Rollup merge of rust-lang#137489 - RalfJung:no-more-rustc_intrinsic_must_be_overridden, r=oli-obk

remove `#[rustc_intrinsic_must_be_overridde]`

In rust-lang#135031, we gained support for just leaving away the body. Now that the bootstrap compiler got bumped, stop using the old style and remove support for it.

r? `@oli-obk`

There are a few more mentions of this attribute in RA code that I didn't touch; Cc `@rust-lang/rust-analyzer`

Author: compiler-errors

alloc/src/boxed.rs

@@ -238,11 +238,8 @@ pub struct Box<
 ///
 /// This is the surface syntax for `box <expr>` expressions.
 #[rustc_intrinsic]
-#[rustc_intrinsic_must_be_overridden]
 #[unstable(feature = "liballoc_internals", issue = "none")]
-pub fn box_new<T>(_x: T) -> Box<T> {
-    unreachable!()
-}
+pub fn box_new<T>(_x: T) -> Box<T>;
 
 impl<T> Box<T> {
     /// Allocates memory on the heap and then places `x` into it.

core/src/ffi/va_list.rs

@@ -305,25 +305,16 @@ impl<'f> Drop for VaListImpl<'f> {
 /// Destroy the arglist `ap` after initialization with `va_start` or
 /// `va_copy`.
 #[rustc_intrinsic]
-#[rustc_intrinsic_must_be_overridden]
 #[rustc_nounwind]
-unsafe fn va_end(_ap: &mut VaListImpl<'_>) {
-    unreachable!()
-}
+unsafe fn va_end(_ap: &mut VaListImpl<'_>);
 
 /// Copies the current location of arglist `src` to the arglist `dst`.
 #[rustc_intrinsic]
-#[rustc_intrinsic_must_be_overridden]
 #[rustc_nounwind]
-unsafe fn va_copy<'f>(_dest: *mut VaListImpl<'f>, _src: &VaListImpl<'f>) {
-    unreachable!()
-}
+unsafe fn va_copy<'f>(_dest: *mut VaListImpl<'f>, _src: &VaListImpl<'f>);
 
 /// Loads an argument of type `T` from the `va_list` `ap` and increment the
 /// argument `ap` points to.
 #[rustc_intrinsic]
-#[rustc_intrinsic_must_be_overridden]
 #[rustc_nounwind]
-unsafe fn va_arg<T: sealed_trait::VaArgSafe>(_ap: &mut VaListImpl<'_>) -> T {
-    unreachable!()
-}
+unsafe fn va_arg<T: sealed_trait::VaArgSafe>(_ap: &mut VaListImpl<'_>) -> T;