Make hir ProjectionKind more precise

This commit also categorizing access as Field, Index, or Subslice.

Ideas are taken from `mir::ProjectionElem`.

Proposed changes: https://github.com/rust-lang/project-rfc-2229/blob/master/hir-place-target.md

Closes: rust-lang/project-rfc-2229#1,
rust-lang/project-rfc-2229#2

Co-authored-by: Aman Arora <me@aman-arora.com>
Co-authored-by: Chris Pardy <chrispardy36@gmail.com>
Co-authored-by: Dhruv Jauhar <dhruvjhr@gmail.com>

src/librustc_typeck/expr_use_visitor.rs

@@ -11,8 +11,10 @@ use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::PatKind;
+use rustc_index::vec::Idx;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::ty::{self, adjustment, TyCtxt};
+use rustc_target::abi::VariantIdx;
 
 use crate::mem_categorization as mc;
 use rustc_span::Span;
@@ -396,6 +398,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                             &*with_expr,
                             with_place.clone(),
                             with_field.ty(self.tcx(), substs),
+                            mc::ProjectionKind::Field(f_index as u32, VariantIdx::new(0)),
                         );
                         self.delegate_consume(&field_place);
                     }

src/librustc_typeck/mem_categorization.rs

@@ -54,11 +54,13 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir as hir;
-use rustc_hir::def::{DefKind, Res};
+use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::PatKind;
+use rustc_index::vec::Idx;
 use rustc_infer::infer::InferCtxt;
 use rustc_span::Span;
+use rustc_target::abi::VariantIdx;
 use rustc_trait_selection::infer::InferCtxtExt;
 
 #[derive(Clone, Debug)]
@@ -77,7 +79,22 @@ pub enum PlaceBase {
 pub enum ProjectionKind {
     /// A dereference of a pointer, reference or `Box<T>` of the given type
     Deref,
-    /// An index or a field
+
+    /// `B.F` where `B` is the base expression and `F` is
+    /// the field. The field is identified by which variant
+    /// it appears in along with a field index. The variant
+    /// is used for enums.
+    Field(u32, VariantIdx),
+
+    /// Some index like `B[x]`, where `B` is the base
+    /// expression. We don't preserve the index `x` because
+    /// we won't need it.
+    Index,
+
+    /// A subslice covering a range of values like `B[x..y]`.
+    Subslice,
+
+    /// Other
     Other,
 }
 
@@ -406,7 +423,20 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
             hir::ExprKind::Field(ref base, _) => {
                 let base = self.cat_expr(&base)?;
                 debug!("cat_expr(cat_field): id={} expr={:?} base={:?}", expr.hir_id, expr, base);
-                Ok(self.cat_projection(expr, base, expr_ty))
+
+                let field_idx = self
+                    .tables
+                    .field_indices()
+                    .get(expr.hir_id)
+                    .cloned()
+                    .expect("Field index not found");
+
+                Ok(self.cat_projection(
+                    expr,
+                    base,
+                    expr_ty,
+                    ProjectionKind::Field(field_idx as u32, VariantIdx::new(0)),
+                ))
             }
 
             hir::ExprKind::Index(ref base, _) => {
@@ -419,7 +449,7 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
                     self.cat_overloaded_place(expr, base)
                 } else {
                     let base = self.cat_expr(&base)?;
-                    Ok(self.cat_projection(expr, base, expr_ty))
+                    Ok(self.cat_projection(expr, base, expr_ty, ProjectionKind::Index))
                 }
             }
 
@@ -533,9 +563,10 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
         node: &N,
         base_place: PlaceWithHirId<'tcx>,
         ty: Ty<'tcx>,
+        kind: ProjectionKind,
     ) -> PlaceWithHirId<'tcx> {
         let mut projections = base_place.place.projections;
-        projections.push(Projection { kind: ProjectionKind::Other, ty: ty });
+        projections.push(Projection { kind: kind, ty: ty });
         let ret = PlaceWithHirId::new(
             node.hir_id(),
             base_place.place.base_ty,
@@ -609,6 +640,62 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
         self.cat_pattern_(place, pat, &mut op)
     }
 
+    fn variant_index_for_adt(
+        &self,
+        qpath: &hir::QPath<'_>,
+        hir_id: hir::HirId,
+        span: Span,
+    ) -> Option<VariantIdx> {
+        let res = self.tables.qpath_res(qpath, hir_id);
+        let ty = self.tables.node_type(hir_id);
+        let adt_def = match ty.kind {
+            ty::Adt(adt_def, _) => adt_def,
+            _ => {
+                self.tcx()
+                    .sess
+                    .delay_span_bug(span, "struct or tuple struct pattern not applied to an ADT");
+
+                return None;
+            }
+        };
+
+        match res {
+            Res::Def(DefKind::Variant, variant_id) => {
+                Some(adt_def.variant_index_with_id(variant_id))
+            }
+            Res::Def(DefKind::Ctor(CtorOf::Variant, ..), variant_ctor_id) => {
+                Some(adt_def.variant_index_with_ctor_id(variant_ctor_id))
+            }
+            Res::Def(DefKind::Ctor(CtorOf::Struct, ..), _)
+            | Res::Def(DefKind::Struct | DefKind::Union | DefKind::TyAlias | DefKind::AssocTy, _)
+            | Res::SelfCtor(..)
+            | Res::SelfTy(..) => {
+                // Structs and Unions have only have one variant.
+                Some(VariantIdx::new(0))
+            }
+            _ => bug!("expected ADT path, found={:?}", res),
+        }
+    }
+
+    fn total_fields_in_adt_variant(
+        &self,
+        hir_id: hir::HirId,
+        variant_index: VariantIdx,
+        span: Span,
+    ) -> Option<usize> {
+        let ty = self.tables.node_type(hir_id);
+        match ty.kind {
+            ty::Adt(adt_def, _) => Some(adt_def.variants[variant_index].fields.len()),
+            _ => {
+                self.tcx()
+                    .sess
+                    .delay_span_bug(span, "struct or tuple struct pattern not applied to an ADT");
+
+                None
+            }
+        }
+    }
+
     // FIXME(#19596) This is a workaround, but there should be a better way to do this
     fn cat_pattern_<F>(
         &self,
@@ -679,20 +766,82 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
         op(&place_with_id, pat);
 
         match pat.kind {
-            PatKind::TupleStruct(_, ref subpats, _) | PatKind::Tuple(ref subpats, _) => {
-                // S(p1, ..., pN) or (p1, ..., pN)
+            PatKind::Tuple(ref subpats, _) => {
+                // (p1, ..., pN)
                 for subpat in subpats.iter() {
                     let subpat_ty = self.pat_ty_adjusted(&subpat)?;
-                    let sub_place = self.cat_projection(pat, place_with_id.clone(), subpat_ty);
+                    let sub_place = self.cat_projection(
+                        pat,
+                        place_with_id.clone(),
+                        subpat_ty,
+                        ProjectionKind::Other,
+                    );
                     self.cat_pattern_(sub_place, &subpat, op)?;
                 }
             }
 
-            PatKind::Struct(_, field_pats, _) => {
+            PatKind::TupleStruct(ref qpath, ref subpats, dots_pos) => {
+                // S(p1, ..., pN)
+                let variant_index = if let Some(variant_index) =
+                    self.variant_index_for_adt(qpath, pat.hir_id, pat.span)
+                {
+                    variant_index
+                } else {
+                    return Err(());
+                };
+
+                // We can unwrap safely here
+                // Any ADT related errors would've been catched when reading the `variant_index`
+                let total_fields =
+                    self.total_fields_in_adt_variant(pat.hir_id, variant_index, pat.span).unwrap();
+
+                for (i, subpat) in subpats.iter().enumerate() {
+                    let subpat_ty = self.pat_ty_adjusted(&subpat)?;
+                    let projection_kind = if let Some(dots_pos) = dots_pos {
+                        if i < dots_pos {
+                            ProjectionKind::Field(i as u32, variant_index)
+                        } else if dots_pos == i {
+                            ProjectionKind::Other
+                        } else {
+                            let field_index = total_fields - subpats.len() + i;
+                            ProjectionKind::Field(field_index as u32, variant_index)
+                        }
+                    } else {
+                        ProjectionKind::Field(i as u32, variant_index)
+                    };
+
+                    let sub_place =
+                        self.cat_projection(pat, place_with_id.clone(), subpat_ty, projection_kind);
+                    self.cat_pattern_(sub_place, &subpat, op)?;
+                }
+            }
+
+            PatKind::Struct(ref qpath, field_pats, _) => {
                 // S { f1: p1, ..., fN: pN }
+
+                let variant_index = if let Some(variant_index) =
+                    self.variant_index_for_adt(qpath, pat.hir_id, pat.span)
+                {
+                    variant_index
+                } else {
+                    return Err(());
+                };
+
                 for fp in field_pats {
                     let field_ty = self.pat_ty_adjusted(&fp.pat)?;
-                    let field_place = self.cat_projection(pat, place_with_id.clone(), field_ty);
+                    let field_index = self
+                        .tables
+                        .field_indices()
+                        .get(fp.hir_id)
+                        .cloned()
+                        .expect("no index for a field");
+
+                    let field_place = self.cat_projection(
+                        pat,
+                        place_with_id.clone(),
+                        field_ty,
+                        ProjectionKind::Field(field_index as u32, variant_index),
+                    );
                     self.cat_pattern_(field_place, &fp.pat, op)?;
                 }
             }
@@ -723,13 +872,23 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
                         return Err(());
                     }
                 };
-                let elt_place = self.cat_projection(pat, place_with_id.clone(), element_ty);
+                let elt_place = self.cat_projection(
+                    pat,
+                    place_with_id.clone(),
+                    element_ty,
+                    ProjectionKind::Subslice,
+                );
                 for before_pat in before {
                     self.cat_pattern_(elt_place.clone(), &before_pat, op)?;
                 }
                 if let Some(ref slice_pat) = *slice {
                     let slice_pat_ty = self.pat_ty_adjusted(&slice_pat)?;
-                    let slice_place = self.cat_projection(pat, place_with_id, slice_pat_ty);
+                    let slice_place = self.cat_projection(
+                        pat,
+                        place_with_id,
+                        slice_pat_ty,
+                        ProjectionKind::Subslice,
+                    );
                     self.cat_pattern_(slice_place, &slice_pat, op)?;
                 }
                 for after_pat in after {