unused_braces lint in Deserialize macro with braced GAT type conversion

[teor2345]

When I derive Deserialize on a struct containing a generic associated type with a braced type conversion, I get an unused_braces warning.

This started happening in my CI when I upgraded to serde 1.0.158 with Rust 1.68. I can reproduce it locally with older serde versions, but I can't reproduce it in a playground with older versions. So I might be missing a reproduction step.

Here is the code:

#[derive(Deserialize)]
struct FooDeserializable {
    item: Generic<{ BAR as u8 }>,
}

And the warning I get:

    warning: unnecessary braces around block return value
      --> src/main.rs:16:19
       |
    16 |     item: Generic<{ BAR as u8 }>,
       |                   ^^         ^^
       |
       = note: `#[warn(unused_braces)]` on by default
    help: remove these braces
       |
    16 -     item: Generic<{ BAR as u8 }>,
    16 +     item: Generic<BAR as u8>,
       |

The fix itself is wrong, so I'll also report a Rust bug.

Here's the full (local) reproduction code, but the playground itself doesn't show the warning:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=98ec53a2a8de5c14af46d88f45feda1c

[teor2345]

Here's the corresponding Rust bug for the incorrect fix suggestion:
rust-lang/rust#109473

[dtolnay]

Fixed in serde_derive 1.0.169.