Expose portable-simd as core::simd

This enables programmers to use a safe alternative to the current
`extern "platform-intrinsics"` API for writing portable SIMD code.
This is `#![feature(portable_simd)]` as tracked in rust-lang#86656

Author: workingjubilee

core/src/lib.rs

@@ -386,4 +386,25 @@ pub mod arch {
     }
 }
 
+// Pull in the `core_simd` crate directly into libcore. The contents of
+// `core_simd` are in a different repository: rust-lang/portable-simd.
+//
+// `core_simd` depends on libcore, but the contents of this module are
+// set up in such a way that directly pulling it here works such that the
+// crate uses this crate as its libcore.
+#[path = "../../portable-simd/crates/core_simd/src/mod.rs"]
+#[allow(missing_debug_implementations, dead_code, unsafe_op_in_unsafe_fn, unused_unsafe)]
+#[allow(rustdoc::bare_urls)]
+#[unstable(feature = "portable_simd", issue = "86656")]
+#[cfg(not(bootstrap))]
+mod core_simd;
+
+#[doc = include_str!("../../portable-simd/crates/core_simd/src/core_simd_docs.md")]
+#[unstable(feature = "portable_simd", issue = "86656")]
+#[cfg(not(bootstrap))]
+pub mod simd {
+    #[unstable(feature = "portable_simd", issue = "86656")]
+    pub use crate::core_simd::simd::*;
+}
+
 include!("primitive_docs.rs");

std/src/lib.rs

@@ -320,6 +320,7 @@
 #![feature(panic_internals)]
 #![feature(panic_unwind)]
 #![feature(pin_static_ref)]
+#![cfg_attr(not(bootstrap), feature(portable_simd))]
 #![feature(prelude_import)]
 #![feature(ptr_internals)]
 #![feature(rustc_attrs)]
@@ -472,6 +473,9 @@ pub use core::pin;
 pub use core::ptr;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::result;
+#[unstable(feature = "portable_simd", issue = "86656")]
+#[cfg(not(bootstrap))]
+pub use core::simd;
 #[unstable(feature = "async_stream", issue = "79024")]
 pub use core::stream;
 #[stable(feature = "i128", since = "1.26.0")]