1.98 beta regression due to name collision with `unroll`

[theemathas]

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This regression was found by the 1.98 beta crater run.

[INFO] [stdout] error[E0659]: `unroll` is ambiguous
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/vers-vecs-1.9.0/src/util/unroll.rs:17:16
[INFO] [stdout]    |
[INFO] [stdout] 17 | pub(crate) use unroll;
[INFO] [stdout]    |                ^^^^^^ ambiguous name
[INFO] [stdout]    |
[INFO] [stdout]    = note: ambiguous because of a name conflict with a builtin attribute
[INFO] [stdout]    = note: `unroll` could refer to a built-in attribute
[INFO] [stdout] note: `unroll` could also refer to the macro defined here
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/vers-vecs-1.9.0/src/util/unroll.rs:5:1
[INFO] [stdout]    |
[INFO] [stdout]  5 | / macro_rules! unroll {
[INFO] [stdout]  6 | |     (1, |$i:ident = {$e:expr}| $s:stmt, $inc:expr) => { let mut $i: usize = $e; $s };
[INFO] [stdout]  7 | |     (2, |$i:ident = {$e:expr}| $s:stmt, $inc:expr) => { unroll!(1, |$i = {$e}| $s, $inc); $inc; $s };
[INFO] [stdout]  8 | |     (3, |$i:ident = {$e:expr}| $s:stmt, $inc:expr) => { unroll!(2, |$i = {$e}| $s, $inc); $inc; $s };
[INFO] [stdout] ...  |
[INFO] [stdout] 13 | |     (8, |$i:ident = {$e:expr}| $s:stmt, $inc:expr) => { unroll!(7, |$i = {$e}| $s, $inc); $inc; $s };
[INFO] [stdout] 14 | | }
[INFO] [stdout]    | |_^

Affects dependents of old versions of vers-vecs:

• (vers-vecs-1.9.0) https://crater-reports.s3.amazonaws.com/beta-1.98-4/1.98.0-beta.1/gh/alvgeppetto.loom/log.txt
• (vers-vecs-1.7.0) https://crater-reports.s3.amazonaws.com/beta-1.98-4/1.98.0-beta.1/gh/feldroop.rust-fmindex-benchmark/log.txt
• (vers-vecs-1.7.0) https://crater-reports.s3.amazonaws.com/beta-1.98-4/1.98.0-beta.1/reg/grafite-0.2.0/log.txt
• (vers-vecs-1.6.3) https://crater-reports.s3.amazonaws.com/beta-1.98-4/1.98.0-beta.1/gh/faassen.colchis/log.txt
• (vers-vecs-1.6.0) https://crater-reports.s3.amazonaws.com/beta-1.98-4/1.98.0-beta.1/gh/sriram98v.Generalized-PD/log.txt
• (vers-vecs-1.5.1) https://crater-reports.s3.amazonaws.com/beta-1.98-4/1.98.0-beta.1/gh/sriram98v.near-linear-cophenetic-distance/log.txt

Presumably regressed by #156816, which added an #[unroll] attribute. cc @saethlin, @JonathanBrouwer, @folkertdev

See also #159428, which is a similar regression.

[folkertdev]

And earlier #[align] which is #[rustc_align] now. Our inability to add new attributes (except over an edition boundary) is really annoying.

[JonathanBrouwer]

The only affected crate already released a new version upstream that fixes this: Cydhra/vers#43
We can either decide to accept this, or to rename this attribute to #[rustc_unroll]. What is the preferred solution?
I'm happy to make a rename PR if needed, should be trivial

[jieyouxu]

#134963 again