Struct MetricRole 

pub struct MetricRole {
    pub name: &'static str,
    pub node: &'static str,
    pub doc: &'static str,
    pub get: fn(&Metrics) -> f32,
    pub set: fn(&mut Metrics, f32),
    pub platform: Option<PlatformValue>,
}

One size the stock widgets are built from (crate::metrics::Metrics), pinned the way ThemeRole pins a colour: a binding iterates the rows to read or write a set, so a field added to Metrics is one row here and nothing anywhere else. The test below destructures the struct exhaustively.

Fields

name: &'static str

The Rust field, and the name used everywhere but Node.

node: &'static str

What Node calls it (camelCase).

doc: &'static str

get: fn(&Metrics) -> f32

set: fn(&mut Metrics, f32)

platform: Option<PlatformValue>

Some for a row whose stock value is the platform’s own rather than a density’s: the value on Windows, then everywhere else. get answers for the running platform; a generator prints the pair, so docs/props.md reads the same whichever machine wrote it. Stock and compact share it — the test below pins that compact() leaves such a row alone.