[red-knot] implement basic call expression inference (astral-sh#13164)

## Summary

Adds basic support for inferring the type resulting from a call
expression. This only works for the *result* of call expressions; it
performs no inference on parameters. It also intentionally does nothing
with class instantiation, `__call__` implementors, or lambdas.

## Test Plan

Adds a test that it infers the right thing!

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/src/types.rs

@@ -320,6 +320,33 @@ impl<'db> Type<'db> {
         }
     }
 
+    /// Return the type resulting from calling an object of this type.
+    ///
+    /// Returns `None` if `self` is not a callable type.
+    #[must_use]
+    pub fn call(&self, db: &'db dyn Db) -> Option<Type<'db>> {
+        match self {
+            Type::Function(function_type) => function_type.returns(db).or(Some(Type::Unknown)),
+
+            // TODO: handle class constructors
+            Type::Class(_class_ty) => Some(Type::Unknown),
+
+            // TODO: handle classes which implement the Callable protocol
+            Type::Instance(_instance_ty) => Some(Type::Unknown),
+
+            // `Any` is callable, and its return type is also `Any`.
+            Type::Any => Some(Type::Any),
+
+            Type::Unknown => Some(Type::Unknown),
+
+            // TODO: union and intersection types, if they reduce to `Callable`
+            Type::Union(_) => Some(Type::Unknown),
+            Type::Intersection(_) => Some(Type::Unknown),
+
+            _ => None,
+        }
+    }
+
     #[must_use]
     pub fn instance(&self) -> Type<'db> {
         match self {
@@ -550,4 +577,25 @@ mod tests {
         let b_file_diagnostics = super::check_types(&db, b_file);
         assert_eq!(&*b_file_diagnostics, &[]);
     }
+
+    #[test]
+    fn invalid_callable() {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            nonsense = 123
+            x = nonsense()
+            ",
+        )
+        .unwrap();
+
+        let a_file = system_path_to_file(&db, "/src/a.py").unwrap();
+        let a_file_diagnostics = super::check_types(&db, a_file);
+        assert_diagnostic_messages(
+            &a_file_diagnostics,
+            &["Object of type 'Literal[123]' is not callable"],
+        );
+    }
 }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -868,19 +868,16 @@ impl<'db> TypeInferenceBuilder<'db> {
             value,
         } = assignment;
 
-        // TODO remove once we infer definitions in unpacking assignment, since that infers the RHS
-        // too, and uses the `infer_expression_types` query to do it
-        self.infer_expression(value);
-
         for target in targets {
-            match target {
-                ast::Expr::Name(name) => {
-                    self.infer_definition(name);
-                }
-                _ => {
-                    // TODO infer definitions in unpacking assignment
-                    self.infer_expression(target);
-                }
+            if let ast::Expr::Name(name) = target {
+                self.infer_definition(name);
+            } else {
+                // TODO infer definitions in unpacking assignment. When we do, this duplication of
+                // the "get `Expression`, call `infer_expression_types` on it, `self.extend`" dance
+                // will be removed; it'll all happen in `infer_assignment_definition` instead.
+                let expression = self.index.expression(value.as_ref());
+                self.extend(infer_expression_types(self.db, expression));
+                self.infer_expression(target);
             }
         }
     }
@@ -1363,7 +1360,8 @@ impl<'db> TypeInferenceBuilder<'db> {
         };
 
         let expr_id = expression.scoped_ast_id(self.db, self.scope);
-        self.types.expressions.insert(expr_id, ty);
+        let previous = self.types.expressions.insert(expr_id, ty);
+        assert!(previous.is_none());
 
         ty
     }
@@ -1746,10 +1744,18 @@ impl<'db> TypeInferenceBuilder<'db> {
         } = call_expression;
 
         self.infer_arguments(arguments);
-        self.infer_expression(func);
-
-        // TODO resolve to return type of `func`, if its a callable type
-        Type::Unknown
+        let function_type = self.infer_expression(func);
+        function_type.call(self.db).unwrap_or_else(|| {
+            self.add_diagnostic(
+                func.as_ref().into(),
+                "call-non-callable",
+                format_args!(
+                    "Object of type '{}' is not callable",
+                    function_type.display(self.db)
+                ),
+            );
+            Type::Unknown
+        })
     }
 
     fn infer_starred_expression(&mut self, starred: &ast::ExprStarred) -> Type<'db> {
@@ -2247,7 +2253,8 @@ impl<'db> TypeInferenceBuilder<'db> {
         };
 
         let expr_id = expression.scoped_ast_id(self.db, self.scope);
-        self.types.expressions.insert(expr_id, ty);
+        let previous = self.types.expressions.insert(expr_id, ty);
+        assert!(previous.is_none());
 
         ty
     }
@@ -2808,6 +2815,25 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn basic_call_expression() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/a.py",
+            "
+            def get_int() -> int:
+                return 42
+
+            x = get_int()
+            ",
+        )?;
+
+        assert_public_ty(&db, "src/a.py", "x", "int");
+
+        Ok(())
+    }
+
     #[test]
     fn resolve_union() -> anyhow::Result<()> {
         let mut db = setup_db();

crates/ruff_benchmark/benches/red_knot.rs

@@ -24,6 +24,7 @@ const TOMLLIB_312_URL: &str = "https://raw.githubusercontent.com/python/cpython/
 // The "unresolved import" is because we don't understand `*` imports yet.
 static EXPECTED_DIAGNOSTICS: &[&str] = &[
     "/src/tomllib/_parser.py:7:29: Module 'collections.abc' has no member 'Iterable'",
+    "/src/tomllib/_parser.py:686:23: Object of type 'Unbound' is not callable",
     "Line 69 is too long (89 characters)",
     "Use double quotes for strings",
     "Use double quotes for strings",