match_single_binding: suggests broken code (missing curly braces)

[matthiaskrgr]

warning: this match could be written as a `let` statement
    --> src/librustc_builtin_macros/deriving/generic/mod.rs:1059:34
     |
1059 |                           .map(|l| match l.next().unwrap() {
     |  __________________________________^
1060 | |                             (.., ex, _) => ex,
1061 | |                         })
     | |_________________________^
     |
     = note: `#[warn(clippy::match_single_binding)]` on by default
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_single_binding
help: consider using `let` statement
     |
1059 |                         .map(|l| let (.., ex, _) = l.next().unwrap();
1060 |                         ex)
     |

The suggested code does not compile,

error: expected one of `)`, `,`, `.`, `?`, or an operator, found `;`
    --> src/librustc_builtin_macros/deriving/generic/mod.rs:1060:64
     |
1060 | ...                   let (.., ex, _) = l.next().unwrap();
     |                                                          ^
     |                                                          |
     |                                                          expected one of `)`, `,`, `.`, `?`, or an operator
     |                                                          help: missing `,`

error[E0425]: cannot find value `ex` in this scope
    --> src/librustc_builtin_macros/deriving/generic/mod.rs:1061:29
     |
1061 | ...                   ex
     |                       ^^ help: a local variable with a similar name exists: `cx`

error[E0658]: `let` expressions in this position are experimental
    --> src/librustc_builtin_macros/deriving/generic/mod.rs:1060:29
     |
1060 | ...                   let (.., ex, _) = l.next().unwrap();
     |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #53667 <https:/rust-lang/rust/issues/53667> for more information
     = help: add `#![feature(let_chains)]` to the crate attributes to enable

we need curly braces around the let + return statement like this:

                        .map(|l| {
                            let (.., ex, _) = l.next().unwrap();
                            ex
                        })

[ThibsG]

Thanks for reporting this, I will work on a fix