Auto merge of #59445 - alexreg:ban-multi-trait-objects-via-aliases, r=oli-obk

Ban multi-trait objects via trait aliases

Obviously, multi-trait objects are not normally supported, so they should not be supported via trait aliases.

This has been factored out from the previous PR https://github.com/rust-lang/rust/pull/55994 (see point 1).

r? @Centril

CC @nikomatsakis

------------------

### RELNOTES:

We now allow `dyn Send + fmt::Debug` with equivalent semantics to `dyn fmt::Debug + Send`.
That is, the order of the mentioned traits does not matter wrt. principal/not-principal traits.
This is a small change that might deserve a mention in the blog post because it is a language change but most likely not.

See https://github.com/rust-lang/rust/blob/ce2ee305f9165c037ecddddb5792588a15ff6c37/src/test/ui/traits/wf-trait-object-reverse-order.rs.

// @Centril

1 files changed, 4 insertions, 4 deletions

diff --git a/src/liballoc/slice.rs b/src/liballoc/slice.rs
index 6eac8487401..8768f1ff081 100644
--- a/src/liballoc/slice.rs
+++ b/src/liballoc/slice.rs
@@ -123,12 +123,12 @@ pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
 ////////////////////////////////////////////////////////////////////////////////
 
 // HACK(japaric) needed for the implementation of `vec!` macro during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use hack::into_vec;
 
 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use hack::to_vec;
 
@@ -376,7 +376,7 @@ impl<T> [T] {
     pub fn to_vec(&self) -> Vec<T>
         where T: Clone
     {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::to_vec(self)
     }
 
@@ -397,7 +397,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn into_vec(self: Box<Self>) -> Vec<T> {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::into_vec(self)
     }