[ty] IDE: add support for object.<CURSOR> completions (#18468)

This PR adds logic for detecting `Name Dot [Name]` token patterns,
finding the corresponding `ExprAttribute`, getting the type of the
object and returning the members available on that object.

Here's a video demonstrating this working:

https://github.com/user-attachments/assets/42ce78e8-5930-4211-a18a-fa2a0434d0eb

Ref astral-sh/ty#86

crates/ty_python_semantic/src/semantic_model.rs

@@ -41,12 +41,18 @@ impl<'db> SemanticModel<'db> {
         resolve_module(self.db, module_name)
     }
 
+    /// Returns completions for symbols available in a `object.<CURSOR>` context.
+    pub fn attribute_completions(&self, node: &ast::ExprAttribute) -> Vec<Name> {
+        let ty = node.value.inferred_type(self);
+        crate::types::all_members(self.db, ty).into_iter().collect()
+    }
+
     /// Returns completions for symbols available in the scope containing the
     /// given expression.
     ///
     /// If a scope could not be determined, then completions for the global
     /// scope of this model's `File` are returned.
-    pub fn completions(&self, node: ast::AnyNodeRef<'_>) -> Vec<Name> {
+    pub fn scoped_completions(&self, node: ast::AnyNodeRef<'_>) -> Vec<Name> {
         let index = semantic_index(self.db, self.file);
 
         // TODO: We currently use `try_expression_scope_id` here as a hotfix for [1].

crates/ty_python_semantic/src/types.rs

@@ -45,6 +45,7 @@ use crate::types::function::{
     DataclassTransformerParams, FunctionSpans, FunctionType, KnownFunction,
 };
 use crate::types::generics::{GenericContext, PartialSpecialization, Specialization};
+pub use crate::types::ide_support::all_members;
 use crate::types::infer::infer_unpack_types;
 use crate::types::mro::{Mro, MroError, MroIterator};
 pub(crate) use crate::types::narrow::infer_narrowing_constraint;

crates/ty_python_semantic/src/types/ide_support.rs

@@ -195,6 +195,6 @@ impl AllMembers {
 
 /// List all members of a given type: anything that would be valid when accessed
 /// as an attribute on an object of the given type.
-pub(crate) fn all_members<'db>(db: &'db dyn Db, ty: Type<'db>) -> FxHashSet<Name> {
+pub fn all_members<'db>(db: &'db dyn Db, ty: Type<'db>) -> FxHashSet<Name> {
     AllMembers::of(db, ty).members
 }