Fix applicability level for cfg suggestion

[chenyukang]

Mark the semicolon, block-wrapping, and if cfg! suggestions for attribute-bearing expressions as MaybeIncorrect.

For example:

fn native_file_attributes(metadata: &std::fs::Metadata) -> u32 {
    #[cfg(unix)]
    std::os::unix::fs::MetadataExt::mode(metadata)
    #[cfg(windows)]
    std::os::windows::fs::MetadataExt::file_attributes(metadata)
}

fn main() -> std::io::Result<()> {
    let metadata = std::fs::metadata(std::env::current_exe()?)?;
    println!(
        "native file attributes: {:#x}",
        native_file_attributes(&metadata)
    );
    Ok(())
}

Adding ; discards the return value on Unix.

Replacing #[cfg] with if cfg! also fails because disabled branches still undergo name resolution and type checking.

Wrapping the expression in a block works here, but still can fail when multiple cfg branches are enabled and the first block returns a non-unit value.

For example:

fn value() -> u8 {
    #[cfg(all())]
    1

    #[cfg(all())]
    2
}

r? @estebank

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[estebank]

@bors r+

[rust-bors]

📌 Commit 2ed1e37 has been approved by estebank

It is now in the queue for this repository.