rust-lang / rust-lang/rfcs

Ref/& Destructuring Inconsistency in Structs

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Description

Rust pattern matching and destructuring allows the use of & and ref.

In the example below, the use of these is intuitive.

fn main() {
    let a: (u32) = (1234);
    let (ref ref_a) = a;

    let b: (&u32) = (&1234);
    let (&deref_b) = b;
}

In this case, we end up with ref_a of type &u32, and deref_b of type u32. Simple enough -- we can go both ways with both ref and & in pattern matching.

In the instance with structs, this consistency breaks down when matching struct field names. In the example that follows, this is shown:

struct Borrowed {
    val: u32
}

fn main() {
    let a = Borrowed {
        val: 1234
    };

    let Borrowed {ref val} = a;
}

In this case, val is borrowed as &u32, mirroring tuple functionality. The following example on the other hand, does not work.

struct Derefed<'a> {
    val: &'a u32
}

fn main() {
    let a = Derefed {
        val: &1234
    };

    let Derefed {&val} = a; // error: expected identifier, found `&`
    let Derefed {val: &val} = a; // works as expected
}

While not functionality-breaking, it is extremely non-intuitive that this matching only works via ref and not via & for structs, when binding directly to the same variable name.

Contributor guide

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reading the issue's tuple and struct pattern-matching examples and compare the ref and & forms described there. Define the intended consistent behavior before identifying the relevant Rust pattern-matching implementation and tests; done means the struct forms have an agreed, documented result and coverage for the accepted syntax.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
35/100

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