nit: update error text to cite tracking issue

Author: nikomatsakis

src/libcore/convert.rs

@@ -556,12 +556,8 @@ impl<T> From<T> for T {
 
 #[stable(feature = "convert_infallible", since = "1.34.0")]
 #[cfg(not(boostrap_stdarch_ignore_this))]
-#[rustc_reservation_impl="a future version of Rust might implement `From<!>` for \
-    all types. \
-    However, it is OK to implement `From<!>` for types you own - \
-    when the blanket impl will be added, coherence will be changed \
-    to make these impls not be an error."
-]
+#[rustc_reservation_impl="permitting this impl would forbid us from adding \
+`impl<T> From<!> for T` later; see rust-lang/rust#64715 for details"]
 impl<T> From<!> for T {
     fn from(t: !) -> T { t }
 }

src/test/ui/never-from-impl-is-reserved.stderr

@@ -7,7 +7,7 @@ LL | // This will conflict with the first impl if we impl `for<T> T: From<!>`.
 LL | impl<T> MyTrait for T where T: From<!> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `MyFoo`
    |
-   = note: a future version of Rust might implement `From<!>` for all types. However, it is OK to implement `From<!>` for types you own - when the blanket impl will be added, coherence will be changed to make these impls not be an error.
+   = note: permitting this impl would forbid us from adding `impl<T> From<!> for T` later; see rust-lang/rust#64715 for details
 
 error: aborting due to previous error