`use_small_heuristics = "Max"` ignores `max_width` for macros

[chrisduerr]

Take the following Rust code:

fn main() {
    println!("Hello, world!");
}

macro_rules! trigger {
    (KeyBinding, $key:literal, $location:expr) => {{ BindingKey::Keycode { key: Key::Character($key.into()), location: $location } }};
    (KeyBinding, $key:literal,) => {{ BindingKey::Keycode { key: Key::Character($key.into()), location: KeyLocation::Any } }};
    (KeyBinding, $key:ident, $location:expr) => {{ BindingKey::Keycode { key: Key::Named(NamedKey::$key), location: $location } }};
    (KeyBinding, $key:ident,) => {{ BindingKey::Keycode { key: Key::Named(NamedKey::$key), location: KeyLocation::Any } }};
    (MouseBinding, $base:ident::$button:ident,) => {{ $base::$button }};
}

enum Key {
    Named(NamedKey),
    Character(char),
}

enum NamedKey {}

enum KeyLocation {
    Any,
}

When formatted with the default formatter, this works properly and fixes the overly long lines in the macro.

However when then switching to the following rustfmt.toml:

use_small_heuristics = "Max"
max_width = 100 # Optional, just for clarity

Suddenly the macro match arms are all reformatted to a single line, even though use_small_heuristics = "Max" is supposed to max out at max_width and not just allow infinitely long lines.

I've had this break one of my macros during the 2024 edition migration, so I believe there might be a regression somewhere?

[ytmimi]

Confirming that I can reproduce this with rustfmt 1.8.0-nightly (fd0ea74 2025-04-12).

required configs to trigger the issue:

use_small_heuristics = "Max"
style_edition = 2024

Style Edition 2024 formatting:

macro_rules! trigger {
    (KeyBinding, $key:literal, $location:expr) => {{ BindingKey::Keycode { key: Key::Character($key.into()), location: $location } }};
    (KeyBinding, $key:literal,) => {{ BindingKey::Keycode { key: Key::Character($key.into()), location: KeyLocation::Any } }};
    (KeyBinding, $key:ident, $location:expr) => {{ BindingKey::Keycode { key: Key::Named(NamedKey::$key), location: $location } }};
    (KeyBinding, $key:ident,) => {{ BindingKey::Keycode { key: Key::Named(NamedKey::$key), location: KeyLocation::Any } }};
    (MouseBinding, $base:ident::$button:ident,) => {{ $base::$button }};
}

Style Edition 2015, 2018, 2021 formatting:

macro_rules! trigger {
    (KeyBinding, $key:literal, $location:expr) => {{
        BindingKey::Keycode { key: Key::Character($key.into()), location: $location }
    }};
    (KeyBinding, $key:literal,) => {{
        BindingKey::Keycode { key: Key::Character($key.into()), location: KeyLocation::Any }
    }};
    (KeyBinding, $key:ident, $location:expr) => {{
        BindingKey::Keycode { key: Key::Named(NamedKey::$key), location: $location }
    }};
    (KeyBinding, $key:ident,) => {{
        BindingKey::Keycode { key: Key::Named(NamedKey::$key), location: KeyLocation::Any }
    }};
    (MouseBinding, $base:ident::$button:ident,) => {{
        $base::$button
    }};
}