Modify the ExprUseVisitor to walk each part of an AutoRef

src/librustc/middle/check_const.rs

@@ -662,7 +662,19 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for CheckCrateVisitor<'a, 'tcx> {
               cmt: mc::cmt<'tcx>,
               _loan_region: ty::Region,
               bk: ty::BorrowKind,
-              loan_cause: euv::LoanCause) {
+              loan_cause: euv::LoanCause)
+    {
+        // Kind of hacky, but we allow Unsafe coercions in constants.
+        // These occur when we convert a &T or *T to a *U, as well as
+        // when making a thin pointer (e.g., `*T`) into a fat pointer
+        // (e.g., `*Trait`).
+        match loan_cause {
+            euv::LoanCause::AutoUnsafe => {
+                return;
+            }
+            _ => { }
+        }
+
         let mut cur = &cmt;
         let mut is_interior = false;
         loop {

src/librustc/middle/expr_use_visitor.rs

@@ -99,6 +99,7 @@ pub enum LoanCause {
     ClosureCapture(Span),
     AddrOf,
     AutoRef,
+    AutoUnsafe,
     RefBinding,
     OverloadedOperator,
     ClosureInvocation,
@@ -800,18 +801,8 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,'tcx,TYPER> {
                             return_if_err!(self.mc.cat_expr_unadjusted(expr));
                         self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
                     }
-                    ty::AdjustDerefRef(ty::AutoDerefRef {
-                        autoref: ref opt_autoref,
-                        autoderefs: n
-                    }) => {
-                        self.walk_autoderefs(expr, n);
-
-                        match *opt_autoref {
-                            None => { }
-                            Some(ref r) => {
-                                self.walk_autoref(expr, r, n);
-                            }
-                        }
+                    ty::AdjustDerefRef(ref adj) => {
+                        self.walk_autoderefref(expr, adj);
                     }
                 }
             }
@@ -852,39 +843,165 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,'tcx,TYPER> {
         }
     }
 
+    fn walk_autoderefref(&mut self,
+                         expr: &ast::Expr,
+                         adj: &ty::AutoDerefRef<'tcx>) {
+        debug!("walk_autoderefref expr={} adj={}",
+               expr.repr(self.tcx()),
+               adj.repr(self.tcx()));
+
+        self.walk_autoderefs(expr, adj.autoderefs);
+
+        // Weird hacky special case: AutoUnsizeUniq, which converts
+        // from a ~T to a ~Trait etc, always comes in a stylized
+        // fashion. In particular, we want to consume the ~ pointer
+        // being dereferenced, not the dereferenced content (as the
+        // content is, at least for upcasts, unsized).
+        match adj.autoref {
+            Some(ty::AutoUnsizeUniq(_)) => {
+                assert!(adj.autoderefs == 1,
+                        format!("Expected exactly 1 deref with Uniq AutoRefs, found: {}",
+                                adj.autoderefs));
+                let cmt_unadjusted =
+                    return_if_err!(self.mc.cat_expr_unadjusted(expr));
+                self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
+                return;
+            }
+            _ => { }
+        }
+
+        let autoref = adj.autoref.as_ref();
+        let cmt_derefd = return_if_err!(
+            self.mc.cat_expr_autoderefd(expr, adj.autoderefs));
+        self.walk_autoref(expr, &cmt_derefd, autoref);
+    }
+
+    /// Walks the autoref `opt_autoref` applied to the autoderef'd
+    /// `expr`. `cmt_derefd` is the mem-categorized form of `expr`
+    /// after all relevant autoderefs have occurred. Because AutoRefs
+    /// can be recursive, this function is recursive: it first walks
+    /// deeply all the way down the autoref chain, and then processes
+    /// the autorefs on the way out. At each point, it returns the
+    /// `cmt` for the rvalue that will be produced by introduced an
+    /// autoref.
     fn walk_autoref(&mut self,
                     expr: &ast::Expr,
-                    autoref: &ty::AutoRef,
-                    n: usize) {
-        debug!("walk_autoref expr={}", expr.repr(self.tcx()));
+                    cmt_derefd: &mc::cmt<'tcx>,
+                    opt_autoref: Option<&ty::AutoRef<'tcx>>)
+                    -> mc::cmt<'tcx>
+    {
+        debug!("walk_autoref(expr.id={} cmt_derefd={} opt_autoref={:?})",
+               expr.id,
+               cmt_derefd.repr(self.tcx()),
+               opt_autoref);
+
+        let autoref = match opt_autoref {
+            Some(autoref) => autoref,
+            None => {
+                // No recursive step here, this is a base case.
+                return cmt_derefd.clone();
+            }
+        };
 
         match *autoref {
-            ty::AutoPtr(r, m, _) => {
-                let cmt_derefd = return_if_err!(
-                    self.mc.cat_expr_autoderefd(expr, n));
-                debug!("walk_adjustment: cmt_derefd={}",
-                       cmt_derefd.repr(self.tcx()));
+            ty::AutoPtr(r, m, ref baseref) => {
+                let cmt_base = self.walk_autoref_recursively(expr, cmt_derefd, baseref);
+
+                debug!("walk_autoref: expr.id={} cmt_base={}",
+                       expr.id,
+                       cmt_base.repr(self.tcx()));
 
                 self.delegate.borrow(expr.id,
                                      expr.span,
-                                     cmt_derefd,
+                                     cmt_base,
                                      r,
                                      ty::BorrowKind::from_mutbl(m),
                                      AutoRef);
             }
-            ty::AutoUnsize(_) |
+
+            ty::AutoUnsize(_) => {
+                // Converting a `[T; N]` to `[T]` or `T` to `Trait`
+                // isn't really a borrow, move, etc, in and of itself.
+                // Also, no recursive step here, this is a base case.
+
+                // It may seem a bit odd to return the cmt_derefd
+                // unmodified here, but in fact I think it's the right
+                // thing to do. Essentially the unsize transformation
+                // isn't really relevant to the borrowing rules --
+                // it's best thought of as a kind of side-modifier to
+                // the autoref, adding additional data that is
+                // attached to the pointer that is produced, but not
+                // affecting the data being borrowed in any other
+                // way. To see what I mean, consider this example:
+                //
+                //    fn foo<'a>(&'a self) -> &'a Trait { self }
+                //
+                // This is valid because the underlying `self` value
+                // lives for the lifetime 'a. If we were to treat the
+                // "unsizing" as e.g. producing an rvalue, that would
+                // only be valid for the temporary scope, which isn't
+                // enough to justify the return value, which have the
+                // lifetime 'a.
+                //
+                // Another option would be to add a variant for
+                // categorization (like downcast) that wraps
+                // cmt_derefd and represents the unsizing operation.
+                // But I don't think there is any particular use for
+                // this (yet). -nmatsakis
+                return cmt_derefd.clone();
+            }
+
             ty::AutoUnsizeUniq(_) => {
-                assert!(n == 1, format!("Expected exactly 1 deref with Uniq \
-                                         AutoRefs, found: {}", n));
-                let cmt_unadjusted =
-                    return_if_err!(self.mc.cat_expr_unadjusted(expr));
-                self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
+                // these are handled via special case above
+                self.tcx().sess.span_bug(expr.span, "nexpected AutoUnsizeUniq");
             }
-            ty::AutoUnsafe(..) => {
+
+            ty::AutoUnsafe(m, ref baseref) => {
+                let cmt_base = self.walk_autoref_recursively(expr, cmt_derefd, baseref);
+
+                debug!("walk_autoref: expr.id={} cmt_base={}",
+                       expr.id,
+                       cmt_base.repr(self.tcx()));
+
+                // Converting from a &T to *T (or &mut T to *mut T) is
+                // treated as borrowing it for the enclosing temporary
+                // scope.
+                let r = ty::ReScope(region::CodeExtent::from_node_id(expr.id));
+
+                self.delegate.borrow(expr.id,
+                                     expr.span,
+                                     cmt_base,
+                                     r,
+                                     ty::BorrowKind::from_mutbl(m),
+                                     AutoUnsafe);
             }
         }
+
+        // Construct the categorization for the result of the autoref.
+        // This is always an rvalue, since we are producing a new
+        // (temporary) indirection.
+
+        let adj_ty =
+            ty::adjust_ty_for_autoref(self.tcx(),
+                                      expr.span,
+                                      cmt_derefd.ty,
+                                      opt_autoref);
+
+        self.mc.cat_rvalue_node(expr.id, expr.span, adj_ty)
     }
 
+    fn walk_autoref_recursively(&mut self,
+                                expr: &ast::Expr,
+                                cmt_derefd: &mc::cmt<'tcx>,
+                                autoref: &Option<Box<ty::AutoRef<'tcx>>>)
+                                -> mc::cmt<'tcx>
+    {
+        // Shuffle from a ref to an optional box to an optional ref.
+        let autoref: Option<&ty::AutoRef<'tcx>> = autoref.as_ref().map(|b| &**b);
+        self.walk_autoref(expr, cmt_derefd, autoref)
+    }
+
+
     // When this returns true, it means that the expression *is* a
     // method-call (i.e. via the operator-overload).  This true result
     // also implies that walk_overloaded_operator already took care of

src/librustc/middle/mem_categorization.rs

@@ -833,6 +833,15 @@ impl<'t,'tcx,TYPER:Typer<'tcx>> MemCategorizationContext<'t,TYPER> {
         ret
     }
 
+    /// Returns the lifetime of a temporary created by expr with id `id`.
+    /// This could be `'static` if `id` is part of a constant expression.
+    pub fn temporary_scope(&self, id: ast::NodeId) -> ty::Region {
+        match self.typer.temporary_scope(id) {
+            Some(scope) => ty::ReScope(scope),
+            None => ty::ReStatic
+        }
+    }
+
     pub fn cat_rvalue_node(&self,
                            id: ast::NodeId,
                            span: Span,
@@ -848,17 +857,12 @@ impl<'t,'tcx,TYPER:Typer<'tcx>> MemCategorizationContext<'t,TYPER> {
             _ => check_const::NOT_CONST
         };
 
+        // Compute maximum lifetime of this rvalue. This is 'static if
+        // we can promote to a constant, otherwise equal to enclosing temp
+        // lifetime.
         let re = match qualif & check_const::NON_STATIC_BORROWS {
-            check_const::PURE_CONST => {
-                // Constant rvalues get promoted to 'static.
-                ty::ReStatic
-            }
-            _ => {
-                match self.typer.temporary_scope(id) {
-                    Some(scope) => ty::ReScope(scope),
-                    None => ty::ReStatic
-                }
-            }
+            check_const::PURE_CONST => ty::ReStatic,
+            _ => self.temporary_scope(id),
         };
         let ret = self.cat_rvalue(id, span, re, expr_ty);
         debug!("cat_rvalue_node ret {}", ret.repr(self.tcx()));

src/librustc_borrowck/borrowck/check_loans.rs

@@ -542,6 +542,7 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                 euv::OverloadedOperator(..) |
                 euv::AddrOf(..) |
                 euv::AutoRef(..) |
+                euv::AutoUnsafe(..) |
                 euv::ClosureInvocation(..) |
                 euv::ForLoop(..) |
                 euv::RefBinding(..) |

src/librustc_borrowck/borrowck/mod.rs

@@ -775,6 +775,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     euv::AddrOf |
                     euv::RefBinding |
                     euv::AutoRef |
+                    euv::AutoUnsafe |
                     euv::ForLoop |
                     euv::MatchDiscriminant => {
                         format!("cannot borrow {} as mutable", descr)
@@ -822,6 +823,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             BorrowViolation(euv::OverloadedOperator) |
             BorrowViolation(euv::AddrOf) |
             BorrowViolation(euv::AutoRef) |
+            BorrowViolation(euv::AutoUnsafe) |
             BorrowViolation(euv::RefBinding) |
             BorrowViolation(euv::MatchDiscriminant) => {
                 "cannot borrow data mutably"

src/librustc_typeck/check/op.rs

@@ -20,7 +20,6 @@ use super::{
     PreferMutLvalue,
     structurally_resolved_type,
 };
-use middle::infer;
 use middle::traits;
 use middle::ty::{self, Ty};
 use syntax::ast;
@@ -314,36 +313,9 @@ fn lookup_op_method<'a, 'tcx>(fcx: &'a FnCtxt<'a, 'tcx>,
 
     let method = match trait_did {
         Some(trait_did) => {
-            // We do eager coercions to make using operators
-            // more ergonomic:
-            //
-            // - If the input is of type &'a T (resp. &'a mut T),
-            //   then reborrow it to &'b T (resp. &'b mut T) where
-            //   'b <= 'a.  This makes things like `x == y`, where
-            //   `x` and `y` are both region pointers, work.  We
-            //   could also solve this with variance or different
-            //   traits that don't force left and right to have same
-            //   type.
-            let (adj_ty, adjustment) = match lhs_ty.sty {
-                ty::ty_rptr(r_in, mt) => {
-                    let r_adj = fcx.infcx().next_region_var(infer::Autoref(lhs_expr.span));
-                    fcx.mk_subr(infer::Reborrow(lhs_expr.span), r_adj, *r_in);
-                    let adjusted_ty = ty::mk_rptr(fcx.tcx(), fcx.tcx().mk_region(r_adj), mt);
-                    let autoptr = ty::AutoPtr(r_adj, mt.mutbl, None);
-                    let adjustment = ty::AutoDerefRef { autoderefs: 1, autoref: Some(autoptr) };
-                    (adjusted_ty, adjustment)
-                }
-                _ => {
-                    (lhs_ty, ty::AutoDerefRef { autoderefs: 0, autoref: None })
-                }
-            };
-
-            debug!("adjusted_ty={} adjustment={:?}",
-                   adj_ty.repr(fcx.tcx()),
-                   adjustment);
-
+            let noop = ty::AutoDerefRef { autoderefs: 0, autoref: None };
             method::lookup_in_trait_adjusted(fcx, expr.span, Some(lhs_expr), opname,
-                                             trait_did, adjustment, adj_ty, Some(other_tys))
+                                             trait_did, noop, lhs_ty, Some(other_tys))
         }
         None => None
     };

src/librustc_typeck/check/regionck.rs

@@ -1119,20 +1119,24 @@ fn link_pattern<'a, 'tcx>(rcx: &Rcx<'a, 'tcx>,
 fn link_autoref(rcx: &Rcx,
                 expr: &ast::Expr,
                 autoderefs: usize,
-                autoref: &ty::AutoRef) {
-
+                autoref: &ty::AutoRef)
+{
     debug!("link_autoref(autoref={:?})", autoref);
     let mc = mc::MemCategorizationContext::new(rcx.fcx);
     let expr_cmt = ignore_err!(mc.cat_expr_autoderefd(expr, autoderefs));
     debug!("expr_cmt={}", expr_cmt.repr(rcx.tcx()));
 
     match *autoref {
         ty::AutoPtr(r, m, _) => {
-            link_region(rcx, expr.span, r,
-                ty::BorrowKind::from_mutbl(m), expr_cmt);
+            link_region(rcx, expr.span, r, ty::BorrowKind::from_mutbl(m), expr_cmt);
+        }
+
+        ty::AutoUnsafe(m, _) => {
+            let r = ty::ReScope(CodeExtent::from_node_id(expr.id));
+            link_region(rcx, expr.span, r, ty::BorrowKind::from_mutbl(m), expr_cmt);
         }
 
-        ty::AutoUnsafe(..) | ty::AutoUnsizeUniq(_) | ty::AutoUnsize(_) => {}
+        ty::AutoUnsizeUniq(_) | ty::AutoUnsize(_) => {}
     }
 }
 

src/test/compile-fail/borrowck-use-uninitialized-in-cast-trait.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Variation on `borrowck-use-uninitialized-in-cast` in which we do a
+// trait cast from an uninitialized source. Issue #20791.
+
+trait Foo { fn dummy(&self) { } }
+impl Foo for i32 { }
+
+fn main() {
+    let x: &i32;
+    let y = x as *const Foo; //~ ERROR use of possibly uninitialized variable: `*x`
+}