Introduce ProjectionElem::try_map.

compiler/rustc_middle/src/lib.rs

@@ -60,6 +60,7 @@
 #![feature(try_trait_v2_residual)]
 #![feature(try_trait_v2_yeet)]
 #![feature(type_alias_impl_trait)]
+#![feature(unwrap_infallible)]
 #![feature(yeet_expr)]
 #![recursion_limit = "256"]
 // tidy-alphabetical-end

compiler/rustc_middle/src/mir/statement.rs

@@ -160,7 +160,11 @@ impl<'tcx> PlaceTy<'tcx> {
     /// Convenience wrapper around `projection_ty_core` for `PlaceElem`,
     /// where we can just use the `Ty` that is already stored inline on
     /// field projection elems.
-    pub fn projection_ty(self, tcx: TyCtxt<'tcx>, elem: PlaceElem<'tcx>) -> PlaceTy<'tcx> {
+    pub fn projection_ty<V: ::std::fmt::Debug>(
+        self,
+        tcx: TyCtxt<'tcx>,
+        elem: ProjectionElem<V, Ty<'tcx>>,
+    ) -> PlaceTy<'tcx> {
         self.projection_ty_core(tcx, &elem, |ty| ty, |_, _, _, ty| ty, |ty| ty)
     }
 
@@ -290,6 +294,36 @@ impl<V, T> ProjectionElem<V, T> {
             Self::UnwrapUnsafeBinder(..) => false,
         }
     }
+
+    /// Returns the `ProjectionKind` associated to this projection.
+    pub fn kind(self) -> ProjectionKind {
+        self.try_map(|_| Some(()), |_| ()).unwrap()
+    }
+
+    /// Apply functions to types and values in this projection and return the result.
+    pub fn try_map<V2, T2>(
+        self,
+        v: impl FnOnce(V) -> Option<V2>,
+        t: impl FnOnce(T) -> T2,
+    ) -> Option<ProjectionElem<V2, T2>> {
+        Some(match self {
+            ProjectionElem::Deref => ProjectionElem::Deref,
+            ProjectionElem::Downcast(name, read_variant) => {
+                ProjectionElem::Downcast(name, read_variant)
+            }
+            ProjectionElem::Field(f, ty) => ProjectionElem::Field(f, t(ty)),
+            ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
+                ProjectionElem::ConstantIndex { offset, min_length, from_end }
+            }
+            ProjectionElem::Subslice { from, to, from_end } => {
+                ProjectionElem::Subslice { from, to, from_end }
+            }
+            ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(t(ty)),
+            ProjectionElem::Subtype(ty) => ProjectionElem::Subtype(t(ty)),
+            ProjectionElem::UnwrapUnsafeBinder(ty) => ProjectionElem::UnwrapUnsafeBinder(t(ty)),
+            ProjectionElem::Index(val) => ProjectionElem::Index(v(val)?),
+        })
+    }
 }
 
 /// Alias for projections as they appear in `UserTypeProjection`, where we

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -7,7 +7,6 @@ use rustc_middle::{bug, span_bug};
 use smallvec::{SmallVec, smallvec};
 use tracing::debug;
 
-use super::abs_domain::Lift;
 use super::{
     Init, InitIndex, InitKind, InitLocation, LocationMap, LookupResult, MoveData, MoveOut,
     MoveOutIndex, MovePath, MovePathIndex, MovePathLookup,
@@ -241,7 +240,7 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
             if union_path.is_none() {
                 // inlined from add_move_path because of a borrowck conflict with the iterator
                 base =
-                    *data.rev_lookup.projections.entry((base, elem.lift())).or_insert_with(|| {
+                    *data.rev_lookup.projections.entry((base, elem.kind())).or_insert_with(|| {
                         new_move_path(
                             &mut data.move_paths,
                             &mut data.path_map,
@@ -272,7 +271,7 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
             tcx,
             ..
         } = self;
-        *rev_lookup.projections.entry((base, elem.lift())).or_insert_with(move || {
+        *rev_lookup.projections.entry((base, elem.kind())).or_insert_with(move || {
             new_move_path(move_paths, path_map, init_path_map, Some(base), mk_place(*tcx))
         })
     }

compiler/rustc_mir_dataflow/src/move_paths/mod.rs

@@ -1,3 +1,16 @@
+//! The move-analysis portion of borrowck needs to work in an abstract
+//! domain of lifted `Place`s. Most of the `Place` variants fall into a
+//! one-to-one mapping between the concrete and abstract (e.g., a
+//! field-deref on a local variable, `x.field`, has the same meaning
+//! in both domains). Indexed projections are the exception: `a[x]`
+//! needs to be treated as mapping to the same move path as `a[y]` as
+//! well as `a[13]`, etc. So we map these `x`/`y` values to `()`.
+//!
+//! (In theory, the analysis could be extended to work with sets of
+//! paths, so that `a[0]` and `a[13]` could be kept distinct, while
+//! `a[x]` would still overlap them both. But that is not this
+//! representation does today.)
+
 use std::fmt;
 use std::ops::{Index, IndexMut};
 
@@ -8,11 +21,8 @@ use rustc_middle::ty::{Ty, TyCtxt};
 use rustc_span::Span;
 use smallvec::SmallVec;
 
-use self::abs_domain::Lift;
 use crate::un_derefer::UnDerefer;
 
-mod abs_domain;
-
 rustc_index::newtype_index! {
     #[orderable]
     #[debug_format = "mp{}"]
@@ -324,7 +334,7 @@ impl<'tcx> MovePathLookup<'tcx> {
         };
 
         for (_, elem) in self.un_derefer.iter_projections(place) {
-            if let Some(&subpath) = self.projections.get(&(result, elem.lift())) {
+            if let Some(&subpath) = self.projections.get(&(result, elem.kind())) {
                 result = subpath;
             } else {
                 return LookupResult::Parent(Some(result));

compiler/rustc_mir_transform/src/gvn.rs

@@ -447,26 +447,9 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
             Projection(base, elem) => {
                 let base = self.evaluated[base].as_ref()?;
-                let elem = match elem {
-                    ProjectionElem::Deref => ProjectionElem::Deref,
-                    ProjectionElem::Downcast(name, read_variant) => {
-                        ProjectionElem::Downcast(name, read_variant)
-                    }
-                    ProjectionElem::Field(f, ()) => ProjectionElem::Field(f, ty.ty),
-                    ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
-                        ProjectionElem::ConstantIndex { offset, min_length, from_end }
-                    }
-                    ProjectionElem::Subslice { from, to, from_end } => {
-                        ProjectionElem::Subslice { from, to, from_end }
-                    }
-                    ProjectionElem::OpaqueCast(()) => ProjectionElem::OpaqueCast(ty.ty),
-                    ProjectionElem::Subtype(()) => ProjectionElem::Subtype(ty.ty),
-                    ProjectionElem::UnwrapUnsafeBinder(()) => {
-                        ProjectionElem::UnwrapUnsafeBinder(ty.ty)
-                    }
-                    // This should have been replaced by a `ConstantIndex` earlier.
-                    ProjectionElem::Index(_) => return None,
-                };
+                // `Index` by constants should have been replaces by `ConstantIndex` by
+                // `simplify_place_projection`.
+                let elem = elem.try_map(|_| None, |()| ty.ty)?;
                 self.ecx.project(base, elem).discard_err()?
             }
             Address { place, kind: _, provenance: _ } => {
@@ -476,13 +459,11 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                 let local = self.locals[place.local]?;
                 let pointer = self.evaluated[local].as_ref()?;
                 let mut mplace = self.ecx.deref_pointer(pointer).discard_err()?;
-                for proj in place.projection.iter().skip(1) {
-                    // We have no call stack to associate a local with a value, so we cannot
-                    // interpret indexing.
-                    if matches!(proj, ProjectionElem::Index(_)) {
-                        return None;
-                    }
-                    mplace = self.ecx.project(&mplace, proj).discard_err()?;
+                for elem in place.projection.iter().skip(1) {
+                    // `Index` by constants should have been replaces by `ConstantIndex` by
+                    // `simplify_place_projection`.
+                    let elem = elem.try_map(|_| None, |ty| ty)?;
+                    mplace = self.ecx.project(&mplace, elem).discard_err()?;
                 }
                 let pointer = mplace.to_ref(&self.ecx);
                 ImmTy::from_immediate(pointer, ty).into()
@@ -902,27 +883,14 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
         proj: ProjectionElem<VnIndex, ()>,
         loc: Location,
     ) -> Option<PlaceElem<'tcx>> {
-        Some(match proj {
-            ProjectionElem::Deref => ProjectionElem::Deref,
-            ProjectionElem::Field(idx, ()) => ProjectionElem::Field(idx, ty),
-            ProjectionElem::Index(idx) => {
-                let Some(local) = self.try_as_local(idx, loc) else {
-                    return None;
-                };
+        proj.try_map(
+            |value| {
+                let local = self.try_as_local(value, loc)?;
                 self.reused_locals.insert(local);
-                ProjectionElem::Index(local)
-            }
-            ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
-                ProjectionElem::ConstantIndex { offset, min_length, from_end }
-            }
-            ProjectionElem::Subslice { from, to, from_end } => {
-                ProjectionElem::Subslice { from, to, from_end }
-            }
-            ProjectionElem::Downcast(symbol, idx) => ProjectionElem::Downcast(symbol, idx),
-            ProjectionElem::OpaqueCast(()) => ProjectionElem::OpaqueCast(ty),
-            ProjectionElem::Subtype(()) => ProjectionElem::Subtype(ty),
-            ProjectionElem::UnwrapUnsafeBinder(()) => ProjectionElem::UnwrapUnsafeBinder(ty),
-        })
+                Some(local)
+            },
+            |()| ty,
+        )
     }
 
     fn simplify_aggregate_to_copy(