Better error if the user tries to do assignment ... else

compiler/rustc_parse/src/parser/stmt.rs

@@ -103,6 +103,16 @@ impl<'a> Parser<'a> {
             } else {
                 self.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs))
             }?;
+            if matches!(e.kind, ExprKind::Assign(..)) && self.eat_keyword(kw::Else) {
+                let bl = self.parse_block()?;
+                // Destructuring assignment ... else.
+                // This is not allowed, but point it out in a nice way.
+                let mut err = self.struct_span_err(
+                    e.span.to(bl.span),
+                    "<assignment> ... else { ... } is not allowed",
+                );
+                err.emit();
+            }
             self.mk_stmt(lo.to(e.span), StmtKind::Expr(e))
         } else {
             self.error_outer_attrs(&attrs.take_for_recovery());

src/test/ui/let-else/let-else-destructuring.rs

@@ -0,0 +1,18 @@
+#![feature(let_else)]
+#[derive(Debug)]
+enum Foo {
+    Done,
+    Nested(Option<&'static Foo>),
+}
+
+fn walk(mut value: &Foo) {
+    loop {
+        println!("{:?}", value);
+        &Foo::Nested(Some(value)) = value else { break }; //~ ERROR invalid left-hand side of assignment
+        //~^ERROR <assignment> ... else { ... } is not allowed
+    }
+}
+
+fn main() {
+    walk(&Foo::Done);
+}

src/test/ui/let-else/let-else-destructuring.stderr

@@ -0,0 +1,17 @@
+error: <assignment> ... else { ... } is not allowed
+  --> $DIR/let-else-destructuring.rs:11:9
+   |
+LL |         &Foo::Nested(Some(value)) = value else { break };
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0070]: invalid left-hand side of assignment
+  --> $DIR/let-else-destructuring.rs:11:35
+   |
+LL |         &Foo::Nested(Some(value)) = value else { break };
+   |         ------------------------- ^
+   |         |
+   |         cannot assign to this expression
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0070`.