add #[allow(deprecated)] to derive implementations

[rcrisanti]

Allow deprecated in the Serialize/Deserialize derive implementations. This allows you to deprecate structs, enums, struct fields, or enum variants and not get compiler warnings/errors about use of deprecated thing.

Resolves #2195

[KarstenB]

I just encountered the same issue and was wondering how that could be implemented. While I am perfectly fine with the given PR, I thought about a more generic way. Like:

#[derive(Serialize, Deserialize)]
#[deprecated]
#[serde(allow = deprecated)]
struct bla {}

[rcrisanti]

@KarstenB that is definitely doable if that is the desired behavior. Personally I'm not sure I can think of a scenario where I do want to see these messages, so I'd at least be in favor of making the default not showing them, with some sort of option to turn them back on.

(For some justification of this, in rust itself they hide deprecation messages in derives, so in my opinion this would put serde more inline with expected behavior.)

[oli-obk]

(For some justification of this, in rust itself they hide deprecation messages in derives, so in my opinion this would put serde more inline with expected behavior.)

in theory that check should work for serde automatically, but making that happen at the proc macro level causes lots of problems inside other diagnostics: #2847 (comment)

[rcrisanti]

@oli-obk is there anything you're waiting to see from me on this? From my perspective it is ready for review

[oli-obk]

No... this is mostly me not wanting to add workarounds that are necessary because of other workarounds. w

[oli-obk]

Considering the low cost to maintenance this PR adds, gonna merge it as is

[dtolnay]

I don't like this workaround because it silently suppresses legitimate deprecations on parts of the generated impl that come from user-specified attributes. Example:

use serde::Deserializer;
use serde_derive::Deserialize;

#[derive(Deserialize)]
pub struct Struct {
    #[serde(deserialize_with = "deprecated_with")]
    pub field: i32,
}

#[deprecated]
fn deprecated_with<'de, D>(_deserializer: D) -> Result<i32, D::Error>
where
    D: Deserializer<'de>,
{
    unimplemented!()
}

warning: use of deprecated function `deprecated_with`
 --> src/main.rs:6:32
  |
6 |     #[serde(deserialize_with = "deprecated_with")]
  |                                ^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(deprecated)]` on by default

Please put allow(deprecated) if and only if the input data structure has either #[deprecated] or #[allow(deprecated)].

[rcrisanti]

I finally got some time to look into this and made some adjustments based on @dtolnay's suggestions. Now we only add the #[allow(deprecated)] if the struct/enum has #[allow(deprecated)] or #[deprecated] or if one of its variants do (if it is an enum). Let me know if there's anything else you'd want to see on this!