String interpolation with a specifier or formatter?

I know we can use string interpolation to format a number:

You know your code causes error under the normal context:

Code Block 
let pi = Double.pi
let str = "\(pi, specifier: "%.2f")" //->Extra argument 'specifier' in call

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specifier: or other extra arguments are based on this evolution proposal.

Fix ExpressibleByStringInterpolation

When there is a type (not String) which conforms to ExpressibleByStringInterpolation and its associated StringInterpolation has a method appendInterpolation(_:specifier:),
you can use String Interpolation literals like "\(pi, specifier: "%.2f")".

The type LocalizedStringKey is the one that matches this condition. (Which is intended to use with Text of SwiftUI.)

Code Block 
import SwiftUI
let pi = Double.pi
let str: LocalizedStringKey = "\(pi, specifier: "%.2f")"

You may need to check the doc of LocalizedStringKey.StringInterpolation, to find what sort of extra arguments you can use.

If you want to use another type conforming to ExpressibleByStringInterpolation, you need to search another place.