Q# 2.0 Tracking Issue

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This issue tracks the tasks we'd like to have done before the release of Q# 2.0.

• Fixed-Length Arrays #2048
• Deprecate Explicit Namespaces (no issue yet)
• Deprecate use qubit blocks
• Deprecate w/ syntax
• Old UDT syntax
• Deprecate Microsoft.Quantum.* libraries
• remove set keyword (see Make set keyword optional and include lint to suggest avoiding it. #1496) (Deprecate set Keyword #2062)
• remove ... syntax from specialization definitions
• asterisk syntax on functors

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Process of Upgrading Q# Syntax

For anything breaking, primarily the removal or modification of existing Q# syntax, we want to be in the following state before Q# 2.0:

1. A lint warning will inform the user that something is deprecated.
2. The v2-preview-syntax feature flag allows the user to opt-in to v2 syntax, meaning that the aforementioned lints become compile errors.
3. There needs to be a clearly documented path to go from deprecated syntax to v2 syntax. Ideally, there's also a quick fix attached to the lint to make this even easier.
4. Update our libraries to reflect the change and make sure they use best practices.

We should define a minimum deprecation time period such that items are deprecated for at least that long before they are removed. During this period, there should also be new syntax available.