Support type annotations and type checking in Starlark

The first goal is: correctness and readability of Starlark code

Other goals are:

• improving efficiency of Starlark interpreter and
• Bazel tech-debt reduction

Roadmap:

Milestone 1 (Q4 2024) - MVP - Bootstraping Starlark types

Design doc: SEP-001 Bootstrapping Starlark types

Starlark specification is incrementally extended under the starlark-with-types branch: https://github.com/bazelbuild/starlark/blob/starlark-with-types/spec.md

Goal: Provide a minimally viable type system in Starlark

• Extend Starlark syntax with type annotations #24407
• Primitive types
• Unions, Collections, functions and constructed types
• Runtime/dynamic type checks
• Bazel/Starlark built-in types (struct)
• Launch MVP

Milestone 2 - Static type checking and type inference

Backlog

• Providers/classes
• Types for Rules & Symbolic macros
• Types for selects
• Types for Java-Starlark interface / Java annotations
• Support types in tooling & IDEs