Auto merge of #10989 - ericmarkmartin:use-placeref-abstraction, r=Manishearth

Use placeref abstraction

rust-lang/rust#80647 suggests refactoring certain patterns with MIR places to use higher-level abstractions provided by the [`Place`](https://doc.rust-lang.org/stable/nightly-rustc/rustc_middle/mir/struct.Place.html)/[`PlaceRef`](https://doc.rust-lang.org/stable/nightly-rustc/rustc_middle/mir/struct.PlaceRef.html). While working on that issue, I found a couple candidates for such refactoring in clippy.

*Please write a short comment explaining your change (or "none" for internal only changes)*

changelog: none

clippy_lints/src/redundant_clone.rs

@@ -320,8 +320,6 @@ fn base_local_and_movability<'tcx>(
     mir: &mir::Body<'tcx>,
     place: mir::Place<'tcx>,
 ) -> (mir::Local, CannotMoveOut) {
-    use rustc_middle::mir::PlaceRef;
-
     // Dereference. You cannot move things out from a borrowed value.
     let mut deref = false;
     // Accessing a field of an ADT that has `Drop`. Moving the field out will cause E0509.
@@ -330,17 +328,14 @@ fn base_local_and_movability<'tcx>(
     // underlying type implements Copy
     let mut slice = false;
 
-    let PlaceRef { local, mut projection } = place.as_ref();
-    while let [base @ .., elem] = projection {
-        projection = base;
+    for (base, elem) in place.as_ref().iter_projections() {
+        let base_ty = base.ty(&mir.local_decls, cx.tcx).ty;
         deref |= matches!(elem, mir::ProjectionElem::Deref);
-        field |= matches!(elem, mir::ProjectionElem::Field(..))
-            && has_drop(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
-        slice |= matches!(elem, mir::ProjectionElem::Index(..))
-            && !is_copy(cx, mir::Place::ty_from(local, projection, &mir.local_decls, cx.tcx).ty);
+        field |= matches!(elem, mir::ProjectionElem::Field(..)) && has_drop(cx, base_ty);
+        slice |= matches!(elem, mir::ProjectionElem::Index(..)) && !is_copy(cx, base_ty);
     }
 
-    (local, deref || field || slice)
+    (place.local, deref || field || slice)
 }
 
 #[derive(Default)]

clippy_utils/src/qualify_min_const_fn.rs

@@ -284,13 +284,10 @@ fn check_operand<'tcx>(tcx: TyCtxt<'tcx>, operand: &Operand<'tcx>, span: Span, b
 }
 
 fn check_place<'tcx>(tcx: TyCtxt<'tcx>, place: Place<'tcx>, span: Span, body: &Body<'tcx>) -> McfResult {
-    let mut cursor = place.projection.as_ref();
-
-    while let [ref proj_base @ .., elem] = *cursor {
-        cursor = proj_base;
+    for (base, elem) in place.as_ref().iter_projections() {
         match elem {
             ProjectionElem::Field(..) => {
-                let base_ty = Place::ty_from(place.local, proj_base, body, tcx).ty;
+                let base_ty = base.ty(body, tcx).ty;
                 if let Some(def) = base_ty.ty_adt_def() {
                     // No union field accesses in `const fn`
                     if def.is_union() {

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -157,3 +157,12 @@ pub fn name(self) -> String {
         self.0
     }
 }
+
+union U {
+    f: u32,
+}
+
+// Do not lint because accessing union fields from const functions is unstable
+fn h(u: U) -> u32 {
+    unsafe { u.f }
+}