Auto merge of rust-lang#145916 - matthiaskrgr:rollup-cnvhq2z, r=matthiaskrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#142215 (Use -Zmir-opt-level=0 in tests for MIR building)
 - rust-lang#143341 (Mention that casting to *const () is a way to roundtrip with from_raw_parts)
 - rust-lang#145078 (Fix wrong cache line size of riscv64)
 - rust-lang#145290 (Improve std::fs::read_dir docs)
 - rust-lang#145335 (Move WTF-8 code from std into core and alloc)
 - rust-lang#145904 (Move `riscv64-gc-unknown-linux-musl` from Tier 2 with Host tools to Tier 2)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

library/alloc/src/lib.rs

@@ -159,6 +159,7 @@
 #![feature(unicode_internals)]
 #![feature(unsize)]
 #![feature(unwrap_infallible)]
+#![feature(wtf8_internals)]
 // tidy-alphabetical-end
 //
 // Language features:
@@ -232,6 +233,8 @@ pub mod sync;
 #[cfg(all(not(no_global_oom_handling), not(no_rc), not(no_sync)))]
 pub mod task;
 pub mod vec;
+#[cfg(all(not(no_rc), not(no_sync), not(no_global_oom_handling)))]
+pub mod wtf8;
 
 #[doc(hidden)]
 #[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]