From 6ce7842dafcda41c9149116f854f719440adf193 Mon Sep 17 00:00:00 2001
From: "Tim (Theemathas Chirananthavat)" <theemathas@gmail.com>
Date: Wed, 12 Aug 2026 17:48:07 +0700
Subject: [PATCH] Remove `MutatingUseContext::Retag`

This variant was never constructed anywhere.
---
 compiler/rustc_borrowck/src/def_use.rs                 | 3 +--
 compiler/rustc_codegen_ssa/src/mir/analyze.rs          | 3 +--
 compiler/rustc_middle/src/mir/visit.rs                 | 2 --
 compiler/rustc_mir_dataflow/src/impls/liveness.rs      | 3 +--
 compiler/rustc_mir_transform/src/deduce_param_attrs.rs | 3 +--
 compiler/rustc_mir_transform/src/known_panics_lint.rs  | 1 -
 compiler/rustc_mir_transform/src/liveness.rs           | 3 +--
 7 files changed, 5 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_borrowck/src/def_use.rs b/compiler/rustc_borrowck/src/def_use.rs
index 8813ef5249262..df20eb0e49c62 100644
--- a/compiler/rustc_borrowck/src/def_use.rs
+++ b/compiler/rustc_borrowck/src/def_use.rs
@@ -59,8 +59,7 @@ pub(crate) fn categorize(context: PlaceContext) -> Option<DefUse> {
         PlaceContext::NonMutatingUse(NonMutatingUseContext::RawBorrow) |
         PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
         PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
-        PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) |
-        PlaceContext::MutatingUse(MutatingUseContext::Retag) => Some(DefUse::Use),
+        PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => Some(DefUse::Use),
 
         ///////////////////////////////////////////////////////////////////////////
         // DROP USES
diff --git a/compiler/rustc_codegen_ssa/src/mir/analyze.rs b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
index 45a9e04986c74..5bf6b44c41ebc 100644
--- a/compiler/rustc_codegen_ssa/src/mir/analyze.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
@@ -211,8 +211,7 @@ impl<'a, 'b, 'tcx, Bx: BuilderMethods<'b, 'tcx>> Visitor<'tcx> for LocalAnalyzer
             }
 
             PlaceContext::NonUse(_)
-            | PlaceContext::NonMutatingUse(NonMutatingUseContext::PlaceMention)
-            | PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
+            | PlaceContext::NonMutatingUse(NonMutatingUseContext::PlaceMention) => {}
 
             PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::Copy
diff --git a/compiler/rustc_middle/src/mir/visit.rs b/compiler/rustc_middle/src/mir/visit.rs
index 429ab7d928fc5..0ae59e99c2b5a 100644
--- a/compiler/rustc_middle/src/mir/visit.rs
+++ b/compiler/rustc_middle/src/mir/visit.rs
@@ -1354,8 +1354,6 @@ pub enum MutatingUseContext {
     /// f(&mut x.y);
     /// ```
     Projection,
-    /// Retagging, a "Stacked Borrows" shadow state operation
-    Retag,
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
diff --git a/compiler/rustc_mir_dataflow/src/impls/liveness.rs b/compiler/rustc_mir_dataflow/src/impls/liveness.rs
index 5c77827a67165..2787365c17be1 100644
--- a/compiler/rustc_mir_dataflow/src/impls/liveness.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/liveness.rs
@@ -180,8 +180,7 @@ impl DefUse {
             PlaceContext::MutatingUse(
                 MutatingUseContext::RawBorrow
                 | MutatingUseContext::Borrow
-                | MutatingUseContext::Drop
-                | MutatingUseContext::Retag,
+                | MutatingUseContext::Drop,
             )
             | PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::RawBorrow
diff --git a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs
index bcd43bc40d8bc..5bba125aefc58 100644
--- a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs
+++ b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs
@@ -74,8 +74,7 @@ impl<'tcx> Visitor<'tcx> for DeduceParamAttrs {
                   MutatingUseContext::Store
                 | MutatingUseContext::SetDiscriminant
                 | MutatingUseContext::AsmOutput
-                | MutatingUseContext::Projection
-                | MutatingUseContext::Retag) => {
+                | MutatingUseContext::Projection) => {
                 self.usage[i] |= UsageSummary::MUTATE;
             }
             | PlaceContext::NonMutatingUse(NonMutatingUseContext::RawBorrow) => {
diff --git a/compiler/rustc_mir_transform/src/known_panics_lint.rs b/compiler/rustc_mir_transform/src/known_panics_lint.rs
index 1a2d47f4cc258..a2f2f8fa063c2 100644
--- a/compiler/rustc_mir_transform/src/known_panics_lint.rs
+++ b/compiler/rustc_mir_transform/src/known_panics_lint.rs
@@ -975,7 +975,6 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
             // whether they'd be fine right now.
             MutatingUse(MutatingUseContext::Yield)
             | MutatingUse(MutatingUseContext::Drop)
-            | MutatingUse(MutatingUseContext::Retag)
             // These can't ever be propagated under any scheme, as we can't reason about indirect
             // mutation.
             | NonMutatingUse(NonMutatingUseContext::SharedBorrow)
diff --git a/compiler/rustc_mir_transform/src/liveness.rs b/compiler/rustc_mir_transform/src/liveness.rs
index c895819a9f8cc..4da8ca0760257 100644
--- a/compiler/rustc_mir_transform/src/liveness.rs
+++ b/compiler/rustc_mir_transform/src/liveness.rs
@@ -1548,8 +1548,7 @@ impl DefUse {
             PlaceContext::MutatingUse(
                 MutatingUseContext::RawBorrow
                 | MutatingUseContext::Borrow
-                | MutatingUseContext::Drop
-                | MutatingUseContext::Retag,
+                | MutatingUseContext::Drop,
             )
             | PlaceContext::NonMutatingUse(
                 NonMutatingUseContext::RawBorrow