Rollup merge of #78603 - petrochenkov:fourdigits, r=matthewjasper

expand: Tweak a comment in implementation of `macro_rules`

The answer to the removed FIXME is that we don't apply mark to the span `sp` just because that span is no longer used. We could apply it, but that would just be unnecessary extra work.

The comments in code tell why the span is unused, it's a span of `$var` literally, which is lost for `tt` variables because their tokens are outputted directly, but kept for other variables which are outputted as [groups](https://doc.rust-lang.org/nightly/proc_macro/struct.Group.html) and `sp` is kept as the group's span.

Closes #2887

Author: m-ou-se

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -232,17 +232,19 @@ pub(super) fn transcribe<'a>(
                 // the meta-var.
                 let ident = MacroRulesNormalizedIdent::new(orignal_ident);
                 if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
-                    if let MatchedNonterminal(ref nt) = cur_matched {
-                        // FIXME #2887: why do we apply a mark when matching a token tree meta-var
-                        // (e.g. `$x:tt`), but not when we are matching any other type of token
-                        // tree?
-                        if let NtTT(ref tt) = **nt {
-                            result.push(tt.clone().into());
+                    if let MatchedNonterminal(nt) = cur_matched {
+                        let token = if let NtTT(tt) = &**nt {
+                            // `tt`s are emitted into the output stream directly as "raw tokens",
+                            // without wrapping them into groups.
+                            tt.clone()
                         } else {
+                            // Other variables are emitted into the output stream as groups with
+                            // `Delimiter::None` to maintain parsing priorities.
+                            // `Interpolated` is currenty used for such groups in rustc parser.
                             marker.visit_span(&mut sp);
-                            let token = TokenTree::token(token::Interpolated(nt.clone()), sp);
-                            result.push(token.into());
-                        }
+                            TokenTree::token(token::Interpolated(nt.clone()), sp)
+                        };
+                        result.push(token.into());
                     } else {
                         // We were unable to descend far enough. This is an error.
                         return Err(cx.struct_span_err(