Auto merge of #130508 - adwinwhite:niche-not-depend-on-order, r=the8472

Get rid of niche selection's dependence on fields's order

Fixes #125630.
Use the optimal niche selection decided in `univariant()` rather than picking niche field manually.

r? `@the8472`

Author: bors

tests/fail/uninit/padding-enum.rs

@@ -17,10 +17,11 @@ fn main() {
         assert!(matches!(*p.as_ptr(), E::None));
 
         // Turns out the discriminant is (currently) stored
-        // in the 2nd pointer, so the first half is padding.
+        // in the 1st pointer, so the second half is padding.
         let c = &p as *const _ as *const u8;
+        let padding_offset = mem::size_of::<&'static ()>();
         // Read a padding byte.
-        let _val = *c.add(0);
+        let _val = *c.add(padding_offset);
         //~^ERROR: uninitialized
     }
 }

tests/fail/uninit/padding-enum.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: using uninitialized data, but this operation requires initialized memory
   --> tests/fail/uninit/padding-enum.rs:LL:CC
    |
-LL |         let _val = *c.add(0);
-   |                    ^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
+LL |         let _val = *c.add(padding_offset);
+   |                    ^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information