Rollup merge of #145879 - Bryanskiy:supertraits-2, r=lcnr

default auto traits: use default supertraits instead of `Self: Trait` bounds on associated items

First commit: the motivation has been discussed [here](https://github.com/rust-lang/rust/pull/144679).

Second commit:  the only new places where new implicit `DefaultAutoTrait` bounds are generated are supertraits and trait object so `?Trait` syntax should be extended to these places only.

r? `@lcnr`

1 files changed, 8 insertions, 7 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 72f20a95ff0..4e2243e8787 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -2101,17 +2101,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 {
                     return;
                 }
-                if self.tcx.features().more_maybe_bounds() {
-                    return;
-                }
             }
             RelaxedBoundPolicy::Forbidden(reason) => {
-                if self.tcx.features().more_maybe_bounds() {
-                    return;
-                }
-
                 match reason {
                     RelaxedBoundForbiddenReason::TraitObjectTy => {
+                        if self.tcx.features().more_maybe_bounds() {
+                            return;
+                        }
+
                         self.dcx().span_err(
                             span,
                             "relaxed bounds are not permitted in trait object types",
@@ -2119,6 +2116,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         return;
                     }
                     RelaxedBoundForbiddenReason::SuperTrait => {
+                        if self.tcx.features().more_maybe_bounds() {
+                            return;
+                        }
+
                         let mut diag = self.dcx().struct_span_err(
                             span,
                             "relaxed bounds are not permitted in supertrait bounds",