Rollup merge of #53932 - matthewjasper:remove-base-path, r=nikomatsakis

[NLL] Remove base_place

This function was supposed to make `Box` less special. But

* I think that the consensus is that MIR borrowck is going to fully special case `Box`
* It wasn't implemented correctly, it's looking at the type of the wrong `Place`, resulting in weird behaviour:

```rust
#![feature(nll)]
type A = Box<i32>; // If this is changed to another type then this will compile.

pub fn foo(x: Box<(String, A)>) {
    let a = x.0; // This will compile if these lines are swapped
    let b = x.1;
}
```

r? @nikomatsakis

src/test/ui/borrowck/borrowck-box-insensitivity.nll.stderr

@@ -1,25 +1,14 @@
-error[E0382]: use of moved value: `a.y`
-  --> $DIR/borrowck-box-insensitivity.rs:46:14
+error: compilation successful
+  --> $DIR/borrowck-box-insensitivity.rs:160:1
    |
-LL |     let _x = a.x;
-   |              --- value moved here
-LL |     //~^ value moved here
-LL |     let _y = a.y; //~ ERROR use of moved
-   |              ^^^ value used here after move
-   |
-   = note: move occurs because `a.x` has type `std::boxed::Box<isize>`, which does not implement the `Copy` trait
+LL | / fn main() {
+LL | |     copy_after_move();
+LL | |     move_after_move();
+LL | |     borrow_after_move();
+...  |
+LL | |     mut_borrow_after_borrow_nested();
+LL | | }
+   | |_^
 
-error[E0382]: use of moved value: `a.y`
-  --> $DIR/borrowck-box-insensitivity.rs:108:14
-   |
-LL |     let _x = a.x.x;
-   |              ----- value moved here
-LL |     //~^ value moved here
-LL |     let _y = a.y; //~ ERROR use of collaterally moved
-   |              ^^^ value used here after move
-   |
-   = note: move occurs because `a.x.x` has type `std::boxed::Box<isize>`, which does not implement the `Copy` trait
+error: aborting due to previous error
 
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0382`.

src/test/ui/borrowck/borrowck-box-insensitivity.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(box_syntax)]
+#![feature(box_syntax, rustc_attrs)]
 
 struct A {
     x: Box<isize>,
@@ -156,6 +156,7 @@ fn mut_borrow_after_borrow_nested() {
     //~^ mutable borrow occurs here
 }
 
+#[rustc_error]
 fn main() {
     copy_after_move();
     move_after_move();