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removed incomplete comment

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as written, I don't believe this comment was helpful; I think it's
better just to steer the reader toward a general understanding of
hygiene.
This commit is contained in:
John Clements 2014-06-27 14:12:13 -07:00
parent 351a5fd2b4
commit 268f6c56c2
1 changed files with 0 additions and 17 deletions
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17
src/libsyntax/fold.rs
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@ -391,23 +391,6 @@ fn fold_arg_<T: Folder>(a: &Arg, fld: &mut T) -> Arg {
// build a new vector of tts by appling the Folder's fold_ident to
// all of the identifiers in the token trees.
//
// This is part of hygiene magic. As far as hygiene is concerned, there
// are three types of let pattern bindings or loop labels:
// - those defined and used in non-macro part of the program
// - those used as part of macro invocation arguments
// - those defined and used inside macro definitions
// Lexically, type 1 and 2 are in one group and type 3 the other. If they
// clash, in order for let and loop label to work hygienically, one group
// or the other needs to be renamed. The problem is that type 2 and 3 are
// parsed together (inside the macro expand function). After being parsed and
// AST being constructed, they can no longer be distinguished from each other.
//
// For that reason, type 2 let bindings and loop labels are actually renamed
// in the form of tokens instead of AST nodes, here. There are wasted effort
// since many token::IDENT are not necessary part of let bindings and most
// token::LIFETIME are certainly not loop labels. But we can't tell in their
// token form. So this is less ideal and hacky but it works.
pub fn fold_tts<T: Folder>(tts: &[TokenTree], fld: &mut T) -> Vec<TokenTree> {
tts.iter().map(|tt| {
match *tt {