Tracking issue: deref patterns

[nrc]

Tracking issue for implementing deref patterns (#[feature(deref_patterns)]).

deref patterns project group repo
lang team initiative issue

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the feature
  • String -> &str: string_deref_patterns implementation: Minimal implementation of implicit deref patterns for Strings Minimal implementation of implicit deref patterns for Strings #98914 (stabilization not started)
  • Deref patterns for other std types
• Adjust documentation (see instructions on rustc-dev-guide)
• Write an RFC
• Stabilization PR (see instructions on rustc-dev-guide)

Status

The current implementation uses a placeholder deref!(<pat>) syntax and is limited to types in the standard library. See the design proposal document for more details.

We limit to types in the standard library using the unstable trait DerefPure (#[feature(deref_pure_trait)]). It is not intended to be stabilized at this stage.

• Supported std types: Box, Rc, Arc, Vec, String, Cow, Pin, ManuallyDrop, Ref, RefMut.
• Unstable types that would be eligible : ThinBox, UniqueRc, LazyCell, LazyLock,

There is also a feature gate just for matching string literals on Strings, under #[feature(string_deref_patterns)].

Unresolved Questions

None at this stage

Implementation history

• Minimal implementation of implicit deref patterns for Strings #98914
• [WIP] implement explicit deref patterns through k#deref #119467
• deref patterns: bare-bones feature gate and typechecking #122222
• Implement macro-based deref!() syntax for deref patterns #122793
• Require DerefMut and DerefPure on deref!() patterns when appropriate #122835
• match lowering: make false edges more precise #123324
• deref patterns: lower deref patterns to MIR #122598
• deref patterns: impl DerefPure for more std types #123480
• Restrict DerefPure for Cow<T> impl to T = impl Clone, [impl Clone], str. #137105
• deref patterns: implement implicit deref patterns #138528
• allow deref patterns to move out of boxes #140022
• deref_patterns: support string and byte string literals in explicit deref!("...") patterns #140028
• deref_patterns: let string and byte string literal patterns peel references and smart pointers before matching #140658
• allow deref patterns to participate in exhaustiveness analysis #140106

[nrc]

@roxelo or @matthewjasper would you be able to mentor @chorman0773 on implementing the compiler parts of this please?

[roxelo]

@nrc I would not consider myself an expert on the pattern matching code but I can try to help mentor...

[AlbertMarashi]

I have this code, it does not compile. I am trying to match a Box<str> inside of a pattern, but the error is:

expected str, found `&str`

but there's no good way to match Box<str> in a deeply nested pattern...

#![feature(box_patterns)]
struct Type {
    pub name: Box<str>,
    pub generics: Option<Vec<Type>>
}

fn main(){
    let kind = Type {
        name: "User".into(),
        generics: Some(Vec::new())
    };

    match kind {
        Type {
            name: box "User",
            generics
        } => {}
        _ => unimplemented!()
    }
}

[chorman0773]

This would be should get solved by deref patterns.

…

On Fri, Feb 4, 2022 at 05:39 Albert Marashi ***@***.***> wrote: I have this code, it does not compile. I am trying to match a Box<str> inside of a pattern, but the error is: expected str, found '&str' but there's no good way to match Box<str> in a deeply nested pattern... #![feature(box_patterns)]struct Type { pub name: Box<str>, pub generics: Option<Vec<Type>> } fn main(){ let kind = Type { name: "User".into(), generics: Some(Vec::new()) }; match kind { Type { name: box "User", generics } => {} _ => unimplemented!() } } — Reply to this email directly, view it on GitHub <#87121 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABGLD27M7YLN2UD7MELBLH3UZOUG5ANCNFSM5AKRWKLA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[AlbertMarashi]

@chorman0773 anything I can do to help move this forward? I'd really love to see this feature in rust, it would be insanely useful for deep matching of things like ASTs.

[rdrpenguin04]

Just bonked my head into this again; how can I help?