Refactor available when new rust features arrive

[Austaras]

Currently I've found 2:

1. if let chain, this is imminent
2. deref pattern, this would bring fundamental change for swc development experience, however little progress has been seen.

[kdy1]

Wow, I really need both of them

[Austaras]

if let chain has been stabilized.

[kdy1]

Wow, great!

[kdy1]

MSRV is now the current stable rust, so we can use it.
#5856

[Austaras]

No, it has been reverted...