Auto merge of #2242 - dtolnay-contrib:rustfmtskip, r=RalfJung

Add rustfmt::skip to some files

Extracted from https://github.com/rust-lang/miri/pull/2097.

Five of the files being skipped here are because rustfmt is buggy (https://github.com/rust-lang/rustfmt/issues/5391; see the error messages below). The other two have clearly preferable manual formatting.

```console
error[internal]: left behind trailing whitespace
  --> tests/fail/validity/transmute_through_ptr.rs:18:18:1
   |
18 |
   | ^^^^
   |

warning: rustfmt has failed to format. See previous 1 errors.

error[internal]: left behind trailing whitespace
  --> tests/fail/stacked_borrows/illegal_read2.rs:10:10:1
   |
10 |
   | ^^^^
   |

warning: rustfmt has failed to format. See previous 1 errors.

error[internal]: left behind trailing whitespace
  --> tests/fail/stacked_borrows/illegal_read5.rs:15:15:1
   |
15 |
   | ^^^^
   |

warning: rustfmt has failed to format. See previous 1 errors.

error[internal]: left behind trailing whitespace
  --> tests/fail/stacked_borrows/illegal_read1.rs:10:10:1
   |
10 |
   | ^^^^
   |

warning: rustfmt has failed to format. See previous 1 errors.

error[internal]: left behind trailing whitespace
 --> /git/miri/tests/fail/erroneous_const2.rs:9:9:1
  |
9 |
  | ^^^^
  |

warning: rustfmt has failed to format. See previous 1 errors.
```

tests/fail/erroneous_const2.rs

@@ -4,6 +4,7 @@ const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
 //~^ERROR any use of this value
 //~|WARN previously accepted
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     println!("{}", FOO); //~ERROR post-monomorphization error
     //~|ERROR evaluation of constant value failed

tests/fail/stacked_borrows/illegal_read1.rs

@@ -1,6 +1,7 @@
 // A callee may not read the destination of our `&mut` without
 // us noticing.
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let mut x = 15;
     let xraw = &mut x as *mut _;

tests/fail/stacked_borrows/illegal_read2.rs

@@ -1,6 +1,7 @@
 // A callee may not read the destination of our `&mut` without
 // us noticing.
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let mut x = 15;
     let xraw = &mut x as *mut _;

tests/fail/stacked_borrows/illegal_read5.rs

@@ -5,6 +5,7 @@
 use std::cell::RefCell;
 use std::{mem, ptr};
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let rc = RefCell::new(0);
     let mut refmut = rc.borrow_mut();

tests/fail/validity/transmute_through_ptr.rs

@@ -7,6 +7,7 @@ fn evil(x: &mut Bool) {
     unsafe { *x = 44; } // out-of-bounds enum tag
 }
 
+#[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {
     let mut x = Bool::True;
     evil(&mut x);

tests/pass/enums.rs

@@ -64,6 +64,7 @@ fn more_discriminant_overflow() {
     // from the `bool` field of `V1`), overflowing for variants with large enough
     // indices (`V3` and `V4`), causing them to be interpreted as other variants.
     #[allow(dead_code)]
+    #[rustfmt::skip] // rustfmt prefers every variant on its own line
     pub enum E2<X> {
         V1 { f: bool },