Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

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

Merged

Conversation

lambinoo
Copy link
Contributor

@lambinoo lambinoo commented Aug 14, 2022

Fixes: #1250

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

Copy link
Contributor

@ehuss ehuss left a comment

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Thanks!

src/attributes/type_system.md Outdated Show resolved Hide resolved
Copy link
Contributor

@ehuss ehuss left a comment

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Thanks!

@ehuss ehuss merged commit e647eb1 into rust-lang:master Aug 16, 2022
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Aug 17, 2022
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 04f3cf0bb2f5a6ee2bfc4b1a6a6cd8c11d1c5531..d3daa1f28e169087becbc5e2b49ac91ca0405a44
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)
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request Aug 17, 2022
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 04f3cf0bb2f5a6ee2bfc4b1a6a6cd8c11d1c5531..d3daa1f28e169087becbc5e2b49ac91ca0405a44
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
Copy link
Contributor

mixy1 commented Sep 9, 2024

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
Copy link
Member

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
Copy link
Member

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
Copy link
Contributor

mixy1 commented Sep 9, 2024

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
Copy link
Member

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
Copy link
Contributor

mixy1 commented Sep 9, 2024

I've opened PR #1607

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Labels
None yet
Projects
None yet
Development

Successfully merging this pull request may close these issues.

Missing documentation around casting foreign non-exhaustive enums
4 participants