Fix docs and tracking issue number for ub_checks

* Change `ub_checks` unstable feature issue from "none" to "123499".
* Change `assert_unsafe_precondition` docs to refer to `check_language_ub`.

library/core/src/lib.rs

@@ -360,7 +360,7 @@ pub mod hint;
 pub mod intrinsics;
 pub mod mem;
 pub mod ptr;
-#[unstable(feature = "ub_checks", issue = "none")]
+#[unstable(feature = "ub_checks", issue = "123499")]
 pub mod ub_checks;
 
 /* Core language traits */

library/core/src/ub_checks.rs

@@ -10,7 +10,7 @@ use crate::intrinsics::{self, const_eval_select};
 /// macro for language UB are always ignored.
 ///
 /// This macro should be called as
-/// `assert_unsafe_precondition!(check_{library,lang}_ub, "message", (ident: type = expr, ident: type = expr) => check_expr)`
+/// `assert_unsafe_precondition!(check_{library,language}_ub, "message", (ident: type = expr, ident: type = expr) => check_expr)`
 /// where each `expr` will be evaluated and passed in as function argument `ident: type`. Then all
 /// those arguments are passed to a function with the body `check_expr`.
 /// Pick `check_language_ub` when this is guarding a violation of language UB, i.e., immediate UB
@@ -49,7 +49,7 @@ use crate::intrinsics::{self, const_eval_select};
 /// debuginfo to have a measurable compile-time impact on debug builds.
 #[allow_internal_unstable(const_ub_checks)] // permit this to be called in stably-const fn
 #[macro_export]
-#[unstable(feature = "ub_checks", issue = "none")]
+#[unstable(feature = "ub_checks", issue = "123499")]
 macro_rules! assert_unsafe_precondition {
     ($kind:ident, $message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => {
         {
@@ -79,11 +79,11 @@ macro_rules! assert_unsafe_precondition {
         }
     };
 }
-#[unstable(feature = "ub_checks", issue = "none")]
+#[unstable(feature = "ub_checks", issue = "123499")]
 pub use assert_unsafe_precondition;
 /// Checking library UB is always enabled when UB-checking is done
 /// (and we use a reexport so that there is no unnecessary wrapper function).
-#[unstable(feature = "ub_checks", issue = "none")]
+#[unstable(feature = "ub_checks", issue = "123499")]
 pub use intrinsics::ub_checks as check_library_ub;
 
 /// Determines whether we should check for language UB.