Handle reinits in match guards

compiler/rustc_typeck/src/check/generator_interior/drop_ranges/cfg_build.rs

@@ -133,11 +133,10 @@ fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
             ExprKind::Match(scrutinee, arms, ..) => {
                 self.visit_expr(scrutinee);
 
-                let fork = self.expr_index;
-                let arm_end_ids = arms
-                    .iter()
-                    .map(|hir::Arm { pat, body, guard, .. }| {
-                        self.drop_ranges.add_control_edge(fork, self.expr_index + 1);
+                let (guard_exit, arm_end_ids) = arms.iter().fold(
+                    (self.expr_index, vec![]),
+                    |(incoming_edge, mut arm_end_ids), hir::Arm { pat, body, guard, .. }| {
+                        self.drop_ranges.add_control_edge(incoming_edge, self.expr_index + 1);
                         self.visit_pat(pat);
                         match guard {
                             Some(Guard::If(expr)) => self.visit_expr(expr),
@@ -147,10 +146,16 @@ fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
                             }
                             None => (),
                         }
+                        let to_next_arm = self.expr_index;
+                        // The default edge does not get added since we also have an explicit edge,
+                        // so we also need to add an edge to the next node as well.
+                        self.drop_ranges.add_control_edge(self.expr_index, self.expr_index + 1);
                         self.visit_expr(body);
-                        self.expr_index
-                    })
-                    .collect::<Vec<_>>();
+                        arm_end_ids.push(self.expr_index);
+                        (to_next_arm, arm_end_ids)
+                    },
+                );
+                self.drop_ranges.add_control_edge(guard_exit, self.expr_index + 1);
                 arm_end_ids.into_iter().for_each(|arm_end| {
                     self.drop_ranges.add_control_edge(arm_end, self.expr_index + 1)
                 });

compiler/rustc_typeck/src/check/generator_interior/drop_ranges/cfg_visualize.rs

@@ -5,6 +5,15 @@
 
 use super::{DropRangesBuilder, PostOrderId};
 
+/// Writes the CFG for DropRangesBuilder to a .dot file for visualization.
+///
+/// It is not normally called, but is kept around to easily add debugging
+/// code when needed.
+#[allow(dead_code)]
+pub(super) fn write_graph_to_file(drop_ranges: &DropRangesBuilder, filename: &str) {
+    dot::render(drop_ranges, &mut std::fs::File::create(filename).unwrap()).unwrap();
+}
+
 impl<'a> dot::GraphWalk<'a> for DropRangesBuilder {
     type Node = PostOrderId;
 

src/test/ui/generator/reinit-in-match-guard.rs

@@ -0,0 +1,25 @@
+// build-pass
+
+#![feature(generators)]
+
+#![allow(unused_assignments, dead_code)]
+
+fn main() {
+    let _ = || {
+        let mut x = vec![22_usize];
+        std::mem::drop(x);
+        match y() {
+            true if {
+                x = vec![];
+                false
+            } => {}
+            _ => {
+                yield;
+            }
+        }
+    };
+}
+
+fn y() -> bool {
+    true
+}