Merge #11598

11598: feat: Parse destructuring assignment r=Veykril a=ChayimFriedman2

Part of #11532.

Lowering is not as easy and may not even be feasible right now as it requires generating identifiers: `(a, b) = (b, a)` is desugared into
```rust
{
    let (<gensym_a>, <gensym_b>) = (b, a);
    a = <gensym_a>;
    b = <gensym_b>;
}
```

rustc uses hygiene to implement that, but we don't support hygiene yet.

However, I think parsing was the main problem as lowering will just affect type inference, and while `{unknown}` is not nice it's much better than a syntax error.

I'm still looking for the best way to do lowering, though.

Fixes #11454.

Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>

crates/ide_db/src/helpers.rs

@@ -187,6 +187,7 @@ pub fn for_each_tail_expr(expr: &ast::Expr, cb: &mut dyn FnMut(&ast::Expr)) {
         | ast::Expr::TupleExpr(_)
         | ast::Expr::WhileExpr(_)
         | ast::Expr::LetExpr(_)
+        | ast::Expr::UnderscoreExpr(_)
         | ast::Expr::YieldExpr(_) => cb(expr),
     }
 }