Better error if a struct defines its own 'de lifetime

[dtolnay]

#[macro_use]
extern crate serde_derive;

#[derive(Deserialize)]
struct S<'de, T: Deserialize<'de>> {
    t: T,
}

error[E0263]: lifetime name `'de` declared twice in the same scope
 --> src/main.rs:6:10
  |
4 | #[derive(Deserialize)]
  |          ^^^^^^^^^^^
  |          |
  |          previous declaration here
  |          declared twice

I would explicitly prefer not to support this use case (see https://github.com/Manishearth/rust-clippy/issues/1689). But it should fail with a helpful message rather than the current behavior of generating broken code.

[dtolnay]

Fixed in 7e3efaf.