[ty] Add basic support for dataclasses.field (#19553)

## Summary

Add basic support for `dataclasses.field`:
* remove fields with `init=False` from the signature of the synthesized
`__init__` method
* infer correct default value types from `default` or `default_factory`
arguments

```py
from dataclasses import dataclass, field

def default_roles() -> list[str]:
    return ["user"]

@dataclass
class Member:
    name: str
    roles: list[str] = field(default_factory=default_roles)
    tag: str | None = field(default=None, init=False)

# revealed: (self: Member, name: str, roles: list[str] = list[str]) -> None
reveal_type(Member.__init__)
```

Support for `kw_only` has **not** been added.

part of https://github.com/astral-sh/ty/issues/111

## Test Plan

New Markdown tests

crates/ty_python_semantic/src/types.rs

@@ -1279,6 +1279,15 @@ impl<'db> Type<'db> {
             // handled above. It's always assignable, though.
             (Type::Dynamic(_), _) | (_, Type::Dynamic(_)) => relation.is_assignability(),
 
+            // Pretend that instances of `dataclasses.Field` are assignable to their default type.
+            // This allows field definitions like `name: str = field(default="")` in dataclasses
+            // to pass the assignability check of the inferred type to the declared type.
+            (Type::KnownInstance(KnownInstanceType::Field(field)), right)
+                if relation.is_assignability() =>
+            {
+                field.default_type(db).has_relation_to(db, right, relation)
+            }
+
             // In general, a TypeVar `T` is not a subtype of a type `S` unless one of the two conditions is satisfied:
             // 1. `T` is a bound TypeVar and `T`'s upper bound is a subtype of `S`.
             //    TypeVars without an explicit upper bound are treated as having an implicit upper bound of `object`.
@@ -5109,6 +5118,10 @@ impl<'db> Type<'db> {
                     invalid_expressions: smallvec::smallvec![InvalidTypeExpression::Deprecated],
                     fallback_type: Type::unknown(),
                 }),
+                KnownInstanceType::Field(__call__) => Err(InvalidTypeExpressionError {
+                    invalid_expressions: smallvec::smallvec![InvalidTypeExpression::Field],
+                    fallback_type: Type::unknown(),
+                }),
                 KnownInstanceType::SubscriptedProtocol(_) => Err(InvalidTypeExpressionError {
                     invalid_expressions: smallvec::smallvec_inline![
                         InvalidTypeExpression::Protocol
@@ -5957,6 +5970,9 @@ pub enum KnownInstanceType<'db> {
 
     /// A single instance of `warnings.deprecated` or `typing_extensions.deprecated`
     Deprecated(DeprecatedInstance<'db>),
+
+    /// A single instance of `dataclasses.Field`
+    Field(FieldInstance<'db>),
 }
 
 fn walk_known_instance_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
@@ -5978,6 +5994,9 @@ fn walk_known_instance_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
         KnownInstanceType::Deprecated(_) => {
             // Nothing to visit
         }
+        KnownInstanceType::Field(field) => {
+            visitor.visit_type(db, field.default_type(db));
+        }
     }
 }
 
@@ -5998,6 +6017,7 @@ impl<'db> KnownInstanceType<'db> {
                 // Nothing to normalize
                 Self::Deprecated(deprecated)
             }
+            Self::Field(field) => Self::Field(field.normalized_impl(db, visitor)),
         }
     }
 
@@ -6007,6 +6027,7 @@ impl<'db> KnownInstanceType<'db> {
             Self::TypeVar(_) => KnownClass::TypeVar,
             Self::TypeAliasType(_) => KnownClass::TypeAliasType,
             Self::Deprecated(_) => KnownClass::Deprecated,
+            Self::Field(_) => KnownClass::Field,
         }
     }
 
@@ -6052,6 +6073,11 @@ impl<'db> KnownInstanceType<'db> {
                     // have a `Type::TypeVar(_)`, which is rendered as the typevar's name.
                     KnownInstanceType::TypeVar(_) => f.write_str("typing.TypeVar"),
                     KnownInstanceType::Deprecated(_) => f.write_str("warnings.deprecated"),
+                    KnownInstanceType::Field(field) => {
+                        f.write_str("dataclasses.Field[")?;
+                        field.default_type(self.db).display(self.db).fmt(f)?;
+                        f.write_str("]")
+                    }
                 }
             }
         }
@@ -6261,6 +6287,8 @@ enum InvalidTypeExpression<'db> {
     Generic,
     /// Same for `@deprecated`
     Deprecated,
+    /// Same for `dataclasses.Field`
+    Field,
     /// Type qualifiers are always invalid in *type expressions*,
     /// but these ones are okay with 0 arguments in *annotation expressions*
     TypeQualifier(SpecialFormType),
@@ -6305,6 +6333,9 @@ impl<'db> InvalidTypeExpression<'db> {
                     InvalidTypeExpression::Deprecated => {
                         f.write_str("`warnings.deprecated` is not allowed in type expressions")
                     }
+                    InvalidTypeExpression::Field => {
+                        f.write_str("`dataclasses.Field` is not allowed in type expressions")
+                    }
                     InvalidTypeExpression::TypeQualifier(qualifier) => write!(
                         f,
                         "Type qualifier `{qualifier}` is not allowed in type expressions \
@@ -6371,6 +6402,36 @@ pub struct DeprecatedInstance<'db> {
 // The Salsa heap is tracked separately.
 impl get_size2::GetSize for DeprecatedInstance<'_> {}
 
+/// Contains information about instances of `dataclasses.Field`, typically created using
+/// `dataclasses.field()`.
+#[salsa::interned(debug)]
+#[derive(PartialOrd, Ord)]
+pub struct FieldInstance<'db> {
+    /// The type of the default value for this field. This is derived from the `default` or
+    /// `default_factory` arguments to `dataclasses.field()`.
+    pub default_type: Type<'db>,
+
+    /// Whether this field is part of the `__init__` signature, or not.
+    pub init: bool,
+}
+
+// The Salsa heap is tracked separately.
+impl get_size2::GetSize for FieldInstance<'_> {}
+
+impl<'db> FieldInstance<'db> {
+    pub(crate) fn normalized_impl(
+        self,
+        db: &'db dyn Db,
+        visitor: &mut TypeTransformer<'db>,
+    ) -> Self {
+        FieldInstance::new(
+            db,
+            self.default_type(db).normalized_impl(db, visitor),
+            self.init(db),
+        )
+    }
+}
+
 /// Whether this typecar was created via the legacy `TypeVar` constructor, or using PEP 695 syntax.
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub enum TypeVarKind {

crates/ty_python_semantic/src/types/call/bind.rs

@@ -28,7 +28,7 @@ use crate::types::generics::{Specialization, SpecializationBuilder, Specializati
 use crate::types::signatures::{Parameter, ParameterForm, Parameters};
 use crate::types::tuple::{Tuple, TupleLength, TupleType};
 use crate::types::{
-    BoundMethodType, ClassLiteral, DataclassParams, KnownClass, KnownInstanceType,
+    BoundMethodType, ClassLiteral, DataclassParams, FieldInstance, KnownClass, KnownInstanceType,
     MethodWrapperKind, PropertyInstanceType, SpecialFormType, TypeMapping, UnionType,
     WrapperDescriptorKind, enums, ide_support, todo_type,
 };
@@ -899,6 +899,36 @@ impl<'db> Bindings<'db> {
                             }
                         }
 
+                        Some(KnownFunction::Field) => {
+                            if let [default, default_factory, init, ..] = overload.parameter_types()
+                            {
+                                let default_ty = match (default, default_factory) {
+                                    (Some(default_ty), _) => *default_ty,
+                                    (_, Some(default_factory_ty)) => default_factory_ty
+                                        .try_call(db, &CallArguments::none())
+                                        .map_or(Type::unknown(), |binding| binding.return_type(db)),
+                                    _ => Type::unknown(),
+                                };
+
+                                let init = init
+                                    .map(|init| !init.bool(db).is_always_false())
+                                    .unwrap_or(true);
+
+                                // `typeshed` pretends that `dataclasses.field()` returns the type of the
+                                // default value directly. At runtime, however, this function returns an
+                                // instance of `dataclasses.Field`. We also model it this way and return
+                                // a known-instance type with information about the field. The drawback
+                                // of this approach is that we need to pretend that instances of `Field`
+                                // are assignable to `T` if the default type of the field is assignable
+                                // to `T`. Otherwise, we would error on `name: str = field(default="")`.
+                                overload.set_return_type(Type::KnownInstance(
+                                    KnownInstanceType::Field(FieldInstance::new(
+                                        db, default_ty, init,
+                                    )),
+                                ));
+                            }
+                        }
+
                         _ => {
                             // Ideally, either the implementation, or exactly one of the overloads
                             // of the function can have the dataclass_transform decorator applied.

crates/ty_python_semantic/src/types/class.rs

@@ -894,6 +894,9 @@ pub(crate) struct DataclassField<'db> {
     /// Whether or not this field is "init-only". If this is true, it only appears in the
     /// `__init__` signature, but is not accessible as a real field
     pub(crate) init_only: bool,
+
+    /// Whether or not this field should appear in the signature of `__init__`.
+    pub(crate) init: bool,
 }
 
 /// Representation of a class definition statement in the AST: either a non-generic class, or a
@@ -1601,9 +1604,15 @@ impl<'db> ClassLiteral<'db> {
                     mut field_ty,
                     mut default_ty,
                     init_only: _,
+                    init,
                 },
             ) in self.fields(db, specialization, field_policy)
             {
+                if name == "__init__" && !init {
+                    // Skip fields with `init=False`
+                    continue;
+                }
+
                 if field_ty
                     .into_nominal_instance()
                     .is_some_and(|instance| instance.class.is_known(db, KnownClass::KwOnly))
@@ -1852,15 +1861,25 @@ impl<'db> ClassLiteral<'db> {
 
                 if let Some(attr_ty) = attr.place.ignore_possibly_unbound() {
                     let bindings = use_def.end_of_scope_symbol_bindings(symbol_id);
-                    let default_ty = place_from_bindings(db, bindings).ignore_possibly_unbound();
+                    let mut default_ty =
+                        place_from_bindings(db, bindings).ignore_possibly_unbound();
+
+                    default_ty =
+                        default_ty.map(|ty| ty.apply_optional_specialization(db, specialization));
+
+                    let mut init = true;
+                    if let Some(Type::KnownInstance(KnownInstanceType::Field(field))) = default_ty {
+                        default_ty = Some(field.default_type(db));
+                        init = field.init(db);
+                    }
 
                     attributes.insert(
                         symbol.name().clone(),
                         DataclassField {
                             field_ty: attr_ty.apply_optional_specialization(db, specialization),
-                            default_ty: default_ty
-                                .map(|ty| ty.apply_optional_specialization(db, specialization)),
+                            default_ty,
                             init_only: attr.is_init_var(),
+                            init,
                         },
                     );
                 }

crates/ty_python_semantic/src/types/class_base.rs

@@ -165,7 +165,8 @@ impl<'db> ClassBase<'db> {
                 KnownInstanceType::SubscriptedProtocol(_) => Some(Self::Protocol),
                 KnownInstanceType::TypeAliasType(_)
                 | KnownInstanceType::TypeVar(_)
-                | KnownInstanceType::Deprecated(_) => None,
+                | KnownInstanceType::Deprecated(_)
+                | KnownInstanceType::Field(_) => None,
             },
 
             Type::SpecialForm(special_form) => match special_form {

crates/ty_python_semantic/src/types/function.rs

@@ -1049,6 +1049,8 @@ pub enum KnownFunction {
 
     /// `dataclasses.dataclass`
     Dataclass,
+    /// `dataclasses.field`
+    Field,
 
     /// `inspect.getattr_static`
     GetattrStatic,
@@ -1127,7 +1129,7 @@ impl KnownFunction {
             Self::AbstractMethod => {
                 matches!(module, KnownModule::Abc)
             }
-            Self::Dataclass => {
+            Self::Dataclass | Self::Field => {
                 matches!(module, KnownModule::Dataclasses)
             }
             Self::GetattrStatic => module.is_inspect(),
@@ -1408,7 +1410,7 @@ pub(crate) mod tests {
 
                 KnownFunction::AbstractMethod => KnownModule::Abc,
 
-                KnownFunction::Dataclass => KnownModule::Dataclasses,
+                KnownFunction::Dataclass | KnownFunction::Field => KnownModule::Dataclasses,
 
                 KnownFunction::GetattrStatic => KnownModule::Inspect,
 

crates/ty_python_semantic/src/types/infer.rs

@@ -10021,6 +10021,15 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
                     }
                     Type::unknown()
                 }
+                KnownInstanceType::Field(_) => {
+                    self.infer_type_expression(&subscript.slice);
+                    if let Some(builder) = self.context.report_lint(&INVALID_TYPE_FORM, subscript) {
+                        builder.into_diagnostic(format_args!(
+                            "`dataclasses.Field` is not allowed in type expressions",
+                        ));
+                    }
+                    Type::unknown()
+                }
                 KnownInstanceType::TypeVar(_) => {
                     self.infer_type_expression(&subscript.slice);
                     todo_type!("TypeVar annotations")