Track what drop obligations are established on match arms.

This is accomplished by:

1. Add `MatchMode` enum to `expr_use_visitor`.

2. Computing the match mode for each pattern via a pre-pass, and then
   passing the mode along when visiting the pattern in
   expr_use_visitor.

3. Adding a `fn matched_pat` callback to expr_use_visitor, which is
   called on interior struct and enum nodes of the pattern (as opposed
   to `fn consume_pat`, which is only invoked for identifiers at the
   leaves of the pattern), and invoking it accordingly.

Of particular interest are the `cat_downcast` instances established
when matching enum variants.

Author: pnkfelix

src/librustc/middle/borrowck/check_loans.rs

@@ -103,6 +103,11 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for CheckLoanCtxt<'a, 'tcx> {
         self.consume_common(consume_id, consume_span, cmt, mode);
     }
 
+    fn matched_pat(&mut self,
+                   _matched_pat: &ast::Pat,
+                   _cmt: mc::cmt,
+                   _mode: euv::MatchMode) { }
+
     fn consume_pat(&mut self,
                    consume_pat: &ast::Pat,
                    cmt: mc::cmt<'tcx>,

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

@@ -88,6 +88,16 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for GatherLoanCtxt<'a, 'tcx> {
         }
     }
 
+    fn matched_pat(&mut self,
+                   matched_pat: &ast::Pat,
+                   cmt: mc::cmt<'tcx>,
+                   mode: euv::MatchMode) {
+        debug!("matched_pat(matched_pat={}, cmt={}, mode={})",
+               matched_pat.repr(self.tcx()),
+               cmt.repr(self.tcx()),
+               mode);
+    }
+
     fn consume_pat(&mut self,
                    consume_pat: &ast::Pat,
                    cmt: mc::cmt<'tcx>,

src/librustc/middle/check_match.rs

@@ -18,6 +18,7 @@ use middle::def::*;
 use middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor, Init};
 use middle::expr_use_visitor::{JustWrite, LoanCause, MutateMode};
 use middle::expr_use_visitor::{WriteAndRead};
+use middle::expr_use_visitor as euv;
 use middle::mem_categorization::cmt;
 use middle::pat_util::*;
 use middle::ty::*;
@@ -1024,6 +1025,7 @@ struct MutationChecker<'a, 'tcx: 'a> {
 }
 
 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
+    fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
     fn consume(&mut self, _: NodeId, _: Span, _: cmt, _: ConsumeMode) {}
     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
     fn borrow(&mut self,

src/librustc/middle/check_rvalues.rs

@@ -59,6 +59,11 @@ impl<'a, 'tcx> euv::Delegate<'tcx> for RvalueContext<'a, 'tcx> {
         }
     }
 
+    fn matched_pat(&mut self,
+                   _matched_pat: &ast::Pat,
+                   _cmt: mc::cmt,
+                   _mode: euv::MatchMode) {}
+
     fn consume_pat(&mut self,
                    _consume_pat: &ast::Pat,
                    _cmt: mc::cmt,

src/librustc/middle/check_static.rs

@@ -325,6 +325,12 @@ impl<'tcx> euv::Delegate<'tcx> for GlobalChecker {
               _assignment_span: Span,
               _assignee_cmt: mc::cmt,
               _mode: euv::MutateMode) {}
+
+    fn matched_pat(&mut self,
+                   _: &ast::Pat,
+                   _: mc::cmt,
+                   _: euv::MatchMode) {}
+
     fn consume_pat(&mut self,
                    _consume_pat: &ast::Pat,
                    _cmt: mc::cmt,