Fix unused_results complaining rustc lint in codegen for adjacently tagged enum

[tyranron]

MRE

#![allow(dead_code)]
#![deny(unused_results)]

use serde::Deserialize; // =1.0.130

#[derive(Deserialize)]
#[serde(tag = "msg", content = "data")]
enum ServerMsg {
    Ping(u32),
    Event {
        room_id: u32,
    },
}

Playground

[tyranron]

I'd like also to add that MRE as a test case, but couldn't figure out where is the right place for it in tests suite.

[dtolnay]

Could you help me understand this PR? The code that is being emitted inside this match arm is:

=> {
    match serde::de::MapAccess::next_value::<serde::de::IgnoredAny>(&mut __map) {
        serde::__private::Ok(__val) => __val,
        serde::__private::Err(__err) => {
            return serde::__private::Err(__err);
        }
    };
    continue;
}

The signature of next_value is: https://docs.rs/serde/1.0.130/serde/de/trait.MapAccess.html#method.next_value. So next_value::<IgnoredAny>() is of type Result<IgnoredAny, E>, and __val (and therefore that whole match expression) is of type IgnoredAny. Where is there an unused result? Isn't this a rustc bug, not a serde_derive bug?

[dtolnay]

Ah it's not saying there is an unused Result — that would be unused_must_use, not unused_result. 😄 It's saying there is a value of some expression, in this case of type IgnoredAny, which is unused.

Thanks, this looks good!

[dtolnay]

I pushed fb2fe40 with an alternative fix because this one solves the rustc lint but causes a clippy lint.

error: calls to `std::mem::drop` with a value that implements `Copy`. Dropping a copy leaves the original intact
   --> test_suite/tests/test_gen.rs:468:25
    |
468 |     #[derive(Serialize, Deserialize)]
    |                         ^^^^^^^^^^^
    |
    = note: `-D clippy::drop-copy` implied by `-D clippy::all`
note: argument has type _::_serde::de::IgnoredAny
   --> test_suite/tests/test_gen.rs:468:25
    |
468 |     #[derive(Serialize, Deserialize)]
    |                         ^^^^^^^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#drop_copy
    = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)