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Irrefutable slice patterns should be inferred as arrays of that length #76342

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scottmcm opened this issue Sep 4, 2020 · 3 comments · Fixed by #113199
Closed

Irrefutable slice patterns should be inferred as arrays of that length #76342

scottmcm opened this issue Sep 4, 2020 · 3 comments · Fixed by #113199
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A-array Area: [T; N] A-const-generics Area: const generics (parameters and arguments) A-inference Area: Type inference C-enhancement Category: An issue proposing an enhancement or a PR with one. E-medium Call for participation: Medium difficulty. Experience needed to fix: Intermediate.

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@scottmcm
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scottmcm commented Sep 4, 2020

Simple demo https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=5b0acd8b21c773c9c4a3ff73bc7a030e:

#![allow(unused_variables)]
struct Zeroes;
impl Into<[usize; 2]> for Zeroes {
    fn into(self) -> [usize; 2] { [0; 2] }
}
impl Into<[usize; 3]> for Zeroes {
    fn into(self) -> [usize; 3] { [0; 3] }
}
impl Into<[usize; 4]> for Zeroes {
    fn into(self) -> [usize; 4] { [0; 4] }
}
fn main() {
    let [a, b, c] = Zeroes.into(); // ERROR: type annotations needed
    let [d, e, f]: [_; 3] = Zeroes.into(); // Works great
}

This is presumably not happening because slice patterns in refutable patterns can be more than just an array of that exact size, but there's no other option for irrefutable patterns.

This would be particularly helpful as we're getting more and more [T; N]: TryFrom<_> implementations with const generics. See the example in #76310 for the place I first noticed this not working the way I'd hoped.

Zulip topic: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Type.20inferrence.20.26.20irrefutable.20slice.20patterns/near/209052937

@scottmcm scottmcm added A-inference Area: Type inference C-feature-request Category: A feature request, i.e: not implemented / a PR. C-enhancement Category: An issue proposing an enhancement or a PR with one. and removed C-feature-request Category: A feature request, i.e: not implemented / a PR. labels Sep 4, 2020
@varkor varkor added the A-const-generics Area: const generics (parameters and arguments) label Sep 18, 2020
@SkiFire13
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but there's no other option for irrefutable patterns.

Actually there's another option, and that's references to arrays:

#![allow(unused_variables)]
struct Zeroes;
impl Into<&'static [usize; 3]> for Zeroes {
    fn into(self) -> &'static [usize; 3] { &[0; 3] }
}
impl Into<[usize; 3]> for Zeroes {
    fn into(self) -> [usize; 3] { [0; 3] }
}
fn main() {
    let [a, b, c] = Zeroes.into(); // Doesn't work: is this an `[usize; 3]` or `&[usize; 3]`?
    let [d, e, f]: [_; 3] = Zeroes.into(); // Works
    let [g, h, i]: &[_; 3] = Zeroes.into(); // Works
}

@jyn514
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jyn514 commented Apr 26, 2023

What are the possible follow-up steps here? It sounds like this really is ambiguous - so either we should add a special-case to the compiler to infer [a, b, c] patterns as arrays by default, or close the issue?

@lcnr
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lcnr commented Jun 6, 2023

we have the same behavior for other patterns as well:

struct Foo(u32);
trait Mk {
    fn mk() -> Self;
}
impl Mk for Foo {
    fn mk() -> Self {
        Foo(0)
    }
}
impl Mk for &'static Foo {
    fn mk() -> Self {
        &Foo(0)
    }
}

fn main() {
    let mut y = Mk::mk();
    // ERRORS, can be fixed by specifying the type of `y` before this statement
    let Foo(_) = y;

    // works
    // let Foo(_): &Foo = y; 
    y = &Foo(1);
}

in nearly all cases this works as expected however:

struct Foo(u32);
struct Bar;
impl Into<&'static Foo> for Bar {
    fn into(self) -> &'static Foo { &Foo(1) }
}
impl Into<Foo> for Bar {
    fn into(self) -> Foo { Foo(1) }
}

fn ty<T>(_: T) {
    println!("{}", std::any::type_name::<T>());
}

fn main() {
    let Foo(i) = Bar.into();
    ty(i); // u32
    let Foo(j) = <Bar as Into<&'static Foo>>::into(Bar);
    ty(j); // &u32
    let Foo(k): Foo = Bar.into();
    ty(k); // u32
    let Foo(l): &Foo = Bar.into();
    ty(l); // &u32
}

I've personally never seen anyone complain about cases where this inference goes wrong. So I think we can do the same thing with arrays, as long as let [a, b] = known_to_be_an_array_reference does not error. As that is the case for struct patterns I don't expect this be an issue and think this should be implemented.

@lcnr lcnr added the E-medium Call for participation: Medium difficulty. Experience needed to fix: Intermediate. label Jun 6, 2023
@b-naber b-naber self-assigned this Jun 21, 2023
@bors bors closed this as completed in fcf3006 Aug 3, 2023
github-actions bot pushed a commit to rust-lang/miri that referenced this issue Aug 4, 2023
Infer type in irrefutable slice patterns with fixed length as array

Fixes rust-lang/rust#76342

In irrefutable slice patterns with a fixed length, we can infer the type as an array type. We now choose to prefer some implementations over others, e.g. in:

```
struct Zeroes;

const ARR: [usize; 2] = [0; 2];
const ARR2: [usize; 2] = [2; 2];

impl Into<&'static [usize; 2]> for Zeroes {
    fn into(self) -> &'static [usize; 2] {
        &ARR
    }
}

impl Into<&'static [usize]> for Zeroes {
    fn into(self) -> &'static [usize] {
        &ARR2
    }
}

fn main() {
    let &[a, b] = Zeroes.into();
}
```

We now prefer the impl candidate `impl Into<&'static [usize; 2]> for Zeroes`, it's not entirely clear to me that this is correct, but given that the slice impl would require a type annotation anyway, this doesn't seem unreasonable.

r? `@lcnr`
lnicola pushed a commit to lnicola/rust-analyzer that referenced this issue Apr 7, 2024
Infer type in irrefutable slice patterns with fixed length as array

Fixes rust-lang/rust#76342

In irrefutable slice patterns with a fixed length, we can infer the type as an array type. We now choose to prefer some implementations over others, e.g. in:

```
struct Zeroes;

const ARR: [usize; 2] = [0; 2];
const ARR2: [usize; 2] = [2; 2];

impl Into<&'static [usize; 2]> for Zeroes {
    fn into(self) -> &'static [usize; 2] {
        &ARR
    }
}

impl Into<&'static [usize]> for Zeroes {
    fn into(self) -> &'static [usize] {
        &ARR2
    }
}

fn main() {
    let &[a, b] = Zeroes.into();
}
```

We now prefer the impl candidate `impl Into<&'static [usize; 2]> for Zeroes`, it's not entirely clear to me that this is correct, but given that the slice impl would require a type annotation anyway, this doesn't seem unreasonable.

r? `@lcnr`
RalfJung pushed a commit to RalfJung/rust-analyzer that referenced this issue Apr 27, 2024
Infer type in irrefutable slice patterns with fixed length as array

Fixes rust-lang/rust#76342

In irrefutable slice patterns with a fixed length, we can infer the type as an array type. We now choose to prefer some implementations over others, e.g. in:

```
struct Zeroes;

const ARR: [usize; 2] = [0; 2];
const ARR2: [usize; 2] = [2; 2];

impl Into<&'static [usize; 2]> for Zeroes {
    fn into(self) -> &'static [usize; 2] {
        &ARR
    }
}

impl Into<&'static [usize]> for Zeroes {
    fn into(self) -> &'static [usize] {
        &ARR2
    }
}

fn main() {
    let &[a, b] = Zeroes.into();
}
```

We now prefer the impl candidate `impl Into<&'static [usize; 2]> for Zeroes`, it's not entirely clear to me that this is correct, but given that the slice impl would require a type annotation anyway, this doesn't seem unreasonable.

r? `@lcnr`
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Labels
A-array Area: [T; N] A-const-generics Area: const generics (parameters and arguments) A-inference Area: Type inference C-enhancement Category: An issue proposing an enhancement or a PR with one. E-medium Call for participation: Medium difficulty. Experience needed to fix: Intermediate.
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7 participants