"undeclared lifetime" when derive Deserialize for an enum with both flatten and 'static fields

[Mingun]

The following code panics with the error:

use serde::Deserialize;

#[derive(Deserialize)]
struct Nested {
    float: f64,
}

#[derive(Deserialize)]
#[serde(tag = "tag")]
enum InternallyTagged {
    Flatten {
        #[serde(flatten)]
        nested: Nested,
        string: &'static str,
    },
}

Commenting out any of fields lead to successful compilation.

Output:

Compiling playground v0.0.1 (/playground)
error[E0261]: use of undeclared lifetime name `'de`
 --> src/lib.rs:8:10
  |
8 | #[derive(Deserialize)]
  |          ^^^^^^^^^^^
  |          |
  |          undeclared lifetime
  |          lifetime `'de` is missing in item created through this procedural macro
  |
  = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider making the bound lifetime-generic with a new `'de` lifetime
  |
8 | #[derive(for<'de> Deserialize)]
  |          ++++++++
help: consider making the bound lifetime-generic with a new `'de` lifetime
  |
8 | #[derive(for<'de> Deserialize)]
  |          ++++++++
help: consider introducing lifetime `'de` here
  |
8 | #[derive(Deserialize<'de>)]
  |                     +++++

For more information about this error, try `rustc --explain E0261`.
error: could not compile `playground` due to previous error

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b3e592812a43c300e2d247c9ac0b0033

[Teajey]

Since &'static str usually represents a string embedded in the binary of your program I don't think it's possible to or even makes sense to Deserialize one? Surely what you want to do is this:

#[derive(Deserialize)]
#[serde(tag = "tag")]
enum InternallyTagged<'a> {
    Flatten {
        #[serde(flatten)]
        nested: Nested,
        string: &'a str,
    },
}

[Mingun]

Of course, this will not work, but serde in that case usually returns an error from Deserialize::deserialize phase instead of incorrect code generation. Moreover, as I said, if you comment nested field, this code is compiled.

So I think it should either:

• always generate compiler error when you try to implement Deserialize with 'static fields
• or never generate such an error

[Mingun]

Of course, this will not work

Actually, I'm not right here. Of course, this will work if you try to deserialize a static string into a type (i.e. you just create a read-only struct around read-only string embedded in binary). This will not work if you try to deserialize heap-allocated string.

[Mingun]

The problem in the DeserializeSeed implementation for the visitor for the deserialized type. The derive generates the following code:

impl DeserializeSeed<'static> for Visitor {
    type Value = InternallyTagged;

    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    where
        D: Deserializer<'de>,// <-- Error here
    {
        deserializer.deserialize_map(self)
    }
}

The cleaned generated code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6c5e32bd6cef06756b0806be7447f827