rust packaging with specific version of rust

[Hamzeh Nasajpour]

Hi,

I'm not expert in `rust`. I need to work with a library that has some 
dependencies to rust and I had to package a few libraries in GuixSD. Anyway now 
I faced with an error in building one rust package:
```
error[E0658]: use of unstable library feature 'inner_deref': newly added
  --> src/http_client/mod.rs:54:25
   |
54 |         self.auth_token.as_deref()
   |                         ^^^^^^^^
   |
   = note: for more information, see 
https://github.com/rust-lang/rust/issues/50264

error[E0658]: use of unstable library feature 'inner_deref': newly added
  --> src/http_client/mod.rs:68:43
   |
68 |         self.imp.get(url, self.auth_token.as_deref()).as_result()
   |                                           ^^^^^^^^
   |
   = note: for more information, see 
https://github.com/rust-lang/rust/issues/50264

error[E0658]: use of unstable library feature 'inner_deref': newly added
  --> src/http_client/mod.rs:72:44
   |
72 |         self.imp.post(url, self.auth_token.as_deref(), body).as_result()
   |                                            ^^^^^^^^
   |
   = note: for more information, see 
https://github.com/rust-lang/rust/issues/50264
...
```

Based on the following document the  `as_deref()` method is available since 
rust version `1.40.0`.
https://doc.rust-lang.org/std/option/enum.Option.html#method.as_deref

Now how can I build/install my rust package with this specific version? is it 
possible? or is there anything that I forget? Please help. 


Regards,
Hamzeh