Add test for option transmutes

Author: oli-obk

tests/mir-opt/const_prop/transmute.option_field.GVN.32bit.diff

@@ -0,0 +1,42 @@
+- // MIR for `option_field` before GVN
++ // MIR for `option_field` after GVN
+  
+  fn option_field(_1: Option<NonNull<()>>) -> *const () {
+      debug x => _1;
+      let mut _0: *const ();
+      let mut _2: isize;
+      let mut _4: std::ptr::NonNull<()>;
+      scope 1 {
+          debug x => _3;
+          let _3: std::ptr::NonNull<()>;
+      }
+  
+      bb0: {
+          _2 = discriminant(_1);
+          switchInt(move _2) -> [1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+-         StorageLive(_3);
++         nop;
+          _3 = copy ((_1 as Some).0: std::ptr::NonNull<()>);
+          StorageLive(_4);
+          _4 = copy _3;
+-         _0 = move _4 as *const () (Transmute);
++         _0 = copy _3 as *const () (Transmute);
+          StorageDead(_4);
+-         StorageDead(_3);
++         nop;
+          goto -> bb3;
+      }
+  
+      bb2: {
+          _0 = const 0_usize as *const () (PointerWithExposedProvenance);
+          goto -> bb3;
+      }
+  
+      bb3: {
+          return;
+      }
+  }
+  

tests/mir-opt/const_prop/transmute.option_field.GVN.64bit.diff

@@ -0,0 +1,42 @@
+- // MIR for `option_field` before GVN
++ // MIR for `option_field` after GVN
+  
+  fn option_field(_1: Option<NonNull<()>>) -> *const () {
+      debug x => _1;
+      let mut _0: *const ();
+      let mut _2: isize;
+      let mut _4: std::ptr::NonNull<()>;
+      scope 1 {
+          debug x => _3;
+          let _3: std::ptr::NonNull<()>;
+      }
+  
+      bb0: {
+          _2 = discriminant(_1);
+          switchInt(move _2) -> [1: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+-         StorageLive(_3);
++         nop;
+          _3 = copy ((_1 as Some).0: std::ptr::NonNull<()>);
+          StorageLive(_4);
+          _4 = copy _3;
+-         _0 = move _4 as *const () (Transmute);
++         _0 = copy _3 as *const () (Transmute);
+          StorageDead(_4);
+-         StorageDead(_3);
++         nop;
+          goto -> bb3;
+      }
+  
+      bb2: {
+          _0 = const 0_usize as *const () (PointerWithExposedProvenance);
+          goto -> bb3;
+      }
+  
+      bb3: {
+          return;
+      }
+  }
+  

tests/mir-opt/const_prop/transmute.rs

@@ -84,4 +84,12 @@ pub unsafe fn unreachable_box() -> ! {
     match *x {}
 }
 
+// EMIT_MIR transmute.option_field.GVN.diff
+pub unsafe fn option_field(x: Option<std::ptr::NonNull<()>>) -> *const () {
+    // CHECK-LABEL: fn option_field(
+    // CHECK: _3 = copy ((_1 as Some).0: std::ptr::NonNull<()>)
+    // CHECK: _0 = copy _3 as *const () (Transmute)
+    if let Some(x) = x { unsafe { transmute(x) } } else { 0 as *const () }
+}
+
 enum Never {}