Auto merge of rust-lang#146666 - Zalathar:rollup-m2b8low, r=Zalathar

Rollup of 14 pull requests

Successful merges:

 - rust-lang#142807 (libtest: expose --fail-fast as an unstable command-line option)
 - rust-lang#144871 (Stabilize `btree_entry_insert` feature)
 - rust-lang#145071 (Update the minimum external LLVM to 20)
 - rust-lang#145181 (remove FIXME block from `has_significant_drop`, it never encounters inference variables)
 - rust-lang#145660 (initial implementation of the darwin_objc unstable feature)
 - rust-lang#145838 (don't apply temporary lifetime extension rules to non-extended `super let`)
 - rust-lang#146259 (Suggest removing Box::new instead of unboxing it)
 - rust-lang#146410 (Iterator repeat: no infinite loop for `last` and `count`)
 - rust-lang#146460 (Add tidy readme)
 - rust-lang#146552 (StateTransform: Do not renumber resume local.)
 - rust-lang#146564 (Remove Rvalue::Len again.)
 - rust-lang#146581 (Detect attempt to use var-args in closure)
 - rust-lang#146588 (tests/run-make: Update list of statically linked musl targets)
 - rust-lang#146631 (cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 3))

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

compiler/rustc_public/src/unstable/convert/stable/mir.rs

@@ -215,7 +215,6 @@ impl<'tcx> Stable<'tcx> for mir::Rvalue<'tcx> {
                 mutability.stable(tables, cx),
                 place.stable(tables, cx),
             ),
-            Len(place) => crate::mir::Rvalue::Len(place.stable(tables, cx)),
             Cast(cast_kind, op, ty) => crate::mir::Rvalue::Cast(
                 cast_kind.stable(tables, cx),
                 op.stable(tables, cx),