Rollup merge of #106012 - JakobDegen:retag-raw, r=RalfJung

Clarify that raw retags are not permitted in Mir

Not sure when this changed, but documentation and the validator needed to be updated. This also removes raw retags from custom mir.

cc rust-lang/miri#2735

r? `@RalfJung`

compiler/rustc_const_eval/src/transform/validate.rs

@@ -9,8 +9,8 @@ use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::{
     traversal, AggregateKind, BasicBlock, BinOp, Body, BorrowKind, CastKind, CopyNonOverlapping,
     Local, Location, MirPass, MirPhase, NonDivergingIntrinsic, Operand, Place, PlaceElem, PlaceRef,
-    ProjectionElem, RuntimePhase, Rvalue, SourceScope, Statement, StatementKind, Terminator,
+    ProjectionElem, RetagKind, RuntimePhase, Rvalue, SourceScope, Statement, StatementKind,
-    TerminatorKind, UnOp, START_BLOCK,
+    Terminator, TerminatorKind, UnOp, START_BLOCK,
 };
 use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeVisitable};
 use rustc_mir_dataflow::impls::MaybeStorageLive;
@@ -667,10 +667,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     self.fail(location, "`Deinit`is not allowed until deaggregation");
                 }
             }
-            StatementKind::Retag(_, _) => {
+            StatementKind::Retag(kind, _) => {
                 // FIXME(JakobDegen) The validator should check that `self.mir_phase <
                 // DropsLowered`. However, this causes ICEs with generation of drop shims, which
                 // seem to fail to set their `MirPhase` correctly.
+                if *kind == RetagKind::Raw || *kind == RetagKind::TwoPhase {
+                    self.fail(location, format!("explicit `{:?}` is forbidden", kind));
+                }
             }
             StatementKind::StorageLive(..)
             | StatementKind::StorageDead(..)

compiler/rustc_middle/src/mir/syntax.rs

@@ -320,8 +320,10 @@ pub enum StatementKind<'tcx> {
     /// <https://internals.rust-lang.org/t/stacked-borrows-an-aliasing-model-for-rust/8153/> for
     /// more details.
     ///
-    /// For code that is not specific to stacked borrows, you should consider retags to read
+    /// For code that is not specific to stacked borrows, you should consider retags to read and
-    /// and modify the place in an opaque way.
+    /// modify the place in an opaque way.
+    ///
+    /// Only `RetagKind::Default` and `RetagKind::FnEntry` are permitted.
     Retag(RetagKind, Box<Place<'tcx>>),
 
     /// Encodes a user's type ascription. These need to be preserved

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

@@ -15,9 +15,6 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
             @call("mir_retag", args) => {
                 Ok(StatementKind::Retag(RetagKind::Default, Box::new(self.parse_place(args[0])?)))
             },
-            @call("mir_retag_raw", args) => {
-                Ok(StatementKind::Retag(RetagKind::Raw, Box::new(self.parse_place(args[0])?)))
-            },
             @call("mir_set_discriminant", args) => {
                 let place = self.parse_place(args[0])?;
                 let var = self.parse_integer_literal(args[1])? as u32;

library/core/src/intrinsics/mir.rs

@@ -259,7 +259,6 @@ define!("mir_drop", fn Drop<T>(place: T, goto: BasicBlock));
 define!("mir_drop_and_replace", fn DropAndReplace<T>(place: T, value: T, goto: BasicBlock));
 define!("mir_call", fn Call<T>(place: T, goto: BasicBlock, call: T));
 define!("mir_retag", fn Retag<T>(place: T));
-define!("mir_retag_raw", fn RetagRaw<T>(place: T));
 define!("mir_move", fn Move<T>(place: T) -> T);
 define!("mir_static", fn Static<T>(s: T) -> &'static T);
 define!("mir_static_mut", fn StaticMut<T>(s: T) -> *mut T);

src/test/mir-opt/building/custom/references.immut_ref.built.after.mir

@@ -6,9 +6,8 @@ fn immut_ref(_1: &i32) -> &i32 {
 
     bb0: {
         _2 = &raw const (*_1);           // scope 0 at $DIR/references.rs:+5:13: +5:29
-        Retag([raw] _2);                 // scope 0 at $DIR/references.rs:+6:13: +6:24
+        _0 = &(*_2);                     // scope 0 at $DIR/references.rs:+6:13: +6:23
-        _0 = &(*_2);                     // scope 0 at $DIR/references.rs:+7:13: +7:23
+        Retag(_0);                       // scope 0 at $DIR/references.rs:+7:13: +7:23
-        Retag(_0);                       // scope 0 at $DIR/references.rs:+8:13: +8:23
+        return;                          // scope 0 at $DIR/references.rs:+8:13: +8:21
-        return;                          // scope 0 at $DIR/references.rs:+9:13: +9:21
     }
 }

src/test/mir-opt/building/custom/references.mut_ref.built.after.mir

@@ -6,9 +6,8 @@ fn mut_ref(_1: &mut i32) -> &mut i32 {
 
     bb0: {
         _2 = &raw mut (*_1);             // scope 0 at $DIR/references.rs:+5:13: +5:33
-        Retag([raw] _2);                 // scope 0 at $DIR/references.rs:+6:13: +6:24
+        _0 = &mut (*_2);                 // scope 0 at $DIR/references.rs:+6:13: +6:26
-        _0 = &mut (*_2);                 // scope 0 at $DIR/references.rs:+7:13: +7:26
+        Retag(_0);                       // scope 0 at $DIR/references.rs:+7:13: +7:23
-        Retag(_0);                       // scope 0 at $DIR/references.rs:+8:13: +8:23
+        return;                          // scope 0 at $DIR/references.rs:+8:13: +8:21
-        return;                          // scope 0 at $DIR/references.rs:+9:13: +9:21
     }
 }

src/test/mir-opt/building/custom/references.rs

@@ -12,7 +12,6 @@ pub fn mut_ref(x: &mut i32) -> &mut i32 {
 
         {
             t = addr_of_mut!(*x);
-            RetagRaw(t);
             RET = &mut *t;
             Retag(RET);
             Return()
@@ -28,7 +27,6 @@ pub fn immut_ref(x: &i32) -> &i32 {
 
         {
             t = addr_of!(*x);
-            RetagRaw(t);
             RET = & *t;
             Retag(RET);
             Return()