Invalid syntax suggested when trying to use `serde::de::Deserializer<'de, Error = E>`

[schneems]

Code

Example repo that demonstrates the problem https://github.com/schneems/serde-deserializer-wrapper-demo

Current output

error[E0308]: mismatched types
   --> src/main.rs:361:33
    |
46  | impl<'de, T, E> serde::de::Deserializer<'de> for ResultDeWrapper<'de, T, E>
    |              - expected this type parameter
    |
356 |     fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    |                                                        ----------------------------- expected `Result<<V as Visitor<'de>>::Value, E>` because of return type
    |
361 |             Ok(deserializer) => deserializer.deserialize_ignored_any(visitor),
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Result<<V as Visitor<'_>>::Value, E>`, found `Result<<V as Visitor<'_>>::Value, ...>`

    = note: expected enum `Result<_, E>`
               found enum `Result<_, <T as _serde::Deserializer<'de>>::Error>`
help: consider further restricting this bound
    |
48  |     T: serde::de::Deserializer<'de><Error = E>,
    |                                    +++++++++++

Desired output

error[E0308]: mismatched types
   --> src/main.rs:361:33
    |
46  | impl<'de, T, E> serde::de::Deserializer<'de> for ResultDeWrapper<'de, T, E>
    |              - expected this type parameter
    |
356 |     fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
    |                                                        ----------------------------- expected `Result<<V as Visitor<'de>>::Value, E>` because of return type
    |
361 |             Ok(deserializer) => deserializer.deserialize_ignored_any(visitor),
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Result<<V as Visitor<'_>>::Value, E>`, found `Result<<V as Visitor<'_>>::Value, ...>`

    = note: expected enum `Result<_, E>`
               found enum `Result<_, <T as _serde::Deserializer<'de>>::Error>`
help: consider further restricting this bound
    |
48  |     T: serde::de::Deserializer<'de, Error = E>,
    |                                    +++++++++++

Rationale and extra context

The suggested change is a syntax error. The goal of the bounds is to convince Rust that the E is the same type as the T impl serde::de::Deserializer::Error.

Other cases

Rust Version

$ rustc --version --verbose
rustc 1.89.0 (29483883e 2025-08-04)
binary: rustc
commit-hash: 29483883eed69d5fb4db01964cdf2af4d86e9cb2
commit-date: 2025-08-04
host: aarch64-apple-darwin
release: 1.89.0
LLVM version: 20.1.7

Anything else?

No response