Auto merge of #95956 - yaahc:stable-in-unstable, r=cjgillot

Support unstable moves via stable in unstable items

part of https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/moving.20items.20to.20core.20unstably and a blocker of rust-lang/rust#90328.

The libs-api team needs the ability to move an already stable item to a new location unstably, in this case for Error in core. Otherwise these changes are insta-stable making them much harder to merge.

This PR attempts to solve the problem by checking the stability of path segments as well as the last item in the path itself, which is currently the only thing checked.

core/src/intrinsics.rs

@@ -1457,6 +1457,7 @@ extern "rust-intrinsic" {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg_attr(not(bootstrap), rustc_allowed_through_unstable_modules)]
     #[rustc_const_stable(feature = "const_transmute", since = "1.56.0")]
     #[rustc_diagnostic_item = "transmute"]
     pub fn transmute<T, U>(e: T) -> U;
@@ -2649,6 +2650,7 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
 /// Here is an example of how this could cause a problem:
 /// ```no_run
 /// #![feature(const_eval_select)]
+/// #![feature(core_intrinsics)]
 /// use std::hint::unreachable_unchecked;
 /// use std::intrinsics::const_eval_select;
 ///

core/src/macros/mod.rs

@@ -1537,7 +1537,7 @@ pub(crate) mod builtin {
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[allow_internal_unstable(core_intrinsics, libstd_sys_internals)]
+    #[allow_internal_unstable(core_intrinsics, libstd_sys_internals, rt)]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
     #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
     pub macro RustcDecodable($item:item) {
@@ -1547,7 +1547,7 @@ pub(crate) mod builtin {
     /// Unstable implementation detail of the `rustc` compiler, do not use.
     #[rustc_builtin_macro]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[allow_internal_unstable(core_intrinsics)]
+    #[allow_internal_unstable(core_intrinsics, rt)]
     #[deprecated(since = "1.52.0", note = "rustc-serialize is deprecated and no longer supported")]
     #[doc(hidden)] // While technically stable, using it is unstable, and deprecated. Hide it.
     pub macro RustcEncodable($item:item) {

core/tests/unicode.rs

@@ -1,5 +1,5 @@
 #[test]
 pub fn version() {
-    let (major, _minor, _update) = core::unicode::UNICODE_VERSION;
+    let (major, _minor, _update) = core::char::UNICODE_VERSION;
     assert!(major >= 10);
 }

std/src/lib.rs

@@ -214,7 +214,7 @@
 #![cfg_attr(not(bootstrap), deny(ffi_unwind_calls))]
 // std may use features in a platform-specific way
 #![allow(unused_features)]
-#![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count))]
+#![cfg_attr(test, feature(internal_output_capture, print_internals, update_panic_count, rt))]
 #![cfg_attr(
     all(target_vendor = "fortanix", target_env = "sgx"),
     feature(slice_index_methods, coerce_unsized, sgx_platform)
@@ -297,6 +297,7 @@
 // Library features (alloc):
 #![feature(alloc_layout_extra)]
 #![feature(alloc_c_string)]
+#![feature(alloc_ffi)]
 #![feature(allocator_api)]
 #![feature(get_mut_unchecked)]
 #![feature(map_try_insert)]

std/src/panic.rs

@@ -11,7 +11,7 @@ use crate::thread::Result;
 
 #[doc(hidden)]
 #[unstable(feature = "edition_panic", issue = "none", reason = "use panic!() instead")]
-#[allow_internal_unstable(libstd_sys_internals, const_format_args, core_panic)]
+#[allow_internal_unstable(libstd_sys_internals, const_format_args, core_panic, rt)]
 #[cfg_attr(not(test), rustc_diagnostic_item = "std_panic_2015_macro")]
 #[rustc_macro_transparency = "semitransparent"]
 pub macro panic_2015 {