no_leading_underscores_for_local_identifiers

The local variable '{0}' starts with an underscore.

The analyzer produces this diagnostic when the name of a local variable starts with an underscore.

Local variables are inherently not visible outside the declaring library, so a leading underscore indicating private adds no value.

The following code produces this diagnostic because the parameter _s starts with an underscore:

int f(String _s) => _s.length;

Remove the underscore:

int f(String s) => s.length;