Add a mir-opt test for an unneeded drop_in_place

Author: scottmcm

tests/mir-opt/remove_unneeded_drops.rs

@@ -22,9 +22,16 @@ fn cannot_opt_generic<T>(x: T) {
     drop(x);
 }
 
+// EMIT_MIR remove_unneeded_drops.slice_in_place.RemoveUnneededDrops.diff
+unsafe fn slice_in_place(ptr: *mut [char]) {
+    std::ptr::drop_in_place(ptr)
+}
+
 fn main() {
     opt(true);
     opt_generic_copy(42);
     cannot_opt_generic(42);
     dont_opt(vec![true]);
+    let mut a = ['o', 'k'];
+    unsafe { slice_in_place(&raw mut a) };
 }

tests/mir-opt/remove_unneeded_drops.slice_in_place.RemoveUnneededDrops.panic-abort.diff

@@ -0,0 +1,20 @@
+- // MIR for `slice_in_place` before RemoveUnneededDrops
++ // MIR for `slice_in_place` after RemoveUnneededDrops
+  
+  fn slice_in_place(_1: *mut [char]) -> () {
+      debug ptr => _1;
+      let mut _0: ();
+      let mut _2: *mut [char];
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = copy _1;
+          _0 = drop_in_place::<[char]>(move _2) -> [return: bb1, unwind unreachable];
+      }
+  
+      bb1: {
+          StorageDead(_2);
+          return;
+      }
+  }
+  

tests/mir-opt/remove_unneeded_drops.slice_in_place.RemoveUnneededDrops.panic-unwind.diff

@@ -0,0 +1,20 @@
+- // MIR for `slice_in_place` before RemoveUnneededDrops
++ // MIR for `slice_in_place` after RemoveUnneededDrops
+  
+  fn slice_in_place(_1: *mut [char]) -> () {
+      debug ptr => _1;
+      let mut _0: ();
+      let mut _2: *mut [char];
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = copy _1;
+          _0 = drop_in_place::<[char]>(move _2) -> [return: bb1, unwind continue];
+      }
+  
+      bb1: {
+          StorageDead(_2);
+          return;
+      }
+  }
+