unnecessary_string_interpolations

Unnecessary use of string interpolation.

The analyzer produces this diagnostic when a string literal contains a single interpolation of a String-valued variable and no other characters.

The following code produces this diagnostic because the string literal contains a single interpolation and doesn't contain any character outside the interpolation:

String f(String s) => '$s';

Replace the string literal with the content of the interpolation:

String f(String s) => s;