Rollup merge of #105484 - nbdd0121:upcast, r=compiler-errors

Implement allow-by-default `multiple_supertrait_upcastable` lint

The lint detects when an object-safe trait has multiple supertraits.

Enabled in libcore and liballoc as they are low-level enough that many embedded programs will use them.

r? `@nikomatsakis`

diff --cc compiler/rustc_feature/src/active.rs
Simple merge

diff --cc compiler/rustc_lint/src/lib.rs
index 1275d6f223c7a38ebe5fbfd42cc50c04c4990149,f9c86f8b5f316ead64067f8a344b00881eb8b5bd..44ee4172675e0cad8517323d9dcd8d10dba32760
--- 1/compiler/rustc_lint/src/lib.rs
--- 2/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@@ -229,8 -230,10 +231,9 @@@ late_lint_methods!
              InvalidAtomicOrdering: InvalidAtomicOrdering,
              NamedAsmLabels: NamedAsmLabels,
              OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
+             MultipleSupertraitUpcastable: MultipleSupertraitUpcastable,
          ]
 -    ],
 -    ['tcx]
 +    ]
  );
  
  pub fn new_lint_store(internal_lints: bool) -> LintStore {

diff --cc compiler/rustc_span/src/symbol.rs
Simple merge

diff --cc library/alloc/src/lib.rs
Simple merge