/// ## What it does
/// Checks for the presence of unused private `typing.TypeAlias` definitions.
///
/// ## Why is this bad?
/// A private `typing.TypeAlias` that is defined but not used is likely a
/// mistake, and should either be used, made public, or removed to avoid
/// confusion.
///
/// ## Example
/// ```python
/// import typing
///
/// _UnusedTypeAlias: typing.TypeAlias = int
/// ```
///
/// Use instead:
/// ```python
/// import typing
///
/// _UsedTypeAlias: typing.TypeAlias = int
///
/// def func(arg: _UsedTypeAlias) -> _UsedTypeAlias:
///     ...
/// ```

…#6142)

In astral-sh#6134 and astral-sh#6136, we see some false positives for "shadowed" class
definitions. For example, here, the first definition is flagged as
unused, since from the perspective of the semantic model (which doesn't
understand branching), it appears to be immediately shadowed in the
`else`, and thus never used:

```python
if sys.version_info >= (3, 11):
    class _RootLoggerConfiguration(TypedDict, total=False):
        level: _Level
        filters: Sequence[str | _FilterType]
        handlers: Sequence[str]

else:
    class _RootLoggerConfiguration(TypedDict, total=False):
        level: _Level
        filters: Sequence[str]
        handlers: Sequence[str]
```

Instead of looking at _all_ bindings, we should instead look at the
"live" bindings, which is similar to how other rules (like unused
variables detection) is structured. We thus move the rule from
`bindings.rs` (which iterates over _all_ bindings, regardless of whether
they're shadowed) to `deferred_scopes.rs`, which iterates over all
"live" bindings once a scope has been fully analyzed.

`cargo test`

Closes astral-sh#5858.

## Summary

Right now, `RUF015` will try to rewrite `x[:1]` as `[next(x)]`. This
isn't equivalent if `x`, for example, is empty, where slicing like
`x[:1]` is forgiving, but `next` raises `StopIteration`. For me this is
a little too much of a deviation to be comfortable with, and most of the
value in this rule is the `x[0]` to `next(x)` conversion anyway.

Closes astral-sh#6148.

/// ## What it does
/// Checks for the presence of unused private `typing.TypeAlias` definitions.
///
/// ## Why is this bad?
/// A private `typing.TypeAlias` that is defined but not used is likely a
/// mistake, and should either be used, made public, or removed to avoid
/// confusion.
///
/// ## Example
/// ```python
/// import typing
///
/// _UnusedTypeAlias: typing.TypeAlias = int
/// ```
///
/// Use instead:
/// ```python
/// import typing
///
/// _UsedTypeAlias: typing.TypeAlias = int
///
/// def func(arg: _UsedTypeAlias) -> _UsedTypeAlias:
///     ...
/// ```

These changes are based on astral-sh#6142