## Summary
This PR tackles a corner case that we'll need to support local symbol
renaming. It relates to a nuance in how we want handle annotations
(i.e., `AnnAssign` statements with no value, like `x: int` in a function
body).
When we see a statement like:
```python
x: int
```
We create a `BindingKind::Annotation` for `x`. This is a special
`BindingKind` that the resolver isn't allowed to return. For example,
given:
```python
x: int
print(x)
```
The second line will yield an `undefined-name` error.
So why does this `BindingKind` exist at all? In Pyflakes, to support the
`unused-annotation` lint:
```python
def f():
x: int # unused-annotation
```
If we don't track `BindingKind::Annotation`, we can't lint for unused
variables that are only "defined" via annotations.
There are a few other wrinkles to `BindingKind::Annotation`. One is
that, if a binding already exists in the scope, we actually just discard
the `BindingKind`. So in this case:
```python
x = 1
x: int
```
When we go to create the `BindingKind::Annotation` for the second
statement, we notice that (1) we're creating an annotation but (2) the
scope already has binding for the name -- so we just drop the binding on
the floor. This has the nice property that annotations aren't considered
to "shadow" another binding, which is important in a bunch of places
(e.g., if we have `import os; os: int`, we still consider `os` to be an
import, as we should). But it also means that these "delayed"
annotations are one of the few remaining references that we don't track
anywhere in the semantic model.
This PR adds explicit support for these via a new `delayed_annotations`
attribute on the semantic model. These should be extremely rare, but we
do need to track them if we want to support local symbol renaming.
### This isn't the right way to model this
This isn't the right way to model this.
Here's an alternative:
- Remove `BindingKind::Annotation`, and treat annotations as their own,
separate concept.
- Instead of storing a map from name to `BindingId` on each `Scope`,
store a map from name to... `SymbolId`.
- Introduce a `Symbol` abstraction, where a symbol can point to a
current binding, and a list of annotations, like:
```rust
pub struct Symbol {
binding: Option<BindingId>,
annotations: Vec<AnnotationId>
}
```
If we did this, we could appropriately model the semantics described
above. When we go to resolve a binding, we ignore annotations (always).
When we try to find unused variables, we look through the list of
symbols, and have sufficient information to discriminate between
annotations and bound variables. Etc.
The main downside of this `Symbol`-based approach is that it's going to
take a lot more work to implement, and it'll be less performant (we'll
be storing more data per symbol, and our binding lookups will have an
added layer of indirection).
## Summary
Small follow-up to #4888 to add a dedicated `ResolvedRead` case for
unbound locals, mostly for clarity and documentation purposes (no
behavior changes).
## Test Plan
`cargo test`
## Summary
Our current mechanism for handling deletions (e.g., `del x`) is to
remove the symbol from the scope's `bindings` table. This "does the
right thing", in that if we then reference a deleted symbol, we're able
to determine that it's unbound -- but it causes a variety of problems,
mostly in that it makes certain bindings and references unreachable
after-the-fact.
Consider:
```python
x = 1
print(x)
del x
```
If we analyze this code _after_ running the semantic model over the AST,
we'll have no way of knowing that `x` was ever introduced in the scope,
much less that it was bound to a value, read, and then deleted --
because we effectively erased `x` from the model entirely when we hit
the deletion.
In practice, this will make it impossible for us to support local symbol
renames. It also means that certain rules that we want to move out of
the model-building phase and into the "check dead scopes" phase wouldn't
work today, since we'll have lost important information about the source
code.
This PR introduces two new `BindingKind` variants to model deletions:
- `BindingKind::Deletion`, which represents `x = 1; del x`.
- `BindingKind::UnboundException`, which represents:
```python
try:
1 / 0
except Exception as e:
pass
```
In the latter case, `e` gets unbound after the exception handler
(assuming it's triggered), so we want to handle it similarly to a
deletion.
The main challenge here is auditing all of our existing `Binding` and
`Scope` usages to understand whether they need to accommodate deletions
or otherwise behave differently. If you look one commit back on this
branch, you'll see that the code is littered with `NOTE(charlie)`
comments that describe the reasoning behind changing (or not) each of
those call sites. I've also augmented our test suite in preparation for
this change over a few prior PRs.
### Alternatives
As an alternative, I considered introducing a flag to `BindingFlags`,
like `BindingFlags::UNBOUND`, and setting that at the appropriate time.
This turned out to be a much more difficult change, because we tend to
match on `BindingKind` all over the place (e.g., we have a bunch of code
blocks that only run when a `BindingKind` is
`BindingKind::Importation`). As a result, introducing these new
`BindingKind` variants requires only a few changes at the client sites.
Adding a flag would've required a much wider-reaching change.
## Summary
This behavior dates back to a Pyflakes commit (5fc37cbd), which was used
to allow this test to pass:
```py
from __future__ import annotations
T: object
def f(t: T): pass
def g(t: 'T'): pass
```
But, I think this is an error. Mypy and Pyright don't accept it -- you
can only use variables as type annotations if they're type aliases
(i.e., annotated with `TypeAlias`), in which case, there has to be an
assignment on the right-hand side (see: [PEP
613](https://peps.python.org/pep-0613/)).
