Auto merge of #55101 - alexreg:trait-aliases, r=nikomatsakis

Implement trait aliases (RFC 1733)

Extends groundwork done in https://github.com/rust-lang/rust/pull/45047, and fully implements https://github.com/rust-lang/rfcs/pull/1733.

CC @durka @nikomatsakis

diff --cc src/librustc/ich/impls_ty.rs
Simple merge

diff --cc src/librustc/traits/mod.rs
Simple merge

diff --cc src/librustc/traits/project.rs
Simple merge

diff --cc src/librustc/traits/select.rs
Simple merge

diff --cc src/librustc/ty/instance.rs
Simple merge

diff --cc src/librustc/ty/mod.rs
Simple merge

diff --cc src/librustc/ty/sty.rs
Simple merge

diff --cc src/librustc/ty/util.rs
Simple merge

diff --cc src/librustc_typeck/astconv.rs
index 46f2c0c76a0c7b457cb19b140428ed69b92b1cb1,e44cfec82ad642f624d224b6e7920c3f372b0977..18f8473b5b56d7d88fb600878aaa16ff8fbbe0b3
--- 1/src/librustc_typeck/astconv.rs
--- 2/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@@ -1009,11 -1009,11 +1009,11 @@@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<
              })
          });
  
-         // check that there are no gross object safety violations,
+         // Check that there are no gross object safety violations;
          // most importantly, that the supertraits don't contain Self,
-         // to avoid ICE-s.
+         // to avoid ICEs.
          let object_safety_violations =
 -            tcx.astconv_object_safety_violations(principal.def_id());
 +            tcx.global_tcx().astconv_object_safety_violations(principal.def_id());
          if !object_safety_violations.is_empty() {
              tcx.report_object_safety_error(
                  span, principal.def_id(), object_safety_violations)

diff --cc src/librustc_typeck/collect.rs
Simple merge