initialize the destination in unit statements

Fixes #46159.

src/librustc_mir/build/expr/into.rs

@@ -273,7 +273,9 @@ pub fn into_expr(&mut self,
             ExprKind::Break { .. } |
             ExprKind::InlineAsm { .. } |
             ExprKind::Return {.. } => {
-                this.stmt_expr(block, expr)
+                unpack!(block = this.stmt_expr(block, expr));
+                this.cfg.push_assign_unit(block, source_info, destination);
+                block.unit()
             }
 
             // these are the cases that are more naturally handled by some other mode

src/test/mir-opt/validate_4.rs

@@ -54,6 +54,7 @@ fn main() {
 //         Validate(Acquire, [_1: &ReFree(DefId(0/1:9 ~ validate_4[317d]::write_42[0]::{{closure}}[0]), BrEnv) [closure@NodeId(22)], _2: *mut i32]);
 //         Validate(Release, [_1: &ReFree(DefId(0/1:9 ~ validate_4[317d]::write_42[0]::{{closure}}[0]), BrEnv) [closure@NodeId(22)], _2: *mut i32]);
 //         (*_2) = const 23i32;
+//         _0 = ();
 //         return;
 //     }
 // }

src/test/run-pass/weird-exprs.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-flags: -Z borrowck=compare
 
 use std::cell::Cell;
 use std::mem::swap;