[Proposal]: Anonymous using declarations

# `using` with a discard

* Specification: [proposals/anonymous-using-declarations.md](https://github.com/dotnet/csharplang/blob/main/proposals/anonymous-using-declarations.md)
* Discussion: https://github.com/dotnet/csharplang/discussions/8605

## Summary
[summary]: #summary

Permit `using` variable declarations with a discard variable:

```cs
void M()
{
using _ = GetDisposable();
}
```

## Design meetings
- https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-12-03.md#discussion
- https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-12-17.md#using-local-declaration-improvements