Auto merge of rust-lang#135709 - lqd:bring-back-len, r=compiler-errors

Temporarily bring back `Rvalue::Len`

r? `@compiler-errors` as requested in rust-lang#135671 (comment)

> However, in the mean time, I'd rather we not crunch trying to find and more importantly validate the soundness of a solution 🤔

Agreed. To fix the IMO P-critical rust-lang#135671 for which we somehow didn't have test coverage, this PR temporarily reverts:
- rust-lang#133734
- its bugfix rust-lang#134371
- rust-lang#134330

cc `@scottmcm`

I added the few samples from that issue as a test, but we can add more in the future, in particular it seems `@steffahn` [will work on that](rust-lang#135671 (comment)).

Fixes rust-lang#135671. And if we want to land this, it should also be nominated for beta backport.

Author: bors

core/src/intrinsics/mir.rs

@@ -233,7 +233,7 @@
 //!
 //!  - Operands implicitly convert to `Use` rvalues.
 //!  - `&`, `&mut`, `addr_of!`, and `addr_of_mut!` all work to create their associated rvalue.
-//!  - [`Discriminant`] and [`CopyForDeref`] have associated functions.
+//!  - [`Discriminant`], [`Len`], and [`CopyForDeref`] have associated functions.
 //!  - Unary and binary operations use their normal Rust syntax - `a * b`, `!c`, etc.
 //!  - The binary operation `Offset` can be created via [`Offset`].
 //!  - Checked binary operations are represented by wrapping the associated binop in [`Checked`].
@@ -401,6 +401,7 @@ define!("mir_storage_dead", fn StorageDead<T>(local: T));
 define!("mir_assume", fn Assume(operand: bool));
 define!("mir_deinit", fn Deinit<T>(place: T));
 define!("mir_checked", fn Checked<T>(binop: T) -> (T, bool));
+define!("mir_len", fn Len<T>(place: T) -> usize);
 define!(
     "mir_ptr_metadata",
     fn PtrMetadata<P: ?Sized>(place: *const P) -> <P as ::core::ptr::Pointee>::Metadata