Auto merge of #39628 - arielb1:shimmir, r=eddyb

Translate shims using MIR

This removes one large remaining part of old trans.

src/libcore/ptr.rs

@@ -37,9 +37,38 @@ pub use intrinsics::copy;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use intrinsics::write_bytes;
 
+#[cfg(stage0)]
 #[stable(feature = "drop_in_place", since = "1.8.0")]
 pub use intrinsics::drop_in_place;
 
+#[cfg(not(stage0))]
+/// Executes the destructor (if any) of the pointed-to value.
+///
+/// This has two use cases:
+///
+/// * It is *required* to use `drop_in_place` to drop unsized types like
+///   trait objects, because they can't be read out onto the stack and
+///   dropped normally.
+///
+/// * It is friendlier to the optimizer to do this over `ptr::read` when
+///   dropping manually allocated memory (e.g. when writing Box/Rc/Vec),
+///   as the compiler doesn't need to prove that it's sound to elide the
+///   copy.
+///
+/// # Undefined Behavior
+///
+/// This has all the same safety problems as `ptr::read` with respect to
+/// invalid pointers, types, and double drops.
+#[stable(feature = "drop_in_place", since = "1.8.0")]
+#[lang="drop_in_place"]
+#[inline]
+#[allow(unconditional_recursion)]
+pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+    // Code here does not matter - this is replaced by the
+    // real drop glue by the compiler.
+    drop_in_place(to_drop);
+}
+
 /// Creates a null raw pointer.
 ///
 /// # Examples