Auto merge of rust-lang#86383 - shamatar:slice_len_lowering, r=bjorn3

Add MIR pass to lower call to `core::slice::len` into `Len` operand

During some larger experiment with range analysis I've found that code like `let l = slice.len()` produces different MIR then one found in bound checks. This optimization pass replaces terminators that are calls to `core::slice::len` with just a MIR operand and Goto terminator.

It uses some heuristics to remove the outer borrow that is made to call `core::slice::len`, but I assume it can be eliminated, just didn't find how.

Would like to express my gratitude to `@oli-obk` who helped me a lot on Zullip

Author: bors

core/src/slice/mod.rs

@@ -96,6 +96,7 @@ impl<T> [T] {
     /// assert_eq!(a.len(), 3);
     /// ```
     #[doc(alias = "length")]
+    #[cfg_attr(not(bootstrap), lang = "slice_len_fn")]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_len", since = "1.39.0")]
     #[inline]

std/src/thread/local/tests.rs

@@ -297,7 +297,7 @@ fn join_orders_after_tls_destructors() {
             .unwrap();
 
         loop {
-            match SYNC_STATE.compare_exchange_weak(
+            match SYNC_STATE.compare_exchange(
                 THREAD1_WAITING,
                 MAIN_THREAD_RENDEZVOUS,
                 Ordering::SeqCst,