Having tinkered around with this a bit, I can't actually get monitoring to work, though.

I have made this little example:

use std::str::FromStr;
use color_eyre::Result;

#[tokio::main]
async fn main() -> Result<()> {    
    let bluetooth = bluer::Session::new().await?;
    let mut adapter = bluetooth.default_adapter().await?;
    adapter.set_powered(true).await?;

    let mut monitor = adapter.monitor().await?.register(
        bluer::monitor::Monitor::default()
    ).await?;

    Ok(())
}

Running this code results in

Error: advertisement monitor could not be activated

Location:
    src/main.rs:10:23

Is this something I should create an issue about?

Initially, I ran into these UnknownMethod errors, but I got rid of those by upgrading my Raspberry Pi OS to Bookworm and building and installing bluez from source with experimental features enabled. I now have bluetoothctl: 5.76, but I am left with the problem mentioned above. Could this be related to #110?

I believe using this bluer::monitor::Monitor::default() for my monitor is what prevented me from activating the monitor.

Specifically, the patterns field of a monitor::Monitor is listed as being

Required if monitor_type is Type::OrPatterns.

Naturally, this is the case, since Type::OrPatterns is the only available monitor_type.

Ideally, I think this requirement should be encoded in the type system somehow. Perhaps patterns could be a mandatory member of Type::OrPatterns, instead of an optional member of monitor::Monitor.
Alternatively, this requirement could at least be clearly stated in an error message.

Would a pull request for one of these options be appreciated?

Now, with the above out of the way, I still have no clue how to actually use these patterns. I think
this https://docs.rs/bluer/latest/bluer/monitor/struct.Pattern.html#structfield.data_type and this https://docs.rs/bluer/latest/bluer/monitor/data_type/index.html assumes some knowledge about the patterns that I don't have. Is there a more beginner friendly description of the patterns? I only found this https://man.archlinux.org/man/org.bluez.AdvertisementMonitor.5.en, which is more or less the same information.

Currently, my best bet is this description generated by ChatGPT, but I don't think that sounds quite right:

Pattern Structure:
Each pattern consists of a tuple with three elements: (uint8, uint8, array{byte}).
The three elements are as follows:
Pattern Type (uint8): Specifies the type of pattern. It can be one of the following:
0: Match the entire advertisement data.
1: Match the manufacturer data.
2: Match the service UUIDs.
3: Match the service data.
Pattern Flags (uint8): Indicates the flags for the pattern. These flags control how the pattern is applied:
0: No flags (exact match).
1: Use a bitmask for matching.
2: Use a bitmask for matching and apply it to the data.
Pattern Data (array{byte}): Contains the actual pattern data to match against.
Examples:
Suppose we want to monitor advertisements from devices with a specific manufacturer ID (e.g., Apple). We would create a pattern like this:
(1, 0, [0x4C, 0x00]) (where 0x4C corresponds to Apple’s manufacturer ID).
To monitor specific service UUIDs (e.g., Bluetooth Low Energy heart rate service), we’d use:
(2, 0, [0x18, 0x0D]) (where 0x18 0x0D represents the heart rate service UUID).
For service data matching (e.g., temperature data from a specific sensor):
(3, 0, [0x18, 0x1C, 0x02, 0x0A, 0x00]) (where 0x18 0x1C is the service UUID, and 0x02 0x0A 0x00 is the specific service data).

Naturally, this is the case, since Type::OrPatterns is the only available monitor_type.

Ideally, I think this requirement should be encoded in the type system somehow. Perhaps patterns could be a mandatory member of Type::OrPatterns, instead of an optional member of monitor::Monitor. Alternatively, this requirement could at least be clearly stated in an error message.

Would a pull request for one of these options be appreciated?

Yes, making the patterns a part of Type::OrPatterns would make sense and a PR would be welcome.

Now, with the above out of the way, I still have no clue how to actually use these patterns. I think this https://docs.rs/bluer/latest/bluer/monitor/struct.Pattern.html#structfield.data_type and this https://docs.rs/bluer/latest/bluer/monitor/data_type/index.html assumes some knowledge about the patterns that I don't have. Is there a more beginner friendly description of the patterns? I only found this https://man.archlinux.org/man/org.bluez.AdvertisementMonitor.5.en, which is more or less the same information.

I think you set Pattern::data_type to the data type you want to match and Pattern::content to the byte values that should be matched. This is done by the controller firmware, so it is low level.