#[non_exhaustive] on variant blocks cross-crate as casts #1249

lambinoo · 2022-08-14T12:21:50Z

Adding missing documentation after changing in rust-lang/rust#91161 (rust-lang/rust#92744)

ehuss

Thanks!

src/attributes/type_system.md

ehuss

Thanks!

Update books ## nomicon 2 commits in 8d1e4dccf71114ff56f328f671f2026d8e6b62a2..8e6aa3448515a0654e347b5e2510f1d4bc4d5a64 2022-07-18 18:12:35 -0400 to 2022-08-15 15:36:13 -0700 - Update the `repr(transparent)` section to reflect the current state (rust-lang/nomicon#376) - [typo] typo on limits of lifetime chapter (rust-lang/nomicon#377) ## reference 5 commits in f3d3953bf3b158d596c96d55ce5366f9f3f972e9..e647eb102890e8927f488bea12672b079eff8d9d 2022-08-01 17:17:37 -0700 to 2022-08-16 11:35:27 -0700 - #[non_exhaustive] on variant blocks cross-crate as casts (rust-lang/reference#1249) - Revert let chains reference docs (rust-lang/reference#1251) - Update subtyping.md (rust-lang/reference#1240) - a fix about .await (rust-lang/reference#1245) - Be less specific about the representation of `+bundle` (rust-lang/reference#1246) ## book 2 commits in 36383b4da21dbd0a0781473bc8ad7ef0ed1b6751..42ca0ef484fcc8437a0682cee23abe4b7c407d52 2022-07-19 21:03:20 -0400 to 2022-08-12 21:52:02 -0400 - Missing period at end of sentence - Fix grammar in ch06-02 ## rust-by-example 8 commits in ee342dc91e1ba1bb1e1f1318f84bbe3bfac04798..03301f8ae55fa6f20f7ea152a517598e6db2cdb7 2022-07-27 11:06:36 -0300 to 2022-08-14 08:51:44 -0300 - Update print.md (rust-lang/rust-by-example#1597) - in Meta, replace 'playpen' with 'playground' (rust-lang/rust-by-example#1596) - Update doc comment to link to name field without compilation warning (rust-lang/rust-by-example#1595) - add line numbers for playpen fixes rust-lang/rust-by-example#1593 (rust-lang/rust-by-example#1594) - clarify that the map-reduce example relies on static data (rust-lang/rust-by-example#1592) - Update flow_control.md (rust-lang/rust-by-example#1591) - Remove duplicate line in the hello/print.md file (rust-lang/rust-by-example#1590) - Make rust editable in chapter on defaults (rust-lang/rust-by-example#1589) ## rustc-dev-guide 15 commits in 04f3cf0..d3daa1f 2022-07-31 07:46:57 +0200 to 2022-08-13 10:00:38 +0900 - Improve the "Diagnostic items" chapter (rust-lang/rustc-dev-guide#1427) - date-check: crates-io - fix/improve compiler-debugging - Update src/compiler-debugging.md - add gdb tips for symbol-mangling-version - move references down to avoid clutter (rust-lang/rustc-dev-guide#1420) - update date-check format on github issue (rust-lang/rustc-dev-guide#1416) - Fix legend colors in dark mode - Add color for downloaded nodes - Add colors to diagram - Add bootstrapping diagram - date-check: rustc_codegen_ssa is still alive - note is now too old to be relevant - date-check: be more strict - make date-check more lightweight (rust-lang/rustc-dev-guide#1394) ## edition-guide 3 commits in c55611dd6c58bdeb52423b5c52fd0f3c93615ba8..6038be9d37d7251c966b486154af621d1794d7af 2022-02-21 14:21:39 +0100 to 2022-08-15 08:12:42 -0700 - use title "The Rust Edition Guide" everywhere (rust-lang/edition-guide#280) - "Creating a new project": add example using 'cargo new --edition YEAR' (rust-lang/edition-guide#279) - fixes rust-lang/edition-guide#277: mention rust 1.0 release month and year (rust-lang/edition-guide#278)

mixy1 · 2024-09-09T20:01:24Z

Is this still the case as of today?

pub use keyboard_types::{Code, Modifiers};

fn main() {
    let key: Code = Code::KeyR;
    println!("{}", key as u32);
}

Works just fine, https://docs.rs/keyboard-types/latest/keyboard_types/enum.Code.html and it's indeed marked as non-exhaustive.

obi1kenobi · 2024-09-09T20:16:10Z

A quick check in the playground confirms the rule is a bit more involved, and the reference is inaccurate: playground (make sure to run "test" not "build")

/// ` ``rust
/// use playground::Example;
///
/// let value = Example::default();
/// println!("{}", value as isize)
/// ` ``
#[non_exhaustive]  // it doesn't matter whether this is commented-out or not
#[derive(Default)]
pub enum Example {
    #[default]
    First,
    
    // uncommenting the next line breaks the doctest:
    // #[non_exhaustive]
    Second,
}

The reference states that "it's also not allowed to cast non-exhaustive types from foreign crates" and gives an example of (failing while) casting the value of a non-exhaustive enum. This is inaccurate.

Per my example above, the rule as implemented today appears to be "it's not allowed to cast enums from foreign crates if the enum contains a non-exhaustive variant."

In the case of keyboard_types, the Code enum is non-exhaustive but all its variants are exhaustive, so under my inferred rule it's allowed. I agree that the reference seems to say that wouldn't work, and I assume a PR would be welcome. Just a note that RustConf is this week, so feedback on the PR would likely be delayed.

obi1kenobi · 2024-09-09T20:22:51Z

It may also be a good idea to update the reference page on enum variants and discriminants, since the section that describes numeric as casting doesn't mention #[non_exhaustive] at all: https://doc.rust-lang.org/reference/items/enumerations.html#casting

In cargo-semver-checks, we were just in the middle of implementing some new lints for numeric as casting of enums based on that reference page. After reading that page, I didn't know about the #[non_exhaustive] rule for variants until trying it in the playground just now, which is less than ideal 😅

mixy1 · 2024-09-09T21:15:54Z

Not sure if this is the right place to ask, but what's the point of marking a variant as non_exhaustive? It intuitively makes sense to mark the enum as non_exhaustive but I can't understand why one would use it for a variant.

obi1kenobi · 2024-09-09T21:18:49Z

It can't be constructed by any downstream crate, and pattern-matching it requires allowing for extra fields with .. since exhaustive matching is disallowed. This means it can have additional fields added to it without causing an exhaustiveness-related major breaking change.

mixy1 · 2024-09-09T21:34:14Z

I've opened PR #1607

cannot cast non-exhaustive enum from foreign crate

3036b18

ehuss reviewed Aug 15, 2022

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ehuss mentioned this pull request Aug 15, 2022

Document that adding #[non_exhaustive] on existing items is breaking. rust-lang/cargo#10877

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Update type_system.md

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ehuss approved these changes Aug 16, 2022

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ehuss merged commit e647eb1 into rust-lang:master Aug 16, 2022

ehuss mentioned this pull request Aug 17, 2022

Update books rust-lang/rust#100655

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#[non_exhaustive] on variant blocks cross-crate as casts #1249

#[non_exhaustive] on variant blocks cross-crate as casts #1249

lambinoo commented Aug 14, 2022 •

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mixy1 commented Sep 9, 2024

#[non_exhaustive] on variant blocks cross-crate as casts #1249

#[non_exhaustive] on variant blocks cross-crate as casts #1249

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mixy1 commented Sep 9, 2024

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lambinoo commented Aug 14, 2022 •

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