silently fails to format match expression if arm pat contains comment

For Tock, we run rustfmt with Travis CI, and some PRs (example1 and example2) are passing a rustfmt check while still using tabs for indentation.

• No config options (we use defaults only).
• Noticed this on nightly-2020-03-06 and nightly-2020-04-10.

I've also tried manually running rustfmt in the relevant crate on my computer and see the same effect. Based on a question on discord, I am opening this issue.

One example (src):

impl Driver for L3gd20Spi<'a> {
    fn command(&self, command_num: usize, data1: usize, data2: usize, _: AppId) -> ReturnCode {
        match command_num {
            0 /* check if present */ => ReturnCode::SUCCESS,
            // Check is sensor is correctly connected
            1 => {
				if self.status.get () == L3gd20Status::Idle {
					self.is_present ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}

			}
			// Power On
            2 => {
				if self.status.get () == L3gd20Status::Idle {
					self.power_on ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
			}
			// Set Scale
            3 => {
				if self.status.get () == L3gd20Status::Idle {
					let scale = data1 as u8;
					self.set_scale (scale);
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
			// Enable High Pass Filter
            4 => {
				if self.status.get () == L3gd20Status::Idle {
					let mode = data1 as u8;
					let divider = data2 as u8;
					self.set_hpf_parameters (mode, divider);
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
			}
			// Set High Pass Filter Mode and Divider
            5 => {
				if self.status.get () == L3gd20Status::Idle {
					let enabled = if data1 == 1 { true } else { false };
					self.enable_hpf (enabled);
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
			// Read XYZ
            6 => {
				if self.status.get () == L3gd20Status::Idle {
					self.read_xyz ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
			}
			// Read Temperature
            7 => {
				if self.status.get () == L3gd20Status::Idle {
					self.read_temperature ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
            // default
            _ => ReturnCode::ENOSUPPORT,
        }
    }

    fn subscribe(
        &self,
        subscribe_num: usize,
        callback: Option<Callback>,
        _app_id: AppId,
    ) -> ReturnCode {
        match subscribe_num {
            0 /* set the one shot callback */ => {
				self.callback.insert (callback);
				ReturnCode::SUCCESS
			},
            // default
            _ => ReturnCode::ENOSUPPORT,
        }
    }
}

Another example (src):

impl Driver for Lsm303dlhcI2C<'a> {
    fn command(&self, command_num: usize, data1: usize, data2: usize, _: AppId) -> ReturnCode {
        match command_num {
            0 /* check if present */ => ReturnCode::SUCCESS,
            // Check is sensor is correctly connected
            1 => {
				if self.state.get () == State::Idle {
					self.is_present ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}

			}
			// Set Accelerometer Power Mode
            2 => {
				if self.state.get () == State::Idle {
                    if let Some (data_rate) = Lsm303dlhcAccelDataRate::from_usize (data1) {
                        self.set_power_mode(data_rate, if data2 != 0 { true } else { false });
                        ReturnCode::SUCCESS
                    }
                    else
                    {
                        ReturnCode::EINVAL
                    }
				}
				else
				{
					ReturnCode::EBUSY
				}
			}
			// Set Accelerometer Scale And Resolution
            3 => {
				if self.state.get () == State::Idle {
					if let Some (scale) = Lsm303dlhcScale::from_usize(data1) {
                        self.set_scale_and_resolution(scale, if data2 != 0 { true } else { false });
                        ReturnCode::SUCCESS
                    }
                    else
                    {
                        ReturnCode::EINVAL
                    }
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
            // Set Magnetometer Temperature Enable and Data Rate 
            4 => {
				if self.state.get () == State::Idle {
                    if let Some (data_rate) = Lsm303dlhcMagnetoDataRate::from_usize (data2) {
                        self.set_temperature_and_magneto_data_rate (if data1 != 0 { true } else { false }, data_rate);
                        ReturnCode::SUCCESS
                    }
                    else
                    {
                        ReturnCode::EINVAL
                    }
				}
				else
				{
					ReturnCode::EBUSY
				}
			}
			// Set Magnetometer Range
            5 => {
				if self.state.get () == State::Idle {
					if let Some (range) = Lsm303dlhcRange::from_usize(data1) {
                        self.set_range(range);
                        ReturnCode::SUCCESS
                    }
                    else
                    {
                        ReturnCode::EINVAL
                    }
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
			// Read Acceleration XYZ
            6 => {
				if self.state.get () == State::Idle {
					self.read_acceleration_xyz ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
            // Read Temperature
            7 => {
				if self.state.get () == State::Idle {
					self.read_temperature ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
			}
			8 => {
				if self.state.get () == State::Idle {
					self.read_magnetometer_xyz ();
					ReturnCode::SUCCESS
				}
				else
				{
					ReturnCode::EBUSY
				}
            }
            // default
            _ => ReturnCode::ENOSUPPORT,
        }
    }

    fn subscribe(
        &self,
        subscribe_num: usize,
        callback: Option<Callback>,
        _app_id: AppId,
    ) -> ReturnCode {
        match subscribe_num {
            0 /* set the one shot callback */ => {
				self.callback.insert (callback);
				ReturnCode::SUCCESS
			},
            // default
            _ => ReturnCode::ENOSUPPORT,
        }
    }
}

Start by reproducing the failure with the Rust match-expression examples from the issue, including comments in arm patterns, using rustfmt's default configuration and the referenced nightly versions. Trace the formatting path for match arms and verify that the affected input is reformatted and no longer silently leaves tab indentation.