The original expand_node() body is migrated as follows:
- Identifier -> fold_identifier()
- FunctionCall -> fold_function_call()
expand_defn() now manages states stack by itself, which simplifies lifetime
parameters.
The templater implementation of FoldableExpression is a stripped-down version
of expand_node(). It's visitor-like because I'm going to write generic alias
substitution rules over abstract expression types (template, revset, fileset.)
Naming comes from rustc.
https://rust-unofficial.github.io/patterns/patterns/creational/fold.html
This is basically the same as the previous patch, but for error types. Some
of these functions could be encoded as "E: From<AliasExpandError<'i>>", but
alias substitution logic is recursive, so it would have to convert E back and
force.
I'm going to extract generic alias substitution functions, and these AST types
will be accessed there. Revset parsing will also be migrated to the generic
functions.
This will help extract common FunctionCallNode<'i, T> type. We don't need
freedom of arbitrary error type choices, but implementing From<_> is the
easiest option I can think of. Another option is to constrain error type by
the expression type T through "T::ParseError: ArgumentsParseError" or
something, but it seemed a bit weird that we have to use trait just for that.
