Rollup merge of #114505 - ouz-a:cleanup_mir, r=RalfJung

Add documentation to has_deref

Documentation of `has_deref` needed some polish to be more clear about where it should be used and what's it's purpose.

cc https://github.com/rust-lang/rust/issues/114401

r? `@RalfJung`

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -441,7 +441,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             LocalRef::Place(place) => place,
             LocalRef::UnsizedPlace(place) => bx.load_operand(place).deref(cx),
             LocalRef::Operand(..) => {
-                if place_ref.has_deref() {
+                if place_ref.is_indirect_first_projection() {
                     base = 1;
                     let cg_base = self.codegen_consume(
                         bx,

compiler/rustc_const_eval/src/interpret/step.rs

@@ -247,7 +247,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             AddressOf(_, place) => {
                 // Figure out whether this is an addr_of of an already raw place.
-                let place_base_raw = if place.has_deref() {
+                let place_base_raw = if place.is_indirect_first_projection() {
                     let ty = self.frame().body.local_decls[place.local].ty;
                     ty.is_unsafe_ptr()
                 } else {

compiler/rustc_middle/src/mir/mod.rs

@@ -1592,14 +1592,13 @@ impl<'tcx> Place<'tcx> {
         self.projection.iter().any(|elem| elem.is_indirect())
     }
 
-    /// If MirPhase >= Derefered and if projection contains Deref,
+    /// Returns `true` if this `Place`'s first projection is `Deref`.
-    /// It's guaranteed to be in the first place
+    ///
-    pub fn has_deref(&self) -> bool {
+    /// This is useful because for MIR phases `AnalysisPhase::PostCleanup` and later,
-        // To make sure this is not accidentally used in wrong mir phase
+    /// `Deref` projections can only occur as the first projection. In that case this method
-        debug_assert!(
+    /// is equivalent to `is_indirect`, but faster.
-            self.projection.is_empty() || !self.projection[1..].contains(&PlaceElem::Deref)
+    pub fn is_indirect_first_projection(&self) -> bool {
-        );
+        self.as_ref().is_indirect_first_projection()
-        self.projection.first() == Some(&PlaceElem::Deref)
     }
 
     /// Finds the innermost `Local` from this `Place`, *if* it is either a local itself or
@@ -1672,9 +1671,16 @@ impl<'tcx> PlaceRef<'tcx> {
         self.projection.iter().any(|elem| elem.is_indirect())
     }
 
-    /// If MirPhase >= Derefered and if projection contains Deref,
+    /// Returns `true` if this `Place`'s first projection is `Deref`.
-    /// It's guaranteed to be in the first place
+    ///
-    pub fn has_deref(&self) -> bool {
+    /// This is useful because for MIR phases `AnalysisPhase::PostCleanup` and later,
+    /// `Deref` projections can only occur as the first projection. In that case this method
+    /// is equivalent to `is_indirect`, but faster.
+    pub fn is_indirect_first_projection(&self) -> bool {
+        // To make sure this is not accidentally used in wrong mir phase
+        debug_assert!(
+            self.projection.is_empty() || !self.projection[1..].contains(&PlaceElem::Deref)
+        );
         self.projection.first() == Some(&PlaceElem::Deref)
     }
 

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -839,7 +839,7 @@ impl Map {
         tail_elem: Option<TrackElem>,
         f: &mut impl FnMut(ValueIndex),
     ) {
-        if place.has_deref() {
+        if place.is_indirect_first_projection() {
             // We do not track indirect places.
             return;
         }

compiler/rustc_mir_transform/src/add_retag.rs

@@ -60,7 +60,7 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
         let basic_blocks = body.basic_blocks.as_mut();
         let local_decls = &body.local_decls;
         let needs_retag = |place: &Place<'tcx>| {
-            !place.has_deref() // we're not really interested in stores to "outside" locations, they are hard to keep track of anyway
+            !place.is_indirect_first_projection() // we're not really interested in stores to "outside" locations, they are hard to keep track of anyway
                 && may_contain_reference(place.ty(&*local_decls, tcx).ty, /*depth*/ 3, tcx)
                 && !local_decls[place.local].is_deref_temp()
         };

compiler/rustc_mir_transform/src/copy_prop.rs

@@ -154,7 +154,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
     fn visit_operand(&mut self, operand: &mut Operand<'tcx>, loc: Location) {
         if let Operand::Move(place) = *operand
             // A move out of a projection of a copy is equivalent to a copy of the original projection.
-            && !place.has_deref()
+            && !place.is_indirect_first_projection()
             && !self.fully_moved.contains(place.local)
         {
             *operand = Operand::Copy(place);

src/tools/clippy/clippy_lints/src/dereference.rs

@@ -1170,7 +1170,7 @@ fn referent_used_exactly_once<'tcx>(
         && let [location] = *local_assignments(mir, local).as_slice()
         && let Some(statement) = mir.basic_blocks[location.block].statements.get(location.statement_index)
         && let StatementKind::Assign(box (_, Rvalue::Ref(_, _, place))) = statement.kind
-        && !place.has_deref()
+        && !place.is_indirect_first_projection()
         // Ensure not in a loop (https://github.com/rust-lang/rust-clippy/issues/9710)
         && TriColorDepthFirstSearch::new(&mir.basic_blocks).run_from(location.block, &mut CycleDetector).is_none()
     {