Disowned the Visitor.

[eddyb]

The primary user of @T/P<T> references from Visitor was ast_map, which in turn had two users (phase 3 in rustc and a step in loading items from metadata).
Both of them have been rewritten to use ast_map as a folder (this might speed up the compilation time of stage2 rustc by 100-200ms just because a fold + a visit are merged together).

[eddyb]

I've missed the second user of @T - borrowck - which used to store @Exprs and @Pats in move_data::MoveKind.
I've attempted giving Visitor an explicit lifetime (which could propagate downwards - the lifetime of every reference is the lifetime of the crate you're vising), but even without considering the refactoring implications, I've hit an ICE cold dead end - see #5121.

cc @pcwalton this might interfere with your plans to remove @mut from libsyntax.

[pcwalton]

It's probably easier to just change the borrow checker to not store reference counted expressions in it.

[alexcrichton]

Closing for now. Sadly we have a very large queue, and I'd like to keep the queue clear of "work in progress" patches that aren't necessarily ready for merging yet.

This is some awesome work though, and thanks!

[eddyb]

@pcwalton turns out the node ID is already there, the @Exprs and @Pats aren't required at all (and looking up the ID is only needed for error reporting).

[emberian]

change to expect with a msg, to make the eventual ICE less unpleasant

[eddyb]

That is curious. I didn't realize ast::Pat are not truly part of the AST map, only node_locals which come from PatIdent (and contain only the Ident) or self.
Because of self being a special case, the best I can do is add an Option<@Pat> in node_local (won't make ast_node larger) and use that when a span is required.