Notes on moving toward newer nightly Rust

[weikengchen]

This is a note that summarizes some changes that would be needed to move stwo into a more recent nightly version of Rust.

• slice_flatten feature used to be defined over flatten. It has been changed to be over as_flattened and has been stablized.
• target_feature = "avx2f" would be considered incorrect. Use target_feature = "avx2" instead
• slice_group_by feature used to be defined over group_by. It has been changed to be over chunk_by and has been stablized.
• One .collect::<Result<_, _>>()? would be considered incompatible with future version because the compiler thinks it would fail if ! and () are no longer considered the same type. This compiler's complaint can be addressed by changing it to .collect::<Result<(), _>>()?.
• array_methods, slice_group_by, slice_flatten, stdsimd, slice_first_last_chunk doesn't need to be brought up as features in the lib.rs.

Remaining issues:

• Since 1.80.0 it seems that the detection of unused features is stricter (or, could be misreporting), so after these changes, there are still some errors suggesting things to be unused. Some of which are reasonable. Could be related to https://github.com/rust-lang/rust/pull/122382/files but it doesn't explain everything, and it could be related to bugs. dead_code lint wrongly warns about public repr(C) structs with private fields but no constructors rust-lang/rust#126169
• Can be addressed by waiving those features from this requirement or implementing some tests around them.