Exit arm scopes correctly in the HIR CFG

When a match evaluates to false we jump to the next arm, when we do so
we need to make sure that we exit the scope for that arm.

src/librustc/cfg/construct.rs

@@ -371,7 +371,8 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
         let expr_exit = self.add_ast_node(id, &[]);
 
         // Keep track of the previous guard expressions
-        let mut prev_guards = Vec::new();
+        let mut prev_guard = None;
+        let match_scope = region::Scope { id, data: region::ScopeData::Node };
 
         for arm in arms {
             // Add an exit node for when we've visited all the
@@ -389,23 +390,23 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                     let guard_start = self.add_dummy_node(&[pat_exit]);
                     // Visit the guard expression
                     let guard_exit = match guard {
-                        hir::Guard::If(ref e) => self.expr(e, guard_start),
+                        hir::Guard::If(ref e) => (&**e, self.expr(e, guard_start)),
                     };
                     // #47295: We used to have very special case code
                     // here for when a pair of arms are both formed
                     // solely from constants, and if so, not add these
                     // edges.  But this was not actually sound without
                     // other constraints that we stopped enforcing at
                     // some point.
-                    while let Some(prev) = prev_guards.pop() {
-                        self.add_contained_edge(prev, guard_start);
+                    if let Some((prev_guard, prev_index)) = prev_guard.take() {
+                        self.add_exiting_edge(prev_guard, prev_index, match_scope, guard_start);
                     }
 
                     // Push the guard onto the list of previous guards
-                    prev_guards.push(guard_exit);
+                    prev_guard = Some(guard_exit);
 
                     // Update the exit node for the pattern
-                    pat_exit = guard_exit;
+                    pat_exit = guard_exit.1;
                 }
 
                 // Add an edge from the exit of this pattern to the

src/test/ui/borrowck/issue-62107-match-arm-scopes.rs

@@ -0,0 +1,12 @@
+fn main() {
+    let e: i32;
+    match e {
+        //~^ ERROR use of possibly uninitialized variable
+        ref u if true => {}
+        ref v if true => {
+            let tx = 0;
+            &tx;
+        }
+        _ => (),
+    }
+}

src/test/ui/borrowck/issue-62107-match-arm-scopes.stderr

@@ -0,0 +1,9 @@
+error[E0381]: use of possibly uninitialized variable: `e`
+  --> $DIR/issue-62107-match-arm-scopes.rs:3:11
+   |
+LL |     match e {
+   |           ^ use of possibly uninitialized `e`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0381`.