Rollup merge of rust-lang#23859 - pnkfelix:fsk-lesser-box, r=nikomats…

…akis

Disallow writing through mutable pointers stored in non-mut Box.

Fix rust-lang#14270 

The fix works by making `cmt::freely_aliasable` result more fine-grained.

Instead of encoding the aliasability (i.e. whether the cmt is uniquely writable or not) as an option, now pass back an enum indicating either: 1. freely-aliasable (thus not uniquely-writable), 2. non-aliasable (thus uniquely writable), or 3. unique but immutable (and thus not uniquely writable, according to proposal from issue rust-lang#14270.)

This is all of course a giant hack that will hopefully go away with an eventually removal of special treatment of `Box<T>` (aka `ty_unique`) from the compiler.

src/librustc/middle/expr_use_visitor.rs

@@ -885,6 +885,11 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,'tcx,TYPER> {
         }
     }
 
+    // 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
+    // recursively processing the input arguments, and thus the caller
+    // should not do so.
     fn walk_overloaded_operator(&mut self,
                                 expr: &ast::Expr,
                                 receiver: &ast::Expr,