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  #98831, so we can have this optimization back.

Author: bors

compiler/rustc_mir_transform/src/remove_zsts.rs

@@ -1,8 +1,7 @@
 //! Removes assignments to ZST places.
 
 use crate::MirPass;
-use rustc_middle::mir::tcx::PlaceTy;
-use rustc_middle::mir::{Body, LocalDecls, Place, StatementKind};
+use rustc_middle::mir::{Body, StatementKind};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
 pub struct RemoveZsts;
@@ -35,9 +34,6 @@ impl<'tcx> MirPass<'tcx> for RemoveZsts {
                     if !layout.is_zst() {
                         continue;
                     }
-                    if involves_a_union(place, local_decls, tcx) {
-                        continue;
-                    }
                     if tcx.consider_optimizing(|| {
                         format!(
                             "RemoveZsts - Place: {:?} SourceInfo: {:?}",
@@ -63,24 +59,3 @@ fn maybe_zst(ty: Ty<'_>) -> bool {
         _ => false,
     }
 }
-
-/// Miri lazily allocates memory for locals on assignment,
-/// so we must preserve writes to unions and union fields,
-/// or it will ICE on reads of those fields.
-fn involves_a_union<'tcx>(
-    place: Place<'tcx>,
-    local_decls: &LocalDecls<'tcx>,
-    tcx: TyCtxt<'tcx>,
-) -> bool {
-    let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
-    if place_ty.ty.is_union() {
-        return true;
-    }
-    for elem in place.projection {
-        place_ty = place_ty.projection_ty(tcx, elem);
-        if place_ty.ty.is_union() {
-            return true;
-        }
-    }
-    return false;
-}

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();
+}