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`

Author: bors

compiler/rustc_borrowck/src/dataflow.rs

@@ -386,6 +386,7 @@ impl<'tcx> rustc_mir_dataflow::GenKillAnalysis<'tcx> for Borrows<'_, 'tcx> {
 
             mir::StatementKind::FakeRead(..)
             | mir::StatementKind::SetDiscriminant { .. }
+            | mir::StatementKind::Deinit(..)
             | mir::StatementKind::StorageLive(..)
             | mir::StatementKind::Retag { .. }
             | mir::StatementKind::AscribeUserType(..)

compiler/rustc_borrowck/src/def_use.rs

@@ -72,5 +72,9 @@ pub fn categorize(context: PlaceContext) -> Option<DefUse> {
 
         // Debug info is neither def nor use.
         PlaceContext::NonUse(NonUseContext::VarDebugInfo) => None,
+
+        PlaceContext::MutatingUse(MutatingUseContext::Deinit | MutatingUseContext::SetDiscriminant) => {
+            bug!("These statements are not allowed in this MIR phase")
+        }
     }
 }