Issue the correct deref count for the implicit deref that occurs

as part of an autoslice operation (rust-lang#6272).

Fixes rust-lang#6272.

src/librustc/middle/borrowck/gather_loans/mod.rs

@@ -260,7 +260,7 @@ pub impl GatherLoanCtxt {
                                              r)
                     }
                     ty::AutoBorrowVec(r, m) | ty::AutoBorrowVecRef(r, m) => {
-                        let cmt_index = mcx.cat_index(expr, cmt);
+                        let cmt_index = mcx.cat_index(expr, cmt, autoderefs+1);
                         self.guarantee_valid(expr.id,
                                              expr.span,
                                              cmt_index,
@@ -574,7 +574,7 @@ pub impl GatherLoanCtxt {
                   let (slice_mutbl, slice_r) =
                       self.vec_slice_info(slice_pat, slice_ty);
                   let mcx = self.bccx.mc_ctxt();
-                  let cmt_index = mcx.cat_index(slice_pat, cmt);
+                  let cmt_index = mcx.cat_index(slice_pat, cmt, 0);
                   self.guarantee_valid(pat.id, pat.span,
                                        cmt_index, slice_mutbl, slice_r);
               }

src/librustc/middle/borrowck/mod.rs

@@ -184,6 +184,16 @@ pub type LoanMap = @mut HashMap<ast::node_id, @Loan>;
 //
 // Note that there is no entry with derefs:3---the type of that expression
 // is T, which is not a box.
+//
+// Note that implicit dereferences also occur with indexing of `@[]`,
+// `@str`, etc.  The same rules apply. So, for example, given a
+// variable `x` of type `@[@[...]]`, if I have an instance of the
+// expression `x[0]` which is then auto-slice'd, there would be two
+// potential entries in the root map, both with the id of the `x[0]`
+// expression. The entry with `derefs==0` refers to the deref of `x`
+// used as part of evaluating `x[0]`. The entry with `derefs==1`
+// refers to the deref of the `x[0]` that occurs as part of the
+// auto-slice.
 #[deriving(Eq, IterBytes)]
 pub struct root_map_key {
     id: ast::node_id,

src/librustc/middle/mem_categorization.rs

@@ -405,7 +405,7 @@ pub impl mem_categorization_ctxt {
             }
 
             let base_cmt = self.cat_expr(base);
-            self.cat_index(expr, base_cmt)
+            self.cat_index(expr, base_cmt, 0)
           }
 
           ast::expr_path(_) => {
@@ -670,7 +670,39 @@ pub impl mem_categorization_ctxt {
 
     fn cat_index<N:ast_node>(&self,
                              elt: N,
-                             base_cmt: cmt) -> cmt {
+                             base_cmt: cmt,
+                             derefs: uint) -> cmt {
+        //! Creates a cmt for an indexing operation (`[]`); this
+        //! indexing operation may occurs as part of an
+        //! AutoBorrowVec, which when converting a `~[]` to an `&[]`
+        //! effectively takes the address of the 0th element.
+        //!
+        //! One subtle aspect of indexing that may not be
+        //! immediately obvious: for anything other than a fixed-length
+        //! vector, an operation like `x[y]` actually consists of two
+        //! disjoint (from the point of view of borrowck) operations.
+        //! The first is a deref of `x` to create a pointer `p` that points
+        //! at the first element in the array. The second operation is
+        //! an index which adds `y*sizeof(T)` to `p` to obtain the
+        //! pointer to `x[y]`. `cat_index` will produce a resulting
+        //! cmt containing both this deref and the indexing,
+        //! presuming that `base_cmt` is not of fixed-length type.
+        //!
+        //! In the event that a deref is needed, the "deref count"
+        //! is taken from the parameter `derefs`. See the comment
+        //! on the def'n of `root_map_key` in borrowck/mod.rs
+        //! for more details about deref counts; the summary is
+        //! that `derefs` should be 0 for an explicit indexing
+        //! operation and N+1 for an indexing that is part of
+        //! an auto-adjustment, where N is the number of autoderefs
+        //! in that adjustment.
+        //!
+        //! # Parameters
+        //! - `elt`: the AST node being indexed
+        //! - `base_cmt`: the cmt of `elt`
+        //! - `derefs`: the deref number to be used for
+        //!   the implicit index deref, if any (see above)
+
         let mt = match ty::index(base_cmt.ty) {
           Some(mt) => mt,
           None => {
@@ -698,7 +730,7 @@ pub impl mem_categorization_ctxt {
             let deref_cmt = @cmt_ {
                 id:elt.id(),
                 span:elt.span(),
-                cat:cat_deref(base_cmt, 0u, ptr),
+                cat:cat_deref(base_cmt, derefs, ptr),
                 mutbl:m,
                 ty:mt.ty
             };
@@ -878,8 +910,8 @@ pub impl mem_categorization_ctxt {
           }
 
           ast::pat_vec(ref before, slice, ref after) => {
+              let elt_cmt = self.cat_index(pat, cmt, 0);
               for before.each |&before_pat| {
-                  let elt_cmt = self.cat_index(pat, cmt);
                   self.cat_pattern(elt_cmt, before_pat, op);
               }
               for slice.each |&slice_pat| {
@@ -888,7 +920,6 @@ pub impl mem_categorization_ctxt {
                   self.cat_pattern(slice_cmt, slice_pat, op);
               }
               for after.each |&after_pat| {
-                  let elt_cmt = self.cat_index(pat, cmt);
                   self.cat_pattern(elt_cmt, after_pat, op);
               }
           }

src/librustc/middle/trans/_match.rs

@@ -885,7 +885,8 @@ pub fn extract_vec_elems(bcx: block,
                       -> ExtractedBlock {
     let _icx = bcx.insn_ctxt("match::extract_vec_elems");
     let vec_datum = match_datum(bcx, val, pat_id);
-    let (bcx, base, len) = vec_datum.get_vec_base_and_len(bcx, pat_span, pat_id);
+    let (bcx, base, len) = vec_datum.get_vec_base_and_len(bcx, pat_span,
+                                                          pat_id, 0);
     let vt = tvec::vec_types(bcx, node_id_type(bcx, pat_id));
 
     let mut elems = do vec::from_fn(elem_count) |i| {

src/librustc/middle/trans/datum.rs

@@ -735,13 +735,14 @@ pub impl Datum {
     fn get_vec_base_and_len(&self,
                             mut bcx: block,
                             span: span,
-                            expr_id: ast::node_id)
+                            expr_id: ast::node_id,
+                            derefs: uint)
                             -> (block, ValueRef, ValueRef) {
         //! Converts a vector into the slice pair. Performs rooting
         //! and write guards checks.
 
         // only imp't for @[] and @str, but harmless
-        bcx = write_guard::root_and_write_guard(self, bcx, span, expr_id, 0);
+        bcx = write_guard::root_and_write_guard(self, bcx, span, expr_id, derefs);
         let (base, len) = self.get_vec_base_and_len_no_root(bcx);
         (bcx, base, len)
     }

src/librustc/middle/trans/expr.rs

@@ -144,10 +144,9 @@ use middle::trans::tvec;
 use middle::trans::type_of;
 use middle::ty::struct_fields;
 use middle::ty::{AutoDerefRef, AutoAddEnv};
-use middle::ty::{AutoPtr, AutoBorrowVec, AutoBorrowVecRef, AutoBorrowFn};
-use middle::ty;
 use middle::ty::{AutoPtr, AutoBorrowVec, AutoBorrowVecRef, AutoBorrowFn,
                  AutoDerefRef, AutoAddEnv, AutoUnsafe};
+use middle::ty;
 use util::common::indenter;
 use util::ppaux::Repr;
 
@@ -215,10 +214,12 @@ pub fn trans_to_datum(bcx: block, expr: @ast::expr) -> DatumBlock {
                     unpack_datum!(bcx, auto_ref(bcx, datum))
                 }
                 Some(AutoBorrowVec(*)) => {
-                    unpack_datum!(bcx, auto_slice(bcx, expr, datum))
+                    unpack_datum!(bcx, auto_slice(bcx, adj.autoderefs,
+                                                  expr, datum))
                 }
                 Some(AutoBorrowVecRef(*)) => {
-                    unpack_datum!(bcx, auto_slice_and_ref(bcx, expr, datum))
+                    unpack_datum!(bcx, auto_slice_and_ref(bcx, adj.autoderefs,
+                                                          expr, datum))
                 }
                 Some(AutoBorrowFn(*)) => {
                     let adjusted_ty = ty::adjust_ty(bcx.tcx(), expr.span,
@@ -246,7 +247,10 @@ pub fn trans_to_datum(bcx: block, expr: @ast::expr) -> DatumBlock {
                                   mode: datum.mode, source: datum.source}}
     }
 
-    fn auto_slice(bcx: block, expr: @ast::expr, datum: Datum) -> DatumBlock {
+    fn auto_slice(bcx: block,
+                  autoderefs: uint,
+                  expr: @ast::expr,
+                  datum: Datum) -> DatumBlock {
         // This is not the most efficient thing possible; since slices
         // are two words it'd be better if this were compiled in
         // 'dest' mode, but I can't find a nice way to structure the
@@ -256,9 +260,8 @@ pub fn trans_to_datum(bcx: block, expr: @ast::expr) -> DatumBlock {
         let tcx = bcx.tcx();
         let unit_ty = ty::sequence_element_type(tcx, datum.ty);
 
-        // FIXME(#6272) need to distinguish "auto-slice" from explicit index?
         let (bcx, base, len) =
-            datum.get_vec_base_and_len(bcx, expr.span, expr.id);
+            datum.get_vec_base_and_len(bcx, expr.span, expr.id, autoderefs+1);
 
         // this type may have a different region/mutability than the
         // real one, but it will have the same runtime representation
@@ -292,9 +295,10 @@ pub fn trans_to_datum(bcx: block, expr: @ast::expr) -> DatumBlock {
     }
 
     fn auto_slice_and_ref(bcx: block,
+                          autoderefs: uint,
                           expr: @ast::expr,
                           datum: Datum) -> DatumBlock {
-        let DatumBlock { bcx, datum } = auto_slice(bcx, expr, datum);
+        let DatumBlock { bcx, datum } = auto_slice(bcx, autoderefs, expr, datum);
         auto_ref(bcx, datum)
     }
 }
@@ -913,7 +917,8 @@ fn trans_lvalue_unadjusted(bcx: block, expr: @ast::expr) -> DatumBlock {
         base::maybe_name_value(bcx.ccx(), scaled_ix, ~"scaled_ix");
 
         let mut (bcx, base, len) =
-            base_datum.get_vec_base_and_len(bcx, index_expr.span, index_expr.id);
+            base_datum.get_vec_base_and_len(bcx, index_expr.span,
+                                            index_expr.id, 0);
 
         if ty::type_is_str(base_ty) {
             // acccount for null terminator in the case of string