Auto merge of rust-lang#120675 - oli-obk:intrinsics3.0, r=pnkfelix

Add a scheme for moving away from `extern "rust-intrinsic"` entirely

All `rust-intrinsic`s can become free functions now, either with a fallback body, or with a dummy body and an attribute, requiring backends to actually implement the intrinsic.

This PR demonstrates the dummy-body scheme with the `vtable_size` intrinsic.

cc rust-lang#63585

follow-up to rust-lang#120500

MCP at rust-lang/compiler-team#720

Author: bors

core/src/intrinsics.rs

@@ -2499,9 +2499,8 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn black_box<T>(dummy: T) -> T;
 
-    /// `ptr` must point to a vtable.
-    /// The intrinsic will return the size stored in that vtable.
     #[rustc_nounwind]
+    #[cfg(bootstrap)]
     pub fn vtable_size(ptr: *const ()) -> usize;
 
     /// `ptr` must point to a vtable.
@@ -2681,6 +2680,17 @@ pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {
 #[cfg_attr(bootstrap, inline)]
 pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
 
+/// `ptr` must point to a vtable.
+/// The intrinsic will return the size stored in that vtable.
+#[rustc_nounwind]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+#[cfg_attr(not(bootstrap), rustc_intrinsic)]
+#[cfg_attr(not(bootstrap), rustc_intrinsic_must_be_overridden)]
+#[cfg(not(bootstrap))]
+pub unsafe fn vtable_size(_ptr: *const ()) -> usize {
+    unreachable!()
+}
+
 // Some functions are defined here because they accidentally got made
 // available in this module on stable. See <https://github.com/rust-lang/rust/issues/15702>.
 // (`transmute` also falls into this category, but it cannot be wrapped due to the