`cfg_select!` in expr position broken after formatting

Summary

I tried to format this code:

fn main() {
    let a = cfg_select! {
        feature = "foo" => {
            {
                let bar = "foo";
                bar.to_owned()
            }
        }
        _ => String::new(),
    };

    println!("{a}");
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=faacc7d972e6b5ab29d965d7df374940

Expected behavior

I expected to see this happen:

Either:

• nothing
• reformat happens without affecting the number of braces

Actual behavior

Instead, this happened: rustfmt removed the braces, leading to incorrect syntax:

fn main() {
    let a = cfg_select! {
        feature = "foo" => {
            let bar = "foo";
            bar.to_owned()
        }
        _ => String::new(),
    };

    println!("{a}");
}

It's now effectively generating let a = let bar = "foo"; ...:

error: expected expression, found `let` statement
 --> src/main.rs:4:13
  |
4 |             let bar = "foo";
  |             ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error: macro expansion ignores `bar` and any tokens following
 --> src/main.rs:5:13
  |
2 |       let a = cfg_select! {
  |  _____________-
3 | |         feature = "foo" => {
4 | |             let bar = "foo";
5 | |             bar.to_owned()
  | |             ^^^
6 | |         }
7 | |         _ => String::new(),
8 | |     };
  | |_____- caused by the macro expansion here
  |
  = note: the usage of `cfg_select!` is likely invalid in expression context

error: trailing semicolon in macro used in expression position
 --> src/main.rs:4:28
  |
4 |             let bar = "foo";
  |                            ^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
  = note: `#[deny(semicolon_in_expressions_from_macros)]` (part of `#[deny(future_incompatible)]`) on by default

Configuration

Default configuration

Meta

Checked both locally and on rust playground (same build):

1.100.0-nightly
(2026-08-21 c656540d6467dee1381f)

Start by reproducing the provided Rust example with rustfmt using the linked Rust Playground configuration, then inspect how cfg_select! branches are formatted in expression position. Done means formatting no longer removes braces in a way that changes the generated syntax, while the reproducer remains valid.