Rollup merge of rust-lang#146564 - cjgillot:mir-nolen, r=scottmcm

Remove Rvalue::Len again.

Now that we have `RawPtrKind::FakeForPtrMetadata`, we can reimplement `Rvalue::Len` using `PtrMetadata(&raw const (fake) place)`.

r? ``@scottmcm``

Author: Zalathar

core/src/intrinsics/mir.rs

@@ -233,7 +233,8 @@
 //!
 //!  - Operands implicitly convert to `Use` rvalues.
 //!  - `&`, `&mut`, `addr_of!`, and `addr_of_mut!` all work to create their associated rvalue.
-//!  - [`Discriminant`], [`Len`], and [`CopyForDeref`] have associated functions.
+//!  - [`CopyForDeref`], [`CastTransmute`], [`CastPtrToPtr`], [`CastUnsize`], and [`Discriminant`]
+//!    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`].
@@ -401,7 +402,6 @@ 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
@@ -491,6 +491,13 @@ define!(
     /// This allows bypassing normal validation to generate strange casts.
     fn CastPtrToPtr<T, U>(operand: T) -> U
 );
+define!(
+    "mir_cast_unsize",
+    /// Emits a `CastKind::PointerCoercion(Unsize)` cast.
+    ///
+    /// This allows bypassing normal validation to generate strange casts.
+    fn CastUnsize<T, U>(operand: T) -> U
+);
 define!(
     "mir_make_place",
     #[doc(hidden)]