1.97 beta rustdoc regression involving `rustc_` attributes

[theemathas]

Regression discovered by the 1.97 beta rustdoc crater run.

---

[INFO] [stdout] error: attributes starting with `rustc` are reserved for use by the `rustc` compiler
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.4/src/backend/linux_raw/io/errno.rs:28:25
[INFO] [stdout]    |
[INFO] [stdout] 28 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_start(0xf001))]
[INFO] [stdout]    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: attributes starting with `rustc` are reserved for use by the `rustc` compiler
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.4/src/backend/linux_raw/io/errno.rs:29:25
[INFO] [stdout]    |
[INFO] [stdout] 29 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_end(0xffff))]
[INFO] [stdout]    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stderr]     Checking quote v1.0.31
[INFO] [stderr]     Checking termcolor v1.2.0
[INFO] [stdout] error: cannot find attribute `rustc_layout_scalar_valid_range_start` in this scope
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.4/src/backend/linux_raw/io/errno.rs:28:25
[INFO] [stdout]    |
[INFO] [stdout] 28 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_start(0xf001))]
[INFO] [stdout]    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: cannot find attribute `rustc_layout_scalar_valid_range_end` in this scope
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.4/src/backend/linux_raw/io/errno.rs:29:25
[INFO] [stdout]    |
[INFO] [stdout] 29 | #[cfg_attr(rustc_attrs, rustc_layout_scalar_valid_range_end(0xffff))]
[INFO] [stdout]    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Affects 45 dependents of old versions of rustix (affects at least versions 0.36.7 - 0.38.4)

• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/reg/jeep-0.0.1/log.txt
• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/gh/museun.twitchchat/log.txt
• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/gh/networkException.oxidetracks/log.txt
• etc.

---

[INFO] [stdout] error: attributes starting with `rustc` are reserved for use by the `rustc` compiler
[INFO] [stdout]    --> src/index.rs:149:41
[INFO] [stdout]     |
[INFO] [stdout] 149 |         #[cfg_attr(feature = "nightly", rustc_layout_scalar_valid_range_end($max))]
[INFO] [stdout]     |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout] ...
[INFO] [stdout] 315 | base_index!(BaseIndex32(u32 <= 0xFFFF_FF00));
[INFO] [stdout]     | -------------------------------------------- in this macro invocation
[INFO] [stdout]     |
[INFO] [stdout]     = note: this error originates in the macro `base_index` (in Nightly builds, run with -Z macro-backtrace for more info)
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error: cannot find attribute `rustc_layout_scalar_valid_range_end` in this scope
[INFO] [stdout]    --> src/index.rs:149:41
[INFO] [stdout]     |
[INFO] [stdout] 149 |         #[cfg_attr(feature = "nightly", rustc_layout_scalar_valid_range_end($max))]
[INFO] [stdout]     |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout] ...
[INFO] [stdout] 315 | base_index!(BaseIndex32(u32 <= 0xFFFF_FF00));
[INFO] [stdout]     | -------------------------------------------- in this macro invocation
[INFO] [stdout]     |
[INFO] [stdout]     = note: this error originates in the macro `base_index` (in Nightly builds, run with -Z macro-backtrace for more info)

Affects solar-data-structures and its 3 dependendents:

• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/reg/solar-data-structures-0.1.8/log.txt
• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/reg/solar-ast-0.1.8/log.txt
• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/reg/solar-interface-0.1.8/log.txt
• https://crater-reports.s3.amazonaws.com/beta-rustdoc-1.97-3/1.97.0-beta.1/reg/solar-parse-0.1.8/log.txt

[Urgau]

How can this be a regression? The attribute is nightly-only and was removed in #155433.

[theemathas]

Maybe something about the rustdoc crater setup allowed using nightly features?

[mejrs]

Those versions of rustix have a feature detection script in build.rs. Doesn't rustdoc use a nightly rustc version?