[red-knot] Remove `Type::None` (#14024)

## Summary

Removes `Type::None` in favor of `KnownClass::NoneType.to_instance(…)`.

closes #13670

## Performance

There is a -4% performance regression on our red-knot benchmark. This is due to the fact that we now have to import `_typeshed` as a module, and infer types.

## Test Plan

Existing tests pass.

crates/red_knot_python_semantic/src/types.rs

@@ -308,8 +308,6 @@ pub enum Type<'db> {
     /// Unknown type (either no annotation, or some kind of type error).
     /// Equivalent to Any, or possibly to object in strict mode
     Unknown,
-    /// The None object -- TODO remove this in favor of Instance(types.NoneType)
-    None,
     /// Temporary type for symbols that can't be inferred yet because of missing implementations.
     /// Behaves equivalently to `Any`.
     ///
@@ -564,10 +562,17 @@ impl<'db> Type<'db> {
     }
 
     /// Return true if this type is equivalent to type `other`.
-    pub(crate) fn is_equivalent_to(self, _db: &'db dyn Db, other: Type<'db>) -> bool {
+    pub(crate) fn is_equivalent_to(self, db: &'db dyn Db, other: Type<'db>) -> bool {
         // TODO equivalent but not identical structural types, differently-ordered unions and
         // intersections, other cases?
+
+        // TODO: The following is a workaround that is required to unify the two different
+        // versions of `NoneType` in typeshed. This should not be required anymore once we
+        // understand `sys.version_info` branches.
         self == other
+            || matches!((self, other),
+                (Type::Instance(self_class), Type::Instance(target_class))
+                    if self_class.is_known(db, KnownClass::NoneType) && target_class.is_known(db, KnownClass::NoneType))
     }
 
     /// Return true if this type and `other` have no common elements.
@@ -603,8 +608,7 @@ impl<'db> Type<'db> {
             }
 
             (
-                left @ (Type::None
-                | Type::BooleanLiteral(..)
+                left @ (Type::BooleanLiteral(..)
                 | Type::IntLiteral(..)
                 | Type::StringLiteral(..)
                 | Type::BytesLiteral(..)
@@ -612,8 +616,7 @@ impl<'db> Type<'db> {
                 | Type::FunctionLiteral(..)
                 | Type::ModuleLiteral(..)
                 | Type::ClassLiteral(..)),
-                right @ (Type::None
-                | Type::BooleanLiteral(..)
+                right @ (Type::BooleanLiteral(..)
                 | Type::IntLiteral(..)
                 | Type::StringLiteral(..)
                 | Type::BytesLiteral(..)
@@ -623,13 +626,20 @@ impl<'db> Type<'db> {
                 | Type::ClassLiteral(..)),
             ) => left != right,
 
-            (Type::None, Type::Instance(class_type)) | (Type::Instance(class_type), Type::None) => {
+            (Type::Instance(class_none), Type::Instance(class_other))
+            | (Type::Instance(class_other), Type::Instance(class_none))
+                if class_none.is_known(db, KnownClass::NoneType) =>
+            {
                 !matches!(
-                    class_type.known(db),
+                    class_other.known(db),
                     Some(KnownClass::NoneType | KnownClass::Object)
                 )
             }
-            (Type::None, _) | (_, Type::None) => true,
+            (Type::Instance(class_none), _) | (_, Type::Instance(class_none))
+                if class_none.is_known(db, KnownClass::NoneType) =>
+            {
+                true
+            }
 
             (Type::BooleanLiteral(..), Type::Instance(class_type))
             | (Type::Instance(class_type), Type::BooleanLiteral(..)) => !matches!(
@@ -687,8 +697,9 @@ impl<'db> Type<'db> {
 
             (Type::Instance(..), Type::Instance(..)) => {
                 // TODO: once we have support for `final`, there might be some cases where
-                // we can determine that two types are disjoint. For non-final classes, we
-                // return false (multiple inheritance).
+                // we can determine that two types are disjoint. Once we do this, some cases
+                // above (e.g. NoneType) can be removed. For non-final classes, we return
+                // false (multiple inheritance).
 
                 // TODO: is there anything specific to do for instances of KnownClass::Type?
 
@@ -726,13 +737,12 @@ impl<'db> Type<'db> {
     ///
     /// Note: This function aims to have no false positives, but might return `false`
     /// for more complicated types that are actually singletons.
-    pub(crate) fn is_singleton(self) -> bool {
+    pub(crate) fn is_singleton(self, db: &'db dyn Db) -> bool {
         match self {
             Type::Any
             | Type::Never
             | Type::Unknown
             | Type::Todo
-            | Type::Instance(..) // TODO some instance types can be singleton types (EllipsisType, NotImplementedType)
             | Type::IntLiteral(..)
             | Type::StringLiteral(..)
             | Type::BytesLiteral(..)
@@ -743,7 +753,14 @@ impl<'db> Type<'db> {
                 // are both of type Literal[345], for example.
                 false
             }
-            Type::None | Type::BooleanLiteral(_) | Type::FunctionLiteral(..) | Type::ClassLiteral(..) | Type::ModuleLiteral(..) => true,
+            Type::BooleanLiteral(_)
+            | Type::FunctionLiteral(..)
+            | Type::ClassLiteral(..)
+            | Type::ModuleLiteral(..) => true,
+            Type::Instance(class) => {
+                // TODO some more instance types can be singleton types (EllipsisType, NotImplementedType)
+                matches!(class.known(db), Some(KnownClass::NoneType))
+            }
             Type::Tuple(..) => {
                 // The empty tuple is a singleton on CPython and PyPy, but not on other Python
                 // implementations such as GraalPy. Its *use* as a singleton is discouraged and
@@ -774,8 +791,7 @@ impl<'db> Type<'db> {
     /// Return true if this type is non-empty and all inhabitants of this type compare equal.
     pub(crate) fn is_single_valued(self, db: &'db dyn Db) -> bool {
         match self {
-            Type::None
-            | Type::FunctionLiteral(..)
+            Type::FunctionLiteral(..)
             | Type::ModuleLiteral(..)
             | Type::ClassLiteral(..)
             | Type::IntLiteral(..)
@@ -835,10 +851,6 @@ impl<'db> Type<'db> {
                 Type::Todo.into()
             }
             Type::Unknown => Type::Unknown.into(),
-            Type::None => {
-                // TODO: attribute lookup on None type
-                Type::Todo.into()
-            }
             Type::FunctionLiteral(_) => {
                 // TODO: attribute lookup on function type
                 Type::Todo.into()
@@ -963,7 +975,6 @@ impl<'db> Type<'db> {
     fn bool(&self, db: &'db dyn Db) -> Truthiness {
         match self {
             Type::Any | Type::Todo | Type::Never | Type::Unknown => Truthiness::Ambiguous,
-            Type::None => Truthiness::AlwaysFalse,
             Type::FunctionLiteral(_) => Truthiness::AlwaysTrue,
             Type::ModuleLiteral(_) => Truthiness::AlwaysTrue,
             Type::ClassLiteral(_) => {
@@ -971,10 +982,15 @@ impl<'db> Type<'db> {
                 // More info in https://docs.python.org/3/library/stdtypes.html#truth-value-testing
                 Truthiness::Ambiguous
             }
-            Type::Instance(_) => {
+            Type::Instance(class) => {
                 // TODO: lookup `__bool__` and `__len__` methods on the instance's class
                 // More info in https://docs.python.org/3/library/stdtypes.html#truth-value-testing
-                Truthiness::Ambiguous
+                // For now, we only special-case some builtin classes
+                if class.is_known(db, KnownClass::NoneType) {
+                    Truthiness::AlwaysFalse
+                } else {
+                    Truthiness::Ambiguous
+                }
             }
             Type::Union(union) => {
                 let union_elements = union.elements(db);
@@ -1164,11 +1180,15 @@ impl<'db> Type<'db> {
             | Type::StringLiteral(_)
             | Type::SliceLiteral(_)
             | Type::Tuple(_)
-            | Type::LiteralString
-            | Type::None => Type::Unknown,
+            | Type::LiteralString => Type::Unknown,
         }
     }
 
+    /// The type `NoneType` / `None`
+    pub fn none(db: &'db dyn Db) -> Type<'db> {
+        KnownClass::NoneType.to_instance(db)
+    }
+
     /// Given a type that is assumed to represent an instance of a class,
     /// return a type that represents that class itself.
     #[must_use]
@@ -1184,7 +1204,6 @@ impl<'db> Type<'db> {
             Type::FunctionLiteral(_) => KnownClass::FunctionType.to_class(db),
             Type::ModuleLiteral(_) => KnownClass::ModuleType.to_class(db),
             Type::Tuple(_) => KnownClass::Tuple.to_class(db),
-            Type::None => KnownClass::NoneType.to_class(db),
             // TODO not accurate if there's a custom metaclass...
             Type::ClassLiteral(_) => KnownClass::Type.to_class(db),
             // TODO can we do better here? `type[LiteralString]`?
@@ -2026,7 +2045,7 @@ mod tests {
             match self {
                 Ty::Never => Type::Never,
                 Ty::Unknown => Type::Unknown,
-                Ty::None => Type::None,
+                Ty::None => Type::none(db),
                 Ty::Any => Type::Any,
                 Ty::Todo => Type::Todo,
                 Ty::IntLiteral(n) => Type::IntLiteral(n),
@@ -2266,7 +2285,7 @@ mod tests {
     fn is_singleton(from: Ty) {
         let db = setup_db();
 
-        assert!(from.into_type(&db).is_singleton());
+        assert!(from.into_type(&db).is_singleton(&db));
     }
 
     #[test_case(Ty::None)]
@@ -2304,7 +2323,7 @@ mod tests {
     fn is_not_singleton(from: Ty) {
         let db = setup_db();
 
-        assert!(!from.into_type(&db).is_singleton());
+        assert!(!from.into_type(&db).is_singleton(&db));
     }
 
     #[test_case(Ty::IntLiteral(1); "is_int_literal_truthy")]

crates/red_knot_python_semantic/src/types/builder.rs

@@ -675,8 +675,8 @@ mod tests {
     fn build_intersection_self_negation() {
         let db = setup_db();
         let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::None)
-            .add_negative(Type::None)
+            .add_positive(Type::none(&db))
+            .add_negative(Type::none(&db))
             .build();
 
         assert_eq!(ty, Type::Never);
@@ -686,18 +686,18 @@ mod tests {
     fn build_intersection_simplify_negative_never() {
         let db = setup_db();
         let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::None)
+            .add_positive(Type::none(&db))
             .add_negative(Type::Never)
             .build();
 
-        assert_eq!(ty, Type::None);
+        assert_eq!(ty, Type::none(&db));
     }
 
     #[test]
     fn build_intersection_simplify_positive_never() {
         let db = setup_db();
         let ty = IntersectionBuilder::new(&db)
-            .add_positive(Type::None)
+            .add_positive(Type::none(&db))
             .add_positive(Type::Never)
             .build();
 
@@ -709,14 +709,14 @@ mod tests {
         let db = setup_db();
 
         let ty = IntersectionBuilder::new(&db)
-            .add_negative(Type::None)
+            .add_negative(Type::none(&db))
             .add_positive(Type::IntLiteral(1))
             .build();
         assert_eq!(ty, Type::IntLiteral(1));
 
         let ty = IntersectionBuilder::new(&db)
             .add_positive(Type::IntLiteral(1))
-            .add_negative(Type::None)
+            .add_negative(Type::none(&db))
             .build();
         assert_eq!(ty, Type::IntLiteral(1));
     }
@@ -875,7 +875,7 @@ mod tests {
         let db = setup_db();
 
         let t1 = Type::IntLiteral(1);
-        let t2 = Type::None;
+        let t2 = Type::none(&db);
 
         let ty = IntersectionBuilder::new(&db)
             .add_positive(t1)

crates/red_knot_python_semantic/src/types/display.rs

@@ -6,7 +6,7 @@ use ruff_db::display::FormatterJoinExtension;
 use ruff_python_ast::str::Quote;
 use ruff_python_literal::escape::AsciiEscape;
 
-use crate::types::{IntersectionType, Type, UnionType};
+use crate::types::{IntersectionType, KnownClass, Type, UnionType};
 use crate::Db;
 use rustc_hash::FxHashMap;
 
@@ -64,7 +64,9 @@ impl Display for DisplayRepresentation<'_> {
             Type::Any => f.write_str("Any"),
             Type::Never => f.write_str("Never"),
             Type::Unknown => f.write_str("Unknown"),
-            Type::None => f.write_str("None"),
+            Type::Instance(class) if class.is_known(self.db, KnownClass::NoneType) => {
+                f.write_str("None")
+            }
             // `[Type::Todo]`'s display should be explicit that is not a valid display of
             // any other type
             Type::Todo => f.write_str("@Todo"),
@@ -380,7 +382,7 @@ mod tests {
             global_symbol(&db, mod_file, "bar").expect_type(),
             global_symbol(&db, mod_file, "B").expect_type(),
             Type::BooleanLiteral(true),
-            Type::None,
+            Type::none(&db),
         ];
         let union = UnionType::from_elements(&db, union_elements).expect_union();
         let display = format!("{}", union.display(&db));

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1149,7 +1149,12 @@ impl<'db> TypeInferenceBuilder<'db> {
                         if exit_ty
                             .call(
                                 self.db,
-                                &[context_manager_ty, Type::None, Type::None, Type::None],
+                                &[
+                                    context_manager_ty,
+                                    Type::none(self.db),
+                                    Type::none(self.db),
+                                    Type::none(self.db),
+                                ],
                             )
                             .return_ty_result(
                                 self.db,
@@ -1877,7 +1882,7 @@ impl<'db> TypeInferenceBuilder<'db> {
 
     fn infer_expression(&mut self, expression: &ast::Expr) -> Type<'db> {
         let ty = match expression {
-            ast::Expr::NoneLiteral(ast::ExprNoneLiteral { range: _ }) => Type::None,
+            ast::Expr::NoneLiteral(ast::ExprNoneLiteral { range: _ }) => Type::none(self.db),
             ast::Expr::NumberLiteral(literal) => self.infer_number_literal_expression(literal),
             ast::Expr::BooleanLiteral(literal) => self.infer_boolean_literal_expression(literal),
             ast::Expr::StringLiteral(literal) => self.infer_string_literal_expression(literal),
@@ -3582,7 +3587,6 @@ impl<'db> TypeInferenceBuilder<'db> {
                 Err(_) => SliceArg::Unsupported,
             },
             Some(Type::BooleanLiteral(b)) => SliceArg::Arg(Some(i32::from(b))),
-            Some(Type::None) => SliceArg::Arg(None),
             Some(Type::Instance(class)) if class.is_known(self.db, KnownClass::NoneType) => {
                 SliceArg::Arg(None)
             }
@@ -3686,7 +3690,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                     .to_instance(self.db)
             }
 
-            ast::Expr::NoneLiteral(_literal) => Type::None,
+            ast::Expr::NoneLiteral(_literal) => Type::none(self.db),
 
             // TODO: parse the expression and check whether it is a string annotation.
             // https://typing.readthedocs.io/en/latest/spec/annotations.html#string-annotations

crates/red_knot_python_semantic/src/types/narrow.rs

@@ -242,7 +242,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
 
                 match if is_positive { *op } else { op.negate() } {
                     ast::CmpOp::IsNot => {
-                        if rhs_ty.is_singleton() {
+                        if rhs_ty.is_singleton(self.db) {
                             let ty = IntersectionBuilder::new(self.db)
                                 .add_negative(rhs_ty)
                                 .build();
@@ -316,7 +316,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             let symbol = self.symbols().symbol_id_by_name(id).unwrap();
 
             let ty = match pattern.value {
-                ast::Singleton::None => Type::None,
+                ast::Singleton::None => Type::none(self.db),
                 ast::Singleton::True => Type::BooleanLiteral(true),
                 ast::Singleton::False => Type::BooleanLiteral(false),
             };