Use const generics for array Default impl

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Currently, we generate array impls for every size up to 32 manually using macros, but with const generics at a suitable level of implementation, we can switch to properly parameterising over all lengths.

Replace most manual impls with const generic impls (PR: Use const generics for array impls [part 1] #62435)
Replace Default impl with const generic impl (more difficult due to Switch libcore array implementations to const generics. #60466 (comment)).

Reimplement impls removed due to bloat using const generic impls (see

rust/src/libcore/array.rs

Lines 221 to 227 in 7840a0b

    
           // NOTE: some less important impls are omitted to reduce code bloat 
        
           __impl_slice_eq1! { [A; $N], [B; $N] } 
        
           __impl_slice_eq2! { [A; $N], [B] } 
        
           __impl_slice_eq2! { [A; $N], &'b [B] } 
        
           __impl_slice_eq2! { [A; $N], &'b mut [B] } 
        
           // __impl_slice_eq2! { [A; $N], &'b [B; $N] } 
        
           // __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }

and

rust/src/liballoc/vec.rs

Lines 2199 to 2204 in bfdfa85

    
           // NOTE: some less important impls are omitted to reduce code bloat 
        
           // FIXME(Centril): Reconsider this? 
        
           //__impl_slice_eq1! { [const N: usize] Vec<A>, &mut [B; N], [B; N]: LengthAtMost32 } 
        
           //__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, [B; N], [B; N]: LengthAtMost32 } 
        
           //__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &[B; N], [B; N]: LengthAtMost32 } 
        
           //__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &mut [B; N], [B; N]: LengthAtMost32 }

)

As I noted in #60466 (comment) and #60466 (comment) I will not sign off on using const generics in stable Rust until such time that const generics are stable. However, we can provide unstable wrapper types meanwhile.

We can probably both keep the exact observable behavior (impls for 0-32) and reduce metadata bloat by using a trick similar to #60466 (comment) (an empty marker trait implemented using macros + the real impl using const generics and a where clause with the marker trait).

This way const generics would be an implementation detail, so their stabilization wouldn't be required.

@petrochenkov

The results from the crater run and perf run for using const generics for array impls are in (#60466).

cargo check performance is increased: Switch libcore array implementations to const generics. #60466 (comment). Otherwise, unchanged.
No regressions: Switch libcore array implementations to const generics. #60466 (comment). (Actually fixes a couple of crates that were relying on impls for larger arrays 😄)

I think there's good reason to go with @petrochenkov's suggestion (#61415 (comment)) and potentially consider lifting the restriction for array sizes.

does T-compiler actually need to be tagged on this issue? Unless its exposing bugs for const generics themselves, I would think this is solely a T-libs issue, maybe T-lang, but not T-compiler?

@petrochenkov

This libs team discussed this yesterday and agreed that we do not want to expand the stable surface API of the standard library, but changing implementaitons to use const generics seems fine so long as it doesn't expand the surface area of what's exposed (e.g. via @petrochenkov's idea)

The @rust-lang/lang team discussed this and we agree with the libs team. =)

No-expanded-surface area (#61415 (comment)) PR up at #62435

@nikomatsakis

We talked about this question -- using const generics for the Default impl on arrays and supporting the unconditional [T; 0]: Default as a special case (for now) -- in the lang call today. We had appetite for seeing this happen. Please nominate the stabilization PR for us, and we'll FCP that along with libs-api.

One thing we talked about is how arrays are already a bit magical, e.g. in how [x; N] for some x: T works for N < 2 even when T: ?Copy, how constants are instantiated and dropped N times, etc. The magic needed for Default doesn't feel categorically different than that.

@nikomatsakis in particular mentioned that there are seemingly many routes open to being able to generalize this and that he wasn't worried about finding coherent and feasible design options. Whether any of those options would carry their weight, at the end of the day, as compared with just accepting the special case here, is something we don't know yet and would need to work out.

Remove Init & introduce ZeroableArray

Linking @WaffleLapkin's analysis here

Use generic const exprs to fix rust-lang#61415

See #88744 (comment)

I still feel like we should just support the [T; 0] impl and can actually do so. We can either do so by relying on specialization, similar to #74254, or by adding a new lang item to std as was done in #84838.

We could also use const generic hacks (GCE or GCI + OGCA)

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Use const generics for array `Default` impl #61415

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	// NOTE: some less important impls are omitted to reduce code bloat
	__impl_slice_eq1! { [A; $N], [B; $N] }
	__impl_slice_eq2! { [A; $N], [B] }
	__impl_slice_eq2! { [A; $N], &'b [B] }
	__impl_slice_eq2! { [A; $N], &'b mut [B] }
	// __impl_slice_eq2! { [A; $N], &'b [B; $N] }
	// __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }

	// NOTE: some less important impls are omitted to reduce code bloat
	// FIXME(Centril): Reconsider this?
	//__impl_slice_eq1! { [const N: usize] Vec<A>, &mut [B; N], [B; N]: LengthAtMost32 }
	//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, [B; N], [B; N]: LengthAtMost32 }
	//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &[B; N], [B; N]: LengthAtMost32 }
	//__impl_slice_eq1! { [const N: usize] Cow<'a, [A]>, &mut [B; N], [B; N]: LengthAtMost32 }

Use const generics for array Default impl #61415

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Centril commented on May 31, 2019

petrochenkov commented on Jun 1, 2019

varkor commented on Jun 3, 2019

pnkfelix commented on Jun 6, 2019

alexcrichton commented on Jun 13, 2019

nikomatsakis commented on Jun 20, 2019

scottmcm commented on Jul 6, 2019

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Use const generics for array `Default` impl #61415