Constraining generic parameter of Rust struct to be Deserialize

Question

I would like to limit a (de)serializable struct to have a generic parameter that is also deserializable. The derive macro Deserialize does not require me to add this constraint, which is nice, but I would like the integration code to get compilation errors even if they never try to deserialize the struct defined in the library.

use failure::Fallible; // 0.1.6 use serde::{Deserialize, Serialize}; // 1.0.104 use serde_json::to_string_pretty; // 1.0.44 #[derive(Deserialize, Serialize)] struct X<T> where // T: Serialize, { a: u8, t: T, } type Main<'a> = &'a dyn Fn() -> Fallible<()>; fn main() -> Fallible<()> { let x = X { a: 1, t: false }; println!("{}", to_string_pretty(&x)?); let y: X<bool> = serde_json::from_str(r#"{"a":2,"t":true}"#)?; println!("{}", y.t); let _z: X<Main> = X { a: 3, t: &main }; // println!("{}", to_string_pretty(&z)?); //let w: X<Main> = serde_json::from_str(r#"{"a":4,"t":NONONO}"#)?; Ok(()) }

playground

The compilation fails if I uncomment the line with to_string_pretty(&z), which is good.
Even without that line, the compilation fails on the let _z = ... line if I uncomment where T: Serialize. This is great, because it helps the library integrators to spot they are using struct X with a non-serializable type parameter even before they actually start serializing it.

I have tried to add where for<'a> T: serde::de::Deserialize<'a> as a constraint to struct X<T>, but that breaks the build even without anything using X

use serde::{Deserialize, Serialize}; #[derive(Deserialize, Serialize)] struct X<T> where for<'a> T: serde::de::Deserialize<'a>, { a: u8, t: T, }

playground

Is there a way to phrase this constraint I am looking for?

Can you post the exact code that you want to work, and add the error message you are getting? I'm not sure which lines you want to uncomment. — trent
– trent, Commented Jan 10, 2020 at 18:59
Okay. Tried to clarify the non-working attempt to the library code. — wigy
– wigy, Commented Jan 10, 2020 at 19:43
That helps, thanks! Unfortunately I'm still as mystified as you are... — trent
– trent, Commented Jan 10, 2020 at 19:47
The 1st code proves that you can instantiate X<Main> without Main being Deserialize. No error unless you actually deserialize X<Main> — wigy
– wigy, Commented Jan 10, 2020 at 20:26

Shepmaster · Accepted Answer · 2020-01-10 20:52:22Z

You need to use #[serde(bound)] to prevent Serde from attempting to automatically determine the bounds on the Deserialize and Serialize implementations:

use serde::{Deserialize, Serialize}; #[derive(Deserialize, Serialize)] #[serde(bound = "T: Serialize, for<'de2> T: Deserialize<'de2>")] struct X<T> where T: Serialize, for<'de2> T: Deserialize<'de2>, { t: T, } struct NotSerializable; fn main() { X { t: true }; // X { t: NotSerializable }; // Generates compiler error }

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