Use ranges for slice notation.

[nrc]

Blocked by #19794. Part of #19148.

Nominating for p-backcompat-lang, 1.0 because its backwards incompatible since we remove the ..foo notation for taking a slice from the start of some range.

[nrc]

Implementation strategy:

• Implement Add range notation #19794, with the special case that range notation inside [] uses the old slice machinery.
• Parse slicing syntax as an index with a range, but check for slicing traits in the compiler.
• Move implementors of slice traits to using index traits/range.
• Remove slicing notation syntax and traits, etc. (breaking change). Check that special treatment of slicing is removed.

[quantheory]

👍
If I understand correctly, I believe that #19794 is doable now, but the changes to indexing traits are blocked by #19129 (and possibly other blockers for associated types in general). I think it's probably possible to do most (all?) of the slicing implementations with the current Index/IndexMut, but then when the new Index lands all of the impls will have to change, so it would be simpler if the Index change happened first.

(Also, the removal of ..j will only occur if RFC 498 lands in its current form, but I guess this change will be backwards-incompatible regardless, due to the removal of the existing slicing traits?)

[pnkfelix]

P-backcompat-lang, 1.0.

[seanmonstar]

With the new array syntax of [u8; ..4], the ..j range doesn't need to be removed, right?

[aturon]

I believe this is done; nominating for closure.

[nrc]

Yeah, should have been closed when I landed the patch.