Auto merge of #95125 - JakobDegen:uninit-variant-rvalue, r=oli-obk

Add new `Deinit` statement

This rvalue replaces `SetDiscriminant` for ADTs. This PR is an alternative to #94590 , which only specifies that the behavior of `SetDiscriminant` is the same as what this rvalue would do. The motivation for this change are discussed in that PR and [on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/SetDiscriminant.20and.20aggregate.20initialization.20.2394590)

r? `@oli-obk`

compiler/rustc_middle/src/mir/mod.rs

@@ -1588,8 +1588,17 @@ pub enum StatementKind<'tcx> {
     FakeRead(Box<(FakeReadCause, Place<'tcx>)>),
 
     /// Write the discriminant for a variant to the enum Place.
+    ///
+    /// This is permitted for both generators and ADTs. This does not necessarily write to the
+    /// entire place; instead, it writes to the minimum set of bytes as required by the layout for
+    /// the type.
     SetDiscriminant { place: Box<Place<'tcx>>, variant_index: VariantIdx },
 
+    /// Deinitializes the place.
+    ///
+    /// This writes `uninit` bytes to the entire place.
+    Deinit(Box<Place<'tcx>>),
+
     /// Start a live range for the storage of the local.
     StorageLive(Local),
 
@@ -1739,6 +1748,7 @@ impl Debug for Statement<'_> {
             SetDiscriminant { ref place, variant_index } => {
                 write!(fmt, "discriminant({:?}) = {:?}", place, variant_index)
             }
+            Deinit(ref place) => write!(fmt, "Deinit({:?})", place),
             AscribeUserType(box (ref place, ref c_ty), ref variance) => {
                 write!(fmt, "AscribeUserType({:?}, {:?}, {:?})", place, variance, c_ty)
             }

compiler/rustc_middle/src/mir/spanview.rs

@@ -243,6 +243,7 @@ pub fn statement_kind_name(statement: &Statement<'_>) -> &'static str {
         Assign(..) => "Assign",
         FakeRead(..) => "FakeRead",
         SetDiscriminant { .. } => "SetDiscriminant",
+        Deinit(..) => "Deinit",
         StorageLive(..) => "StorageLive",
         StorageDead(..) => "StorageDead",
         Retag(..) => "Retag",

compiler/rustc_middle/src/mir/visit.rs

@@ -395,10 +395,17 @@ macro_rules! make_mir_visitor {
                     StatementKind::SetDiscriminant { place, .. } => {
                         self.visit_place(
                             place,
-                            PlaceContext::MutatingUse(MutatingUseContext::Store),
+                            PlaceContext::MutatingUse(MutatingUseContext::SetDiscriminant),
                             location
                         );
                     }
+                    StatementKind::Deinit(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::MutatingUse(MutatingUseContext::Deinit),
+                            location
+                        )
+                    }
                     StatementKind::StorageLive(local) => {
                         self.visit_local(
                             local,
@@ -1174,6 +1181,10 @@ pub enum NonMutatingUseContext {
 pub enum MutatingUseContext {
     /// Appears as LHS of an assignment.
     Store,
+    /// Appears on `SetDiscriminant`
+    SetDiscriminant,
+    /// Appears on `Deinit`
+    Deinit,
     /// Output operand of an inline assembly block.
     AsmOutput,
     /// Destination of a call.