Rustfmt: Assignment operator RHS indentation

Summary

In the 2024 Edition, rustfmt now indents the right-hand side of an assignment operator relative to the last line of the left-hand side, providing a clearer delineation and making it easier to notice the assignment operator.

Details

In Rust 2021 and before, if an assignment operator has a multi-line left-hand side, the indentation of the right-hand side will visually run together with the left-hand side:

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info =
            long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;

        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info = Some(ExtraInfo {
            parent,
            count: count as u16,
            children: children.into_boxed_slice(),
        });
    }
}

In the 2024 Edition, rustfmt now indents the right-hand side relative to the last line of the left-hand side:

impl SomeType {
    fn method(&mut self) {
        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info =
                long_long_long_long_long_long_long_long_long_long_long_long_long_long_long;

        self.array[array_index as usize]
            .as_mut()
            .expect("thing must exist")
            .extra_info = Some(ExtraInfo {
                parent,
                count: count as u16,
                children: children.into_boxed_slice(),
            });
    }
}

Migration

The change can be applied automatically by running cargo fmt or rustfmt with the 2024 Edition. See the Style edition chapter for more information on migrating and how style editions work.