Admin comment: this is being implemented, feature specification, implementation issue, experiment flag wildcard-variables.

Pattern matching brought a new way to declare variables. Inside patterns, any variable whose name is _ is considered a "wildcard". It behaves like a variable syntactically, but doesn't actually create a variable with that name. That means you can use _ multiple times in a pattern without a name collision.

That leads to an irregularity:

var (_, _) = (1, 2); // OK.

var _ = 1;
var _ = 2; // Error. Already a variable in scope with this name.

Also, it's annoying that _ binds a name. When you have a lambda with more than one parameter that you don't use, you end up having to do:

takesCallback((_, __, ___) { ... });

I have a proposal to fix both by saying that local variables and parameters named _ are also non-binding. This is the tracking issue for that proposal.