Rollup merge of #114837 - RalfJung:error_in_core, r=cuviper

add missing feature(error_in_core)

Needed to fix feature gate errors in https://github.com/rust-lang/miri-test-libstd/actions/runs/5862810459/job/15895203359. I don't know how doctests are passing in-tree without this feature gate...

library/core/src/error.rs

@@ -130,6 +130,7 @@ fn cause(&self) -> Option<&dyn Error> {
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     /// use core::fmt;
     /// use core::error::{request_ref, Request};
     ///
@@ -360,6 +361,7 @@ pub fn sources(&self) -> Source<'_> {
 ///
 /// ```rust
 /// # #![feature(error_generic_member_access)]
+/// # #![feature(error_in_core)]
 /// use std::error::Error;
 /// use core::error::request_value;
 ///
@@ -383,6 +385,7 @@ pub fn request_value<'a, T>(err: &'a (impl Error + ?Sized)) -> Option<T>
 ///
 /// ```rust
 /// # #![feature(error_generic_member_access)]
+/// # #![feature(error_in_core)]
 /// use core::error::Error;
 /// use core::error::request_ref;
 ///
@@ -454,6 +457,7 @@ fn request_by_type_tag<'a, I>(err: &'a (impl Error + ?Sized)) -> Option<I::Reifi
 ///
 /// ```
 /// #![feature(error_generic_member_access)]
+/// #![feature(error_in_core)]
 /// use core::fmt;
 /// use core::error::Request;
 /// use core::error::request_ref;
@@ -524,6 +528,7 @@ fn new<'b>(erased: &'b mut (dyn Erased<'a> + 'a)) -> &'b mut Request<'a> {
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     ///
     /// use core::error::Request;
     ///
@@ -558,6 +563,7 @@ pub fn provide_value<T>(&mut self, value: T) -> &mut Self
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     ///
     /// use core::error::Request;
     ///
@@ -593,6 +599,7 @@ pub fn provide_value_with<T>(&mut self, fulfil: impl FnOnce() -> T) -> &mut Self
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     ///
     /// use core::error::Request;
     ///
@@ -625,6 +632,7 @@ pub fn provide_ref<T: ?Sized + 'static>(&mut self, value: &'a T) -> &mut Self {
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     ///
     /// use core::error::Request;
     ///
@@ -691,6 +699,7 @@ fn provide_with<I>(&mut self, fulfil: impl FnOnce() -> I::Reified) -> &mut Self
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     ///
     /// use core::error::Request;
     /// use core::error::request_value;
@@ -778,6 +787,7 @@ pub fn would_be_satisfied_by_value_of<T>(&self) -> bool
     ///
     /// ```rust
     /// #![feature(error_generic_member_access)]
+    /// #![feature(error_in_core)]
     ///
     /// use core::error::Request;
     /// use core::error::request_ref;