rustfmt is not idempotent over some blocks that consist of a series of empty statements finished by an expression

Describe the bug

rustfmt is not idempotent over some blocks that consist of a series of empty statements finished by an expression. The first pass strips the empty statements and the second pass applies any applicable transformation to the block such as unwrapping the expression in it or rearranging it into a single line.

To Reproduce

Example input:

fn foo(x: Option<i32>) -> i32 {
    let y = {
        ;
        x
    };
    match y {
        Some(y) => {
            ;
            ;
            y
        }
        None => 0,
    }
}

Calling rustfmt once results in

fn foo(x: Option<i32>) -> i32 {
    let y = {
        x
    };
    match y {
        Some(y) => {
            y
        }
        None => 0,
    }
}

Calling rustfmt on the previous output then results in

fn foo(x: Option<i32>) -> i32 {
    let y = { x };
    match y {
        Some(y) => y,
        None => 0,
    }
}

Expected behavior

rustfmt should return the output of the second pass in the first pass.

Meta

• rustfmt version: rustfmt 1.4.36-nightly (7de6968e 2021-02-07)
• From where did you install rustfmt?: rustup
• How do you run rustfmt?: via rust-analyzer in vscode, but this was tested with rustfmt too

Start by running rustfmt twice on the Rust example in the issue and compare the outputs. Trace the block-formatting path responsible for removing empty statements and applying block transformations; done means the second-pass output is produced by the first pass and the behavior is covered by a regression test.