FR: Metadata for template/revset aliases (types, documentation)

[jakobhellermann]

the idea came up in #5541 (review)

Currently, template aliases (builtin_op_log_compact) and template alias functions ('format_short_id(id)' = 'id.shortest(8)') are defined without any type information available.
If you could (optionally) describe what type they expect, that would enable a few potential use cases:

• jj log -T <tab> could only suggest template aliases that accept a commit
• if there was an LSP for the template it would benefit from having this information available

---

How could this look like?

Functions could simply be extended with the obvious syntax for declaring types:

'commit_timestamp(commit: Commit)' = 'commit.committer().timestamp()'

Pure symbols like builtin_log_compact that still take a self parameter (with implicit property access) could maybe look like this:

'builtin_log_compact(self: Commit)' = '''
if(root,
  format_root_commit(self),
  foo # implicitly self.foo
  ...
)
'''

If you wanted to add a docstring, maybe you could just have a comment in the first line (given #5317 is implemented)

'commit_timestamp(commit: Commit)' = 'commit.committer().timestamp()'

That's basically how python does it: https://realpython.com/documenting-python-code/#basics-of-commenting-code

[yuja]

'builtin_log_compact(self: Commit)' = '''

nit: Aliases are kinda syntactic macro, so f() and f are different. Suppose : Type would be processed as a comment during alias substitution, it's weird that the implicit self type is annotated as a function argument.