This PR adds support for the "selection range" language server feature.
This feature was recently requested by a ty user in [this feature
request](https://github.com/astral-sh/ty/issues/882).
This feature allows a client to implement "smart selection expansion"
based on the structure of the parse tree. For example, if you type
"shift-ctrl-right-arrow" in VS Code, the current selection will be
expanded to include the parent AST node. Conversely,
"shift-ctrl-left-arrow" shrinks the selection.
We will probably need to tune the granularity of selection expansion
based on user feedback. The initial implementation includes most AST
nodes, but users may find this to be too fine-grained. We have the
option of skipping some AST nodes that are not as meaningful when
editing code.
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR adds support for "document symbols" and "workspace symbols"
language server features. Most of the logic to implement these features
is shared.
The "document symbols" feature returns a list of all symbols within a
specified source file. Clients can specify whether they want a flat or
hierarchical list. Document symbols are typically presented by a client
in an "outline" form. Here's what this looks like in VS Code, for
example.
<img width="240" height="249" alt="image"
src="https://github.com/user-attachments/assets/82b11f4f-32ec-4165-ba01-d6496ad13bdf"
/>
The "workspace symbols" feature returns a list of all symbols across the
entire workspace that match some user-supplied query string. This allows
the user to quickly find and navigate to any symbol within their code.
<img width="450" height="134" alt="image"
src="https://github.com/user-attachments/assets/aac131e0-9464-4adf-8a6c-829da028c759"
/>
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR fixes bug [#879](https://github.com/astral-sh/ty/issues/879)
where the signature help popup remains visible after typing the closing
paren in a call expression.
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR adds support for the "document highlights" language server
feature.
This feature allows a client to highlight all instances of a selected
name within a document. Without this feature, editors perform
highlighting based on a simple text match. This adds semantic knowledge.
The implementation of this feature largely overlaps that of the
recently-added "references" feature. This PR refactors the existing
"references.rs" module, separating out the functionality and tests that
are specific to the other language feature into a "goto_references.rs"
module. The "references.rs" module now contains the functionality that
is common to "goto references", "document highlights" and "rename"
(which is not yet implemented).
As part of this PR, I also created a new `ReferenceTarget` type which is
similar to the existing `NavigationTarget` type but better suited for
references. This idea was suggested by @MichaReiser in [this code review
feedback](https://github.com/astral-sh/ruff/pull/19475#discussion_r2224061006)
from a previous PR. Notably, this new type contains a field that
specifies the "kind" of the reference (read, write or other). This
"kind" is needed for the document highlights feature.
Before: all textual instances of `foo` are highlighted
<img width="156" height="126" alt="Screenshot 2025-07-23 at 12 51 09 PM"
src="https://github.com/user-attachments/assets/37ccdb2f-d48a-473d-89d5-8e89cb6c394e"
/>
After: only semantic matches are highlighted
<img width="164" height="157" alt="Screenshot 2025-07-23 at 12 52 05 PM"
src="https://github.com/user-attachments/assets/2efadadd-4691-4815-af04-b031e74c81b7"
/>
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR updates our call binding logic to handle splatted arguments.
Complicating matters is that we have separated call bind analysis into
two phases: parameter matching and type checking. Parameter matching
looks at the arity of the function signature and call site, and assigns
arguments to parameters. Importantly, we don't yet know the type of each
argument! This is needed so that we can decide whether to infer the type
of each argument as a type form or value form, depending on the
requirements of the parameter that the argument was matched to.
This is an issue when splatting an argument, since we need to know how
many elements the splatted argument contains to know how many positional
parameters to match it against. And to know how many elements the
splatted argument has, we need to know its type.
To get around this, we now make the assumption that splatted arguments
can only be used with value-form parameters. (If you end up splatting an
argument into a type-form parameter, we will silently pass in its
value-form type instead.) That allows us to preemptively infer the
(value-form) type of any splatted argument, so that we have its arity
available during parameter matching. We defer inference of non-splatted
arguments until after parameter matching has finished, as before.
We reuse a lot of the new tuple machinery to make this happen — in
particular resizing the tuple spec representing the number of arguments
passed in with the tuple length representing the number of parameters
the splat was matched with.
This work also shows that we might need to change how we are performing
argument expansion during overload resolution. At the moment, when we
expand parameters, we assume that each argument will still be matched to
the same parameters as before, and only retry the type-checking phase.
With splatted arguments, this is no longer the case, since the inferred
arity of each union element might be different than the arity of the
union as a whole, which can affect how many parameters the splatted
argument is matched to. See the regression test case in
`mdtest/call/function.md` for more details.
Summary
--
This PR tweaks Ruff's internal usage of the new diagnostic model to more
closely
match the intended use, as I understand it. Specifically, it moves the
fix/help
suggestion from the primary annotation's message to a subdiagnostic. In
turn, it
adds the secondary/noqa code as the new primary annotation message. As
shown in
the new `ruff_db` tests, this more closely mirrors Ruff's current
diagnostic
output.
I also added `Severity::Help` to render the fix suggestion with a
`help:` prefix
instead of `info:`.
These changes don't have any external impact now but should help a bit
with #19415.
Test Plan
--
New full output format tests in `ruff_db`
Rendered Diagnostics
--
Full diagnostic output from `annotate-snippets` in this PR:
```
error[unused-import]: `os` imported but unused
--> fib.py:1:8
|
1 | import os
| ^^
|
help: Remove unused import: `os`
```
Current Ruff output for the same code:
```
fib.py:1:8: F401 [*] `os` imported but unused
|
1 | import os
| ^^ F401
|
= help: Remove unused import: `os`
```
Proposed final output after #19415:
```
F401 [*] `os` imported but unused
--> fib.py:1:8
|
1 | import os
| ^^
|
help: Remove unused import: `os`
```
These are slightly updated from
https://github.com/astral-sh/ruff/pull/19464#issuecomment-3097377634
below to remove the extra noqa codes in the primary annotation messages
for the first and third cases.
This implements mapping of definitions in stubs to definitions in the
"real" implementation using the approach described in
https://github.com/astral-sh/ty/issues/788#issuecomment-3097000287
I've tested this with goto-definition in vscode with code that uses
`colorama` and `types-colorama`.
Notably this implementation does not add support for stub-mapping stdlib
modules, which can be done as an essentially orthogonal followup in the
implementation of `resolve_real_module`.
Part of https://github.com/astral-sh/ty/issues/788
I noticed that the semantic token implementation was not handling
identifiers in a few cases. This adds support for identifiers that
appear in `except`, `case`, `nonlocal`, and `global` statements.
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR extends the "go to declaration" and "go to definition"
functionality to support import statements — both standard imports and
"from" import forms.
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR builds upon #19371. It addresses a few additional code review
suggestions and adds support for attribute accesses (expressions of the
form `x.y`) and keyword arguments within call expressions.
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
## Summary
This PR updates the server to keep track of open files both system and
virtual files.
This is done by updating the project by adding the file in the open file
set in `didOpen` notification and removing it in `didClose`
notification.
This does mean that for workspace diagnostics, ty will only check open
files because the behavior of different diagnostic builder is to first
check `is_file_open` and only add diagnostics for open files. So, this
required updating the `is_file_open` model to be `should_check_file`
model which validates whether the file needs to be checked based on the
`CheckMode`. If the check mode is open files only then it will check
whether the file is open. If it's all files then it'll return `true` by
default.
Closes: astral-sh/ty#619
## Test Plan
### Before
There are two files in the project: `__init__.py` and `diagnostics.py`.
In the video, I'm demonstrating the old behavior where making changes to
the (open) `diagnostics.py` file results in re-parsing the file:
https://github.com/user-attachments/assets/c2ac0ecd-9c77-42af-a924-c3744b146045
### After
Same setup as above.
In the video, I'm demonstrating the new behavior where making changes to
the (open) `diagnostics.py` file doesn't result in re-parting the file:
https://github.com/user-attachments/assets/7b82fe92-f330-44c7-b527-c841c4545f8f
This fixes https://github.com/astral-sh/ty/issues/832.
New tests were added to prevent future regressions.
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
This PR implements "go to definition" and "go to declaration"
functionality for name nodes only. Future PRs will add support for
attributes, module names in import statements, keyword argument names,
etc.
This PR:
* Registers a declaration and definition request handler for the
language server.
* Splits out the `goto_type_definition` into its own module. The `goto`
module contains functionality that is common to `goto_type_definition`,
`goto_declaration` and `goto_definition`.
* Roughs in a new module `stub_mapping` that is not yet implemented. It
will be responsible for mapping a definition in a stub file to its
corresponding definition(s) in an implementation (source) file.
* Adds a new IDE support function `definitions_for_name` that collects
all of the definitions associated with a name and resolves any imports
(recursively) to find the original definitions associated with that
name.
* Adds a new `VisibleAncestorsIter` stuct that iterates up the scope
hierarchy but skips scopes that are not visible to starting scope.
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
## Summary
Add a new `Type::EnumLiteral(…)` variant and infer this type for member
accesses on enums.
**Example**: No more `@Todo` types here:
```py
from enum import Enum
class Answer(Enum):
YES = 1
NO = 2
def is_yes(self) -> bool:
return self == Answer.YES
reveal_type(Answer.YES) # revealed: Literal[Answer.YES]
reveal_type(Answer.YES == Answer.NO) # revealed: Literal[False]
reveal_type(Answer.YES.is_yes()) # revealed: bool
```
## Test Plan
* Many new Markdown tests for the new type variant
* Added enum literal types to property tests, ran property tests
## Ecosystem analysis
Summary:
Lots of false positives removed. All of the new diagnostics are
either new true positives (the majority) or known problems. Click for
detailed analysis</summary>
Details:
```diff
AutoSplit (https://github.com/Toufool/AutoSplit)
+ error[call-non-callable] src/capture_method/__init__.py:137:9: Method `__getitem__` of type `bound method CaptureMethodDict.__getitem__(key: Never, /) -> type[CaptureMethodBase]` is not callable on object of type `CaptureMethodDict`
+ error[call-non-callable] src/capture_method/__init__.py:147:9: Method `__getitem__` of type `bound method CaptureMethodDict.__getitem__(key: Never, /) -> type[CaptureMethodBase]` is not callable on object of type `CaptureMethodDict`
+ error[call-non-callable] src/capture_method/__init__.py:148:1: Method `__getitem__` of type `bound method CaptureMethodDict.__getitem__(key: Never, /) -> type[CaptureMethodBase]` is not callable on object of type `CaptureMethodDict`
```
New true positives. That `__getitem__` method is apparently annotated
with `Never` to prevent developers from using it.
```diff
dd-trace-py (https://github.com/DataDog/dd-trace-py)
+ error[invalid-assignment] ddtrace/vendor/psutil/_common.py:29:5: Object of type `None` is not assignable to `Literal[AddressFamily.AF_INET6]`
+ error[invalid-assignment] ddtrace/vendor/psutil/_common.py:33:5: Object of type `None` is not assignable to `Literal[AddressFamily.AF_UNIX]`
```
Arguably true positives:
e0a772c28b/ddtrace/vendor/psutil/_common.py (L29)
```diff
ignite (https://github.com/pytorch/ignite)
+ error[invalid-argument-type] tests/ignite/engine/test_custom_events.py:190:34: Argument to bound method `__call__` is incorrect: Expected `((...) -> Unknown) | None`, found `Literal["123"]`
+ error[invalid-argument-type] tests/ignite/engine/test_custom_events.py:220:37: Argument to function `default_event_filter` is incorrect: Expected `Engine`, found `None`
+ error[invalid-argument-type] tests/ignite/engine/test_custom_events.py:220:43: Argument to function `default_event_filter` is incorrect: Expected `int`, found `None`
+ error[call-non-callable] tests/ignite/engine/test_custom_events.py:561:9: Object of type `CustomEvents` is not callable
+ error[invalid-argument-type] tests/ignite/metrics/test_frequency.py:50:38: Argument to bound method `attach` is incorrect: Expected `Events`, found `CallableEventWithFilter`
```
All true positives. Some of them are inside `pytest.raises(TypeError,
…)` blocks 🙃
```diff
meson (https://github.com/mesonbuild/meson)
+ error[invalid-argument-type] unittests/internaltests.py:243:51: Argument to bound method `__init__` is incorrect: Expected `bool`, found `Literal[MachineChoice.HOST]`
+ error[invalid-argument-type] unittests/internaltests.py:271:51: Argument to bound method `__init__` is incorrect: Expected `bool`, found `Literal[MachineChoice.HOST]`
```
New true positives. Enum literals can not be assigned to `bool`, even if
their value types are `0` and `1`.
```diff
poetry (https://github.com/python-poetry/poetry)
+ error[invalid-assignment] src/poetry/console/exceptions.py:101:5: Object of type `Literal[""]` is not assignable to `InitVar[str]`
```
New false positive, missing support for `InitVar`.
```diff
prefect (https://github.com/PrefectHQ/prefect)
+ error[invalid-argument-type] src/integrations/prefect-dask/tests/test_task_runners.py:193:17: Argument is incorrect: Expected `StateType`, found `Literal[StateType.COMPLETED]`
```
This is confusing. There are two definitions
([one](74d8cd93ee/src/prefect/client/schemas/objects.py (L89-L100)),
[two](https://github.com/PrefectHQ/prefect/blob/main/src/prefect/server/schemas/states.py#L40))
of the `StateType` enum. Here, we're trying to assign one to the other.
I don't think that should be allowed, so this is a true positive (?).
```diff
python-htmlgen (https://github.com/srittau/python-htmlgen)
+ error[invalid-assignment] test_htmlgen/form.py:51:9: Object of type `str` is not assignable to attribute `autocomplete` of type `Autocomplete | None`
+ error[invalid-assignment] test_htmlgen/video.py:38:9: Object of type `str` is not assignable to attribute `preload` of type `Preload | None`
```
True positives. [The stubs are
wrong](01e3b911ac/htmlgen/form.pyi (L8-L10)).
These should not contain type annotations, but rather just `OFF = ...`.
```diff
rotki (https://github.com/rotki/rotki)
+ error[invalid-argument-type] rotkehlchen/tests/unit/test_serialization.py:62:30: Argument to bound method `deserialize` is incorrect: Expected `str`, found `Literal[15]`
```
New true positive.
```diff
vision (https://github.com/pytorch/vision)
+ error[unresolved-attribute] test/test_extended_models.py:302:17: Type `type[WeightsEnum]` has no attribute `DEFAULT`
+ error[unresolved-attribute] test/test_extended_models.py:302:58: Type `type[WeightsEnum]` has no attribute `DEFAULT`
```
Also new true positives. No `DEFAULT` member exists on `WeightsEnum`.
Basically, we weren't quite using `Type::member` in every case
correctly. Specifically, this example from @sharkdp:
```
class Meta(type):
@property
def meta_attr(self) -> int:
return 0
class C(metaclass=Meta): ...
C.<CURSOR>
```
While we would return `C.meta_attr` here, we were claiming its type was
`property`. But its type should be `int`.
Ref https://github.com/astral-sh/ruff/pull/19216#discussion_r2197065241
While we did previously support submodule completions via our
`all_members` API, that only works when submodules are attributes of
their parent module. For example, `os.path`. But that didn't work when
the submodule was not an attribute of its parent. For example,
`http.client`. To make the latter work, we read the directory of the
parent module to discover its submodules.
This PR includes:
* Implemented core signature help logic
* Added new docstring method on Definition that returns a docstring for
function and class definitions
* Modified the display code for Signature that allows a signature string
to be broken into text ranges that correspond to each parameter in the
signature
* Augmented Signature struct so it can track the Definition for a
signature when available; this allows us to find the docstring
associated with the signature
* Added utility functions for parsing parameter documentation from three
popular docstring formats (Google, NumPy and reST)
* Implemented tests for all of the above
"Signature help" is displayed by an editor when you are typing a
function call expression. It is typically triggered when you type an
open parenthesis. The language server provides information about the
target function's signature (or multiple signatures), documentation, and
parameters.
Here is how this appears:
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
Co-authored-by: Micha Reiser <micha@reiser.io>
Since we generally need (so far) to get the type information of each
suggestion to figure out its boundness anyway, we might as well expose
it here. Completions want to use this information to enhance the
metadata on each suggestion for a more pleasant user experience.
For the most part, this was pretty straight-forward. The most exciting
part was in computing the types for instance attributes. I'm not 100%
sure it's correct or is the best way to do it.
This commit doesn't change any behavior, but makes it so `all_members`
returns a `Vec<Member>` instead of `Vec<Name>`, where a `Member`
contains a `Name`. This gives us an expansion point to include other
data (such as the type of the `Name`).
This PR implements a basic semantic token provider for ty's language
server. This allows for more accurate semantic highlighting / coloring
within editors that support this LSP functionality.
Here are screen shots that show how code appears in VS Code using the
"rainbow" theme both before and after this change.
The token types and modifier tags in this implementation largely mirror
those used in Microsoft's default language server for Python.
The implementation supports two LSP interfaces. The first provides
semantic tokens for an entire document, and the second returns semantic
tokens for a requested range within a document.
The PR includes unit tests. It also includes comments that document
known limitations and areas for future improvements.
---------
Co-authored-by: UnboundVariable <unbound@gmail.com>
This implements filtering of private symbols from stub files based on
type information as discussed in
https://github.com/astral-sh/ruff/pull/19102. It extends the previous
implementation to apply to all stub files, instead of just the
`builtins` module, and uses type information to retain private names
that are may be relevant at runtime.
Most of the work here was doing some light refactoring to facilitate
sensible testing. That is, we don't want to list every builtin included
in most tests, so we add some structure to the completion type returned.
Tests can now filter based on whether a completion is a builtin or not.
Otherwise, builtins are found using the existing infrastructure for
`object.attr` completions (where we hard-code the module name
`builtins`).
I did consider changing the sort order based on whether a completion
suggestion was a builtin or not. In particular, it seemed like it might
be a good idea to sort builtins after other scope based completions,
but before the dunder and sunder attributes. Namely, it seems likely
that there is an inverse correlation between the size of a scope and
the likelihood of an item in that scope being used at any given point.
So it *might* be a good idea to prioritize the likelier candidates in
the completions returned.
Additionally, the number of items introduced by adding builtins is quite
large. So I wondered whether mixing them in with everything else would
become too noisy.
However, it's not totally clear to me that this is the right thing to
do. Right now, I feel like there is a very obvious lexicographic
ordering that makes "finding" the right suggestion to activate
potentially easier than if the ranking mechanism is less clear.
(Technically, the dunder and sunder attributes are not sorted
lexicographically, but I'd put forward that most folks don't have an
intuitive understanding of where `_` ranks lexicographically with
respect to "regular" letters. Moreover, since dunder and sunder
attributes are all grouped together, I think the ordering here ends up
being very obvious after even a quick glance.)
This commit does a small refactor to combine the file and
cursor offset into a single type. I think this makes it
clearer that even if there are multiple files in the cursor
test, this one in particular corresponds to the file that
contains the `<CURSOR>` marker.
This doesn't change any functionality of the cursor tests, but does
re-arrange the code a bit. Firstly, it's now in a builder. And secondly,
there's an API to add multiple files to the test (but exactly one must
have a `<CURSOR>` marker).
There were two main challenges in this PR.
The first was mostly just figuring out how to get the symbols
corresponding to `module`. It turns out that we do this in a couple
of places in ty already, but through different means. In one approach,
we use [`exported_names`]. In another approach, we get a `Type`
corresponding to the module. We take the latter approach here, which is
consistent with how we do completions elsewhere. (I looked into
factoring this logic out into its own function, but it ended up being
pretty constrained. e.g., There's only one other place where we want to
go from `ast::StmtImportFrom` to a module `Type`, and that code also
wants the module name.)
The second challenge was recognizing the `from module import <CURSOR>`
pattern in the code. I initially started with some fixed token patterns
to get a proof of concept working. But I ended up switching to mini
state machine over tokens. I looked at the parser for `StmtImportFrom`
to determine what kinds of tokens we can expect.
[`exported_names`]:
23a3b6ef23/crates/ty_python_semantic/src/semantic_index/re_exports.rs (L47)
## Summary
* Completely removes the concept of visibility constraints. Reachability
constraints are now used to model the static visibility of bindings and
declarations. Reachability constraints are *much* easier to reason about
/ work with, since they are applied at the beginning of a branch, and
not applied retroactively. Removing the duplication between visibility
and reachability constraints also leads to major code simplifications
[^1]. For an overview of how the new constraint system works, see the
updated doc comment in `reachability_constraints.rs`.
* Fixes a [control-flow modeling bug
(panic)](https://github.com/astral-sh/ty/issues/365) involving `break`
statements in loops
* Fixes a [bug where](https://github.com/astral-sh/ty/issues/624) where
`elif` branches would have wrong reachability constraints
* Fixes a [bug where](https://github.com/astral-sh/ty/issues/648) code
after infinite loops would not be considered unreachble
* Fixes a panic on the `pywin32` ecosystem project, which we should be
able to move to `good.txt` once this has been merged.
* Removes some false positives in unreachable code because we infer
`Never` more often, due to the fact that reachability constraints now
apply retroactively to *all* active bindings, not just to bindings
inside a branch.
* As one example, this removes the `division-by-zero` diagnostic from
https://github.com/astral-sh/ty/issues/443 because we now infer `Never`
for the divisor.
* Supersedes and includes similar test changes as
https://github.com/astral-sh/ruff/pull/18392
closes https://github.com/astral-sh/ty/issues/365
closes https://github.com/astral-sh/ty/issues/624
closes https://github.com/astral-sh/ty/issues/642
closes https://github.com/astral-sh/ty/issues/648
## Benchmarks
Benchmarks on black, pandas, and sympy showed that this is neither a
performance improvement, nor a regression.
## Test Plan
Regression tests for:
- [x] https://github.com/astral-sh/ty/issues/365
- [x] https://github.com/astral-sh/ty/issues/624
- [x] https://github.com/astral-sh/ty/issues/642
- [x] https://github.com/astral-sh/ty/issues/648
[^1]: I'm afraid this is something that @carljm advocated for since the
beginning, and I'm not sure anymore why we have never seriously tried
this before. So I suggest we do *not* attempt to do a historical deep
dive to find out exactly why this ever became so complicated, and just
enjoy the fact that we eventually arrived here.
---------
Co-authored-by: Carl Meyer <carl@astral.sh>
Specifically, this PR reverts "Make completions an opt-in LSP feature
(#17921)",
corresponding to commit 51e2effd2d.
In practice, this means you don't need to opt into completions working
by enabling experimental features. i.e., I was able to remove this from
my LSP configuration:
```
"experimental": {
"completions": {
"enable": true
}
},
```
There's still a lot of work left to do to make completions awesome, but
I think it's in a state where it would be useful to get real user
feedback. It's also meaningfully using ty to provide completions that
use type information.
Ref astral-sh/ty#86
This makes it work for a number of additional cases, like nested
attribute access and things like `[].<CURSOR>`.
The basic idea is that instead of selecting a covering node closest to a
leaf that contains the cursor, we walk up the tree as much as we can.
This lets us access the correct `ExprAttribute` node when performing
nested access.
This routine lets us climb up the AST tree when we find
a contiguous sequence of nodes that satisfy our predicate.
This will be useful for making things like `a.b.<CURSOR>`
work. That is, we don't want the `ExprAttribute` closest
to a leaf. We also don't always want the `ExprAttribute`
closest to the root. Rather, (I think) we want the
`ExprAttribute` closest to the root that has an unbroken
chain to the `ExprAttribute` closest to the leaf.
This commit doesn't change any functionality, but instead changes the
representation of `CoveringNode` to make the implementation simpler (as
well as planned future additions). By putting the found node last in the
list of ancestors (now just generically called `nodes`), we reduce the
amount of special case handling we need.
The downside is that the representation now allows invalid states (a
`CoveringNode` with no elements). But I think this is well mitigated by
encapsulation.
## Summary
https://github.com/astral-sh/ty/issues/214 will require a couple
invasive changes that I would like to get merged even before garbage
collection is fully implemented (to avoid rebasing):
- `ParsedModule` can no longer be dereferenced directly. Instead you
need to load a `ParsedModuleRef` to access the AST, which requires a
reference to the salsa database (as it may require re-parsing the AST if
it was collected).
- `AstNodeRef` can only be dereferenced with the `node` method, which
takes a reference to the `ParsedModuleRef`. This allows us to encode the
fact that ASTs do not live as long as the database and may be collected
as soon a given instance of a `ParsedModuleRef` is dropped. There are a
number of places where we currently merge the `'db` and `'ast`
lifetimes, so this requires giving some types/functions two separate
lifetime parameters.
