Auto merge of #105229 - saethlin:zst-writes-to-unions, r=oli-obk

Re-enable removal of ZST writes to unions

This was previously disabled because Miri was lazily allocating unsized locals. But we aren't doing that anymore since  https://github.com/rust-lang/rust/pull/98831, so we can have this optimization back.

src/test/mir-opt/remove_zsts.get_union.PreCodegen.after.mir

@@ -0,0 +1,10 @@
+// MIR for `get_union` after PreCodegen
+
+fn get_union() -> Foo {
+    let mut _0: Foo;                     // return place in scope 0 at $DIR/remove_zsts.rs:+0:19: +0:22
+
+    bb0: {
+        Deinit(_0);                      // scope 0 at $DIR/remove_zsts.rs:+1:5: +1:18
+        return;                          // scope 0 at $DIR/remove_zsts.rs:+2:2: +2:2
+    }
+}

src/test/mir-opt/remove_zsts.get_union.RemoveZsts.diff

@@ -0,0 +1,19 @@
+- // MIR for `get_union` before RemoveZsts
++ // MIR for `get_union` after RemoveZsts
+  
+  fn get_union() -> Foo {
+      let mut _0: Foo;                     // return place in scope 0 at $DIR/remove_zsts.rs:+0:19: +0:22
+      let mut _1: ();                      // in scope 0 at $DIR/remove_zsts.rs:+1:14: +1:16
+  
+      bb0: {
+          StorageLive(_1);                 // scope 0 at $DIR/remove_zsts.rs:+1:14: +1:16
+-         Deinit(_1);                      // scope 0 at $DIR/remove_zsts.rs:+1:14: +1:16
++         nop;                             // scope 0 at $DIR/remove_zsts.rs:+1:14: +1:16
+          Deinit(_0);                      // scope 0 at $DIR/remove_zsts.rs:+1:5: +1:18
+-         (_0.0: ()) = move _1;            // scope 0 at $DIR/remove_zsts.rs:+1:5: +1:18
++         nop;                             // scope 0 at $DIR/remove_zsts.rs:+1:5: +1:18
+          StorageDead(_1);                 // scope 0 at $DIR/remove_zsts.rs:+1:17: +1:18
+          return;                          // scope 0 at $DIR/remove_zsts.rs:+2:2: +2:2
+      }
+  }
+  

src/test/mir-opt/remove_zsts.rs

@@ -0,0 +1,14 @@
+union Foo {
+    x: (),
+    y: u64,
+}
+
+// EMIT_MIR remove_zsts.get_union.RemoveZsts.diff
+// EMIT_MIR remove_zsts.get_union.PreCodegen.after.mir
+fn get_union() -> Foo {
+    Foo { x: () }
+}
+
+fn main() {
+    get_union();
+}

src/test/mir-opt/remove_zsts_dont_touch_unions.get_union.RemoveZsts.after.mir

@@ -1,15 +0,0 @@
-// MIR for `get_union` after RemoveZsts
-
-fn get_union() -> Foo {
-    let mut _0: Foo;                     // return place in scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+0:19: +0:22
-    let mut _1: ();                      // in scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+1:14: +1:16
-
-    bb0: {
-        StorageLive(_1);                 // scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+1:14: +1:16
-        nop;                             // scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+1:14: +1:16
-        Deinit(_0);                      // scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+1:5: +1:18
-        (_0.0: ()) = move _1;            // scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+1:5: +1:18
-        StorageDead(_1);                 // scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+1:17: +1:18
-        return;                          // scope 0 at $DIR/remove_zsts_dont_touch_unions.rs:+2:2: +2:2
-    }
-}

src/test/mir-opt/remove_zsts_dont_touch_unions.rs

@@ -1,19 +0,0 @@
-// unit-test: RemoveZsts
-
-// Ensure RemoveZsts doesn't remove ZST assignments to union fields,
-// which causes problems in Miri.
-
-union Foo {
-    x: (),
-    y: u64,
-}
-
-// EMIT_MIR remove_zsts_dont_touch_unions.get_union.RemoveZsts.after.mir
-fn get_union() -> Foo {
-    Foo { x: () }
-}
-
-
-fn main() {
-    get_union();
-}