Fix incorrect double assignment in MIR for while loops

src/librustc_mir/build/expr/into.rs

@@ -184,15 +184,23 @@ pub fn into_expr(
                                 loop_block_end = this.as_local_operand(loop_block, cond_expr)
                             );
                             body_block = this.cfg.start_new_block();
-                            let term =
-                                TerminatorKind::if_(this.hir.tcx(), cond, body_block, exit_block);
+                            let false_block = this.cfg.start_new_block();
+                            let term = TerminatorKind::if_(
+                                this.hir.tcx(),
+                                cond,
+                                body_block,
+                                false_block,
+                            );
                             this.cfg.terminate(loop_block_end, source_info, term);
 
                             // if the test is false, there's no `break` to assign `destination`, so
-                            // we have to do it; this overwrites any `break`-assigned value but it's
-                            // always `()` anyway
-                            this.cfg
-                                .push_assign_unit(exit_block, source_info, destination);
+                            // we have to do it
+                            this.cfg.push_assign_unit(false_block, source_info, destination);
+                            this.cfg.terminate(
+                                false_block,
+                                source_info,
+                                TerminatorKind::Goto { target: exit_block },
+                            );
                         } else {
                             body_block = this.cfg.start_new_block();
                             let diverge_cleanup = this.diverge_cleanup();

src/test/ui/nll/assign-while-to-immutable.rs

+// We used to incorrectly assign to `x` twice when generating MIR for this
+// function, preventing this from compiling.
+
+// check-pass
+
+fn main() {
+    let x = while false {
+        break;
+    };
+    let y = 'l: while break 'l {};
+}