diff options
Diffstat (limited to 'compiler/rustc_expand/src/mbe/transcribe.rs')
| -rw-r--r-- | compiler/rustc_expand/src/mbe/transcribe.rs | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/compiler/rustc_expand/src/mbe/transcribe.rs b/compiler/rustc_expand/src/mbe/transcribe.rs index 3901b82eb52..8a084dcb4fe 100644 --- a/compiler/rustc_expand/src/mbe/transcribe.rs +++ b/compiler/rustc_expand/src/mbe/transcribe.rs @@ -253,8 +253,23 @@ pub(super) fn transcribe<'a>( mbe::TokenTree::MetaVar(mut sp, mut original_ident) => { // Find the matched nonterminal from the macro invocation, and use it to replace // the meta-var. + // + // We use `Spacing::Alone` everywhere here, because that's the conservative choice + // and spacing of declarative macros is tricky. E.g. in this macro: + // ``` + // macro_rules! idents { + // ($($a:ident,)*) => { stringify!($($a)*) } + // } + // ``` + // `$a` has no whitespace after it and will be marked `JointHidden`. If you then + // call `idents!(x,y,z,)`, each of `x`, `y`, and `z` will be marked as `Joint`. So + // if you choose to use `$x`'s spacing or the identifier's spacing, you'll end up + // producing "xyz", which is bad because it effectively merges tokens. + // `Spacing::Alone` is the safer option. Fortunately, `space_between` will avoid + // some of the unnecessary whitespace. let ident = MacroRulesNormalizedIdent::new(original_ident); if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) { + // njn: explain the use of alone here let tt = match cur_matched { MatchedSingle(ParseNtResult::Tt(tt)) => { // `tt`s are emitted into the output stream directly as "raw tokens", |