pub use std::simd::StdFloat;

Make available the remaining float intrinsics that require runtime support
from a platform's libm, and thus cannot be included in a no-deps libcore,
by exposing them through a sealed trait, `std::simd::StdFloat`.

We might use the trait approach a bit more in the future, or maybe not.
Ideally, this trait doesn't stick around, even if so.
If we don't need to intermesh it with std, it can be used as a crate,
but currently that is somewhat uncertain.

library/std/src/lib.rs

@@ -313,6 +313,7 @@
 #![feature(panic_internals)]
 #![feature(panic_unwind)]
 #![feature(pin_static_ref)]
+#![feature(platform_intrinsics)]
 #![feature(portable_simd)]
 #![feature(prelude_import)]
 #![feature(ptr_as_uninit)]
@@ -465,8 +466,6 @@ 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")]
-pub use core::simd;
 #[unstable(feature = "async_stream", issue = "79024")]
 pub use core::stream;
 #[stable(feature = "i128", since = "1.26.0")]
@@ -513,6 +512,25 @@ pub mod time;
 #[unstable(feature = "once_cell", issue = "74465")]
 pub mod lazy;
 
+// Pull in `std_float` crate  into libstd. The contents of
+// `std_float` are in a different repository: rust-lang/portable-simd.
+#[path = "../../portable-simd/crates/std_float/src/lib.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(all(miri, doctest)))] // Miri does not support all SIMD intrinsics
+mod std_float;
+
+#[cfg(not(all(miri, doctest)))] // Miri does not support all SIMD intrinsics
+#[doc = include_str!("../../portable-simd/crates/core_simd/src/core_simd_docs.md")]
+#[unstable(feature = "portable_simd", issue = "86656")]
+pub mod simd {
+    #[doc(inline)]
+    pub use crate::std_float::StdFloat;
+    #[doc(inline)]
+    pub use core::simd::*;
+}
+
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub mod task {
     //! Types and Traits for working with asynchronous tasks.

src/test/ui/simd/libm_std_can_float.rs

@@ -0,0 +1,23 @@
+// run-pass
+
+// This is the converse of the other libm test.
+#![feature(portable_simd)]
+use std::simd::f32x4;
+use std::simd::StdFloat;
+
+// For SIMD float ops, the LLIR version which is used to implement the portable
+// forms of them may become calls to math.h AKA libm. So, we can't guarantee
+// we can compile them for #![no_std] crates.
+//
+// However, we can expose some of these ops via an extension trait.
+fn main() {
+    let x = f32x4::from_array([0.1, 0.5, 0.6, -1.5]);
+    let x2 = x + x;
+    let _xc = x.ceil();
+    let _xf = x.floor();
+    let _xr = x.round();
+    let _xt = x.trunc();
+    let _xfma = x.mul_add(x, x);
+    let _xsqrt = x.sqrt();
+    let _ = x2.abs() * x2;
+}

src/test/ui/simd/portable-intrinsics-arent-exposed.rs

@@ -1,7 +1,8 @@
 // May not matter, since people can use them with a nightly feature.
 // However this tests to guarantee they don't leak out via portable_simd,
 // and thus don't accidentally get stabilized.
-use std::simd::intrinsics; //~ERROR E0603
+use core::simd::intrinsics; //~ERROR E0433
+use std::simd::intrinsics; //~ERROR E0432
 
 fn main() {
     ()

src/test/ui/simd/portable-intrinsics-arent-exposed.stderr

@@ -1,15 +1,16 @@
-error[E0603]: module `intrinsics` is private
-  --> $DIR/portable-intrinsics-arent-exposed.rs:4:16
+error[E0433]: failed to resolve: maybe a missing crate `core`?
+  --> $DIR/portable-intrinsics-arent-exposed.rs:4:5
+   |
+LL | use core::simd::intrinsics;
+   |     ^^^^ maybe a missing crate `core`?
+
+error[E0432]: unresolved import `std::simd::intrinsics`
+  --> $DIR/portable-intrinsics-arent-exposed.rs:5:5
    |
 LL | use std::simd::intrinsics;
-   |                ^^^^^^^^^^ private module
-   |
-note: the module `intrinsics` is defined here
-  --> $SRC_DIR/core/src/lib.rs:LL:COL
-   |
-LL |     pub use crate::core_simd::simd::*;
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     ^^^^^^^^^^^^^^^^^^^^^ no `intrinsics` in `simd`
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0603`.
+Some errors have detailed explanations: E0432, E0433.
+For more information about an error, try `rustc --explain E0432`.