[Feature Request] general warning annotations on declarations

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Add a general purpose annotation that allows attaching custom warnings to declarations (functions, methods, structs, traits, etc.), surfaced when the symbol is used.

This would generalize the existing @deprecated mechanism without implying removal, allowing library authors to communicate information such as experimental status or behavioral notes.

What is your motivation for this change?

A general warning annotation would improve API clarity and user experience while keeping changes without breaking.

Any other details?

@mark(status="experimental", "May change without notice")
struct Foo:
    ...

or

@status("experimental", "May change without notice")
struct Foo:
    ...

or

@warning("private", "This is intended for internal use and is not part of the public API.")
fn internalHelper():
    ...

Exact naming, syntax are open for discussion.

Start by reviewing Mojo's existing @deprecated mechanism and how declaration-use warnings are surfaced. The issue is complete when an agreed general-purpose annotation design can attach custom warnings to declarations and display them when the symbol is used; naming and syntax remain open for discussion.