Auto merge of #139916 - RalfJung:intrinsic-wrappers, r=Mark-Simulacrum

make std::intrinsics functions actually be intrinsics

Most of the functions in `std::intrinsics` are actually intrinsics, but some are not: for historical reasons, `std::intrinsics::{copy,copy_nonoverlapping,write_bytes}` are accessible on stable, and the versions in `std::ptr` are just re-exports. These functions are not intrinsics, but wrappers around the intrinsic, because they add extra debug assertions.

This PR makes the functions in `std::intrinsics` actually be intrinsics.
- The advantage is that we can now use it in tests that need to directly call the intrinsic, thus removing a footgun for compiler development. We also remove the extended user-facing doc comments of these functions out of a file that should be largely internal documentation.
- The downside is that if users are using those functions directly, they will not get the debug assertions any more. Note however that those users are already ignoring a deprecation warning, so I think this is fine. Furthermore, if someone imports the `intrinsic` name of this function and turns that into a function pointer, that will no longer work, since only the wrapper functions can be turned into a function pointer. I would be rather surprised if anyone did this, though... and again, they must have already ignored a deprecation warning. Still, seems worth a crater run, if there's general agreement that we want to go ahead with this change.

(`intrinsics::drop_in_place` also remains not-an-intrinsic, which bugs me, but oh well, not much we can do about it; we can't remove it from the module as the path is accidentally-stable.)

Cc `@rust-lang/libs-api` `@saethlin`

Author: bors

tests/fail/intrinsics/copy_overlapping.rs

@@ -1,14 +1,11 @@
-#![feature(intrinsics)]
-
-// Directly call intrinsic to avoid debug assertions in libstd
-#[rustc_intrinsic]
-unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
+#![feature(core_intrinsics)]
 
 fn main() {
     let mut data = [0u8; 16];
     unsafe {
         let a = data.as_mut_ptr();
         let b = a.wrapping_offset(1) as *mut _;
-        copy_nonoverlapping(a, b, 2); //~ ERROR: `copy_nonoverlapping` called on overlapping ranges
+        // Directly call intrinsic to avoid debug assertions in the `std::ptr` version.
+        std::intrinsics::copy_nonoverlapping(a, b, 2); //~ ERROR: `copy_nonoverlapping` called on overlapping ranges
     }
 }

tests/fail/intrinsics/copy_overlapping.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: `copy_nonoverlapping` called on overlapping ranges
   --> tests/fail/intrinsics/copy_overlapping.rs:LL:CC
    |
-LL |         copy_nonoverlapping(a, b, 2);
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `copy_nonoverlapping` called on overlapping ranges
+LL |         std::intrinsics::copy_nonoverlapping(a, b, 2);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `copy_nonoverlapping` called on overlapping ranges
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

tests/fail/intrinsics/copy_unaligned.rs

@@ -1,14 +1,11 @@
-#![feature(intrinsics)]
-
-// Directly call intrinsic to avoid debug assertions in libstd
-#[rustc_intrinsic]
-unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
+#![feature(core_intrinsics)]
 
 fn main() {
     let mut data = [0u16; 8];
     let ptr = (&mut data[0] as *mut u16 as *mut u8).wrapping_add(1) as *mut u16;
     // Even copying 0 elements to something unaligned should error
     unsafe {
-        copy_nonoverlapping(&data[5], ptr, 0); //~ ERROR: accessing memory with alignment 1, but alignment 2 is required
+        // Directly call intrinsic to avoid debug assertions in the `std::ptr` version.
+        std::intrinsics::copy_nonoverlapping(&data[5], ptr, 0); //~ ERROR: accessing memory with alignment 1, but alignment 2 is required
     }
 }

tests/fail/intrinsics/copy_unaligned.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: accessing memory with alignment ALIGN, but alignment ALIGN is required
   --> tests/fail/intrinsics/copy_unaligned.rs:LL:CC
    |
-LL |         copy_nonoverlapping(&data[5], ptr, 0);
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
+LL |         std::intrinsics::copy_nonoverlapping(&data[5], ptr, 0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information