Like python, and Rust with the format! macro (which doesn't even support arbitrary expressions), C# the full syntax for interpolated/formatted strings is this:

Like python, and Rust with the format! macro (which doesn't even support arbitrary expressions), C# the full syntax for interpolated/formatted strings is this: {<interpolationExpression>[,<alignment>][:<formatString>]}, ie there is more going on then just a simple wrapper around concat or StringBuilder.