an invalid original-enum tag. Restrict copied discriminants to a compiler-defined, repeatable grammar of integer and byte literals with primitive arithmetic and bitwise operations. Reject paths, macros, calls, casts, attributes, and other context-bearing forms before emitting any unsafe impl. Preserve the skip-on-error dependency chain for exhaustive enums, including fieldful variants whose zero tag cannot be identified syntactically, and add cross-toolchain regressions. Closes #3619 *Authored by an AI agent acting on Josh Liebow-Feeser's behalf.* gherrit-pr-id: Gifjr6pbpu4fh6t7qbzuuqczj3odny3ck
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Derives that validate enum tags copy explicit discriminants into a helper
enum. Context-dependent expressions can evaluate differently in that helper,
allowing the generated validator to accept an invalid original-enum tag.
Restrict copied discriminants to a compiler-defined, repeatable grammar of
integer and byte literals with primitive arithmetic and bitwise operations.
Reject paths, macros, calls, casts, attributes, and other context-bearing
forms before emitting any unsafe impl. Preserve the skip-on-error dependency
chain for exhaustive enums, including fieldful variants whose zero tag cannot
be identified syntactically, and add cross-toolchain regressions.
Closes #3619
Authored by an AI agent acting on Josh Liebow-Feeser's behalf.
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git fetch origin refs/heads/Gifjr6pbpu4fh6t7qbzuuqczj3odny3ck && git checkout -b pr-Gifjr6pbpu4fh6t7qbzuuqczj3odny3ck FETCH_HEADCheckout
git fetch origin refs/heads/Gifjr6pbpu4fh6t7qbzuuqczj3odny3ck && git checkout FETCH_HEADCherry Pick
git fetch origin refs/heads/Gifjr6pbpu4fh6t7qbzuuqczj3odny3ck && git cherry-pick FETCH_HEADPull
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