Switch from SmallVec to 100% safe TinyVec

[Shnatsel]

Replaces uses of SmallVec with TinyVec, which is the same idea implemented in 100% safe code.

Performance is generally in line with SmallVec in this case, with some benchmarks being slightly slower and some slightly faster:

SmallVec 1.1.0

test bench_is_nfc_ascii                      ... bench:          15 ns/iter (+/- 0)
test bench_is_nfc_normalized                 ... bench:          26 ns/iter (+/- 0)
test bench_is_nfc_not_normalized             ... bench:         455 ns/iter (+/- 7)
test bench_is_nfc_stream_safe_ascii          ... bench:          14 ns/iter (+/- 0)
test bench_is_nfc_stream_safe_normalized     ... bench:          39 ns/iter (+/- 0)
test bench_is_nfc_stream_safe_not_normalized ... bench:         526 ns/iter (+/- 9)
test bench_is_nfd_ascii                      ... bench:          17 ns/iter (+/- 3)
test bench_is_nfd_normalized                 ... bench:          43 ns/iter (+/- 0)
test bench_is_nfd_not_normalized             ... bench:          15 ns/iter (+/- 0)
test bench_is_nfd_stream_safe_ascii          ... bench:          17 ns/iter (+/- 3)
test bench_is_nfd_stream_safe_normalized     ... bench:          72 ns/iter (+/- 1)
test bench_is_nfd_stream_safe_not_normalized ... bench:          15 ns/iter (+/- 0)
test bench_nfc_ascii                         ... bench:         557 ns/iter (+/- 6)
test bench_nfc_long                          ... bench:     235,422 ns/iter (+/- 2,928)
test bench_nfd_ascii                         ... bench:         325 ns/iter (+/- 6)
test bench_nfd_long                          ... bench:     133,196 ns/iter (+/- 1,167)
test bench_nfkc_ascii                        ... bench:         557 ns/iter (+/- 19)
test bench_nfkc_long                         ... bench:     252,428 ns/iter (+/- 2,076)
test bench_nfkd_ascii                        ... bench:         300 ns/iter (+/- 3)
test bench_nfkd_long                         ... bench:     137,857 ns/iter (+/- 1,435)
test bench_streamsafe_adversarial            ... bench:         471 ns/iter (+/- 7)
test bench_streamsafe_ascii                  ... bench:          81 ns/iter (+/- 1)

TinyVec 0.3.2

test bench_is_nfc_ascii                      ... bench:          14 ns/iter (+/- 0)
test bench_is_nfc_normalized                 ... bench:          26 ns/iter (+/- 0)
test bench_is_nfc_not_normalized             ... bench:         440 ns/iter (+/- 3)
test bench_is_nfc_stream_safe_ascii          ... bench:          14 ns/iter (+/- 0)
test bench_is_nfc_stream_safe_normalized     ... bench:          38 ns/iter (+/- 0)
test bench_is_nfc_stream_safe_not_normalized ... bench:         524 ns/iter (+/- 5)
test bench_is_nfd_ascii                      ... bench:          16 ns/iter (+/- 2)
test bench_is_nfd_normalized                 ... bench:          42 ns/iter (+/- 0)
test bench_is_nfd_not_normalized             ... bench:          14 ns/iter (+/- 0)
test bench_is_nfd_stream_safe_ascii          ... bench:          16 ns/iter (+/- 0)
test bench_is_nfd_stream_safe_normalized     ... bench:          53 ns/iter (+/- 0)
test bench_is_nfd_stream_safe_not_normalized ... bench:          15 ns/iter (+/- 0)
test bench_nfc_ascii                         ... bench:         595 ns/iter (+/- 8)
test bench_nfc_long                          ... bench:     216,145 ns/iter (+/- 3,176)
test bench_nfd_ascii                         ... bench:         361 ns/iter (+/- 5)
test bench_nfd_long                          ... bench:     141,053 ns/iter (+/- 552)
test bench_nfkc_ascii                        ... bench:         587 ns/iter (+/- 8)
test bench_nfkc_long                         ... bench:     225,678 ns/iter (+/- 2,640)
test bench_nfkd_ascii                        ... bench:         333 ns/iter (+/- 7)
test bench_nfkd_long                         ... bench:     149,023 ns/iter (+/- 613)
test bench_streamsafe_adversarial            ... bench:         485 ns/iter (+/- 2)
test bench_streamsafe_ascii                  ... bench:          68 ns/iter (+/- 0)

As you can see, e.g. bench_is_nfd_stream_safe_normalized and bench_nfkc_long have improved while bench_nfkd_long and bench_nfc_ascii have regressed, but none of the changes are dramatic.

cargo test passes, I haven't run any parametric or fuzz testing (not sure if this crate has any). TinyVec itself has been fuzz-tested for equivalence to std::Vec using arbitrary-model-tests framework.

[Manishearth]

I'm usually not super worried about unsafe, however given that we're able to make this change without affecting anything else, I don't mind.

[8573]

I noticed that my CI builds for https://github.com/8573/irc-bot.rs are failing on trying to build, as a transitive dependency, smallvec 1.2, whose minimum supported Rust version (MSRV), at 1.36, is higher than mine. cargo tree traces the smallvec 1.2 dependency to unicode-normalization. I see that this crate's MSRV rose to 1.36 because of smallvec (commit cd63ded). With smallvec gone from this crate's dependencies, will this crate's MSRV go back down, or has this crate otherwise come to depend on newer Rust features since commit cd63ded?

[8573]

Oh, my apologies! I should have checked tinyvec's MSRV — also 1.36.

[Shnatsel]

Could you please make a new release on crates.io incorporating the changes? I see the version has already been bumped in master, and it's been months since the merge.

[Manishearth]

Done

[Shnatsel]

Thanks a lot!