`SimplifyArmIdentity` can cause use-after-moves in MIR

[jonas-schievink]

Reduced example diff from <core::slice::Iter<'a, T> as Iterator>::find_map:

-// MIR for `slice::<impl at src/libcore/slice/mod.rs:3293:9: 3475:10>::find_map` before SimplifyArmIdentity
+// MIR for `slice::<impl at src/libcore/slice/mod.rs:3293:9: 3475:10>::find_map` after SimplifyArmIdentity
 
 fn slice::<impl at src/libcore/slice/mod.rs:3293:9: 3475:10>::find_map(_1: &mut slice::Iter<T>, _2: F) -> option::Option<B> {
     debug self => _1;                    // in scope 0 at src/libcore/slice/mod.rs:3420:31: 3420:40
     debug f => _2;                       // in scope 0 at src/libcore/slice/mod.rs:3420:42: 3420:47
     let mut _0: option::Option<B>;       // return place in scope 0 at src/libcore/slice/mod.rs:3420:55: 3420:64
     let _3: ();                          // in scope 0 at src/libcore/slice/mod.rs:3425:17: 3429:18
     let mut _4: ();                      // in scope 0 at src/libcore/slice/mod.rs:3420:13: 3431:14
     let mut _5: option::Option<&T>;      // in scope 0 at src/libcore/slice/mod.rs:3425:37: 3425:48
     let mut _6: &mut slice::Iter<T>;     // in scope 0 at src/libcore/slice/mod.rs:3425:37: 3425:41
     let mut _7: isize;                   // in scope 0 at src/libcore/slice/mod.rs:3425:27: 3425:34
     let _8: &T;                          // in scope 0 at src/libcore/slice/mod.rs:3425:32: 3425:33
     let mut _9: option::Option<B>;       // in scope 0 at src/libcore/slice/mod.rs:3426:38: 3426:42
     let mut _10: &mut F;                 // in scope 0 at src/libcore/slice/mod.rs:3426:38: 3426:39
     let mut _11: (&T,);                  // in scope 0 at src/libcore/slice/mod.rs:3426:38: 3426:42
     let mut _12: &T;                     // in scope 0 at src/libcore/slice/mod.rs:3426:40: 3426:41
     let mut _13: isize;                  // in scope 0 at src/libcore/slice/mod.rs:3426:28: 3426:35
     let mut _15: !;                      // in scope 0 at src/libcore/slice/mod.rs:3426:43: 3428:22
     let mut _16: B;                      // in scope 0 at src/libcore/slice/mod.rs:3427:37: 3427:38
     let mut _17: !;                      // in scope 0 at src/libcore/slice/mod.rs:3425:17: 3429:18
     let mut _18: isize;                  // in scope 0 at src/libcore/slice/mod.rs:3429:17: 3429:18
     let mut _19: isize;                  // in scope 0 at src/libcore/slice/mod.rs:3429:17: 3429:18
     scope 1 {
         debug x => _8;                   // in scope 1 at src/libcore/slice/mod.rs:3425:32: 3425:33
         let _14: B;                      // in scope 1 at src/libcore/slice/mod.rs:3426:33: 3426:34
         scope 2 {
             debug y => _14;              // in scope 2 at src/libcore/slice/mod.rs:3426:33: 3426:34
         }
     }
 
     bb0: {
         StorageLive(_3);                 // scope 0 at src/libcore/slice/mod.rs:3425:17: 3429:18
         goto -> bb1;                     // scope 0 at src/libcore/slice/mod.rs:3425:17: 3429:18
     }

     // <...>
 
     bb7: {
-        StorageLive(_14);                // scope 1 at src/libcore/slice/mod.rs:3426:33: 3426:34
-        _14 = move ((_9 as Some).0: B);  // scope 1 at src/libcore/slice/mod.rs:3426:33: 3426:34
-        StorageLive(_16);                // scope 2 at src/libcore/slice/mod.rs:3427:37: 3427:38
-        _16 = move _14;                  // scope 2 at src/libcore/slice/mod.rs:3427:37: 3427:38
-        ((_0 as Some).0: B) = move _16;  // scope 2 at src/libcore/slice/mod.rs:3427:32: 3427:39
-        discriminant(_0) = 1;            // scope 2 at src/libcore/slice/mod.rs:3427:32: 3427:39
-        StorageDead(_16);                // scope 2 at src/libcore/slice/mod.rs:3427:38: 3427:39
-        StorageDead(_14);                // scope 1 at src/libcore/slice/mod.rs:3428:21: 3428:22
+        _0 = move _9;                    // scope 2 at src/libcore/slice/mod.rs:3427:32: 3427:39
         _18 = discriminant(_9);          // scope 1 at src/libcore/slice/mod.rs:3429:17: 3429:18
         StorageDead(_9);                 // scope 1 at src/libcore/slice/mod.rs:3429:17: 3429:18
         StorageDead(_8);                 // scope 0 at src/libcore/slice/mod.rs:3429:17: 3429:18
         StorageDead(_5);                 // scope 0 at src/libcore/slice/mod.rs:3429:17: 3429:18
         StorageDead(_3);                 // scope 0 at src/libcore/slice/mod.rs:3429:17: 3429:18
         goto -> bb9;                     // scope 0 at src/libcore/slice/mod.rs:3431:13: 3431:14
     }
 
     bb9: {
         return;                          // scope 0 at src/libcore/slice/mod.rs:3431:14: 3431:14
     }
 }

Here, the move _9 makes _9 uninitialized (in MaybeInitializedLocals), but the next statement immediately uses it again via discriminant(_9).

I do not believe that this is currently a soundness issue, as no analyses other than the generator transform (which runs earlier) should rely on this data. It is a problem for #72632 though, since that runs after SimplifyArmIdentity. Granted, the example above is unlikely to be problematic even for that, but other cases might be.