[macro_metavar_expr_concat] Allow concat in repetitions

Author: c410-f3r

compiler/rustc_expand/src/mbe/macro_check.rs

@@ -352,10 +352,10 @@ fn check_occurrences(
             check_ops_is_prefix(psess, node_id, macros, binders, ops, span, name);
         }
         TokenTree::MetaVarExpr(dl, ref mve) => {
-            let Some(name) = mve.ident().map(MacroRulesNormalizedIdent::new) else {
-                return;
-            };
-            check_ops_is_prefix(psess, node_id, macros, binders, ops, dl.entire(), name);
+            mve.idents((), |_, ident| {
+                let name = MacroRulesNormalizedIdent::new(*ident);
+                check_ops_is_prefix(psess, node_id, macros, binders, ops, dl.entire(), name);
+            });
         }
         TokenTree::Delimited(.., ref del) => {
             check_nested_occurrences(psess, node_id, &del.tts, macros, binders, ops, guar);

compiler/rustc_expand/src/mbe/metavar_expr.rs

@@ -111,10 +111,18 @@ impl MetaVarExpr {
         Ok(rslt)
     }
 
-    pub(crate) fn ident(&self) -> Option<Ident> {
-        match *self {
-            MetaVarExpr::Count(ident, _) | MetaVarExpr::Ignore(ident) => Some(ident),
-            MetaVarExpr::Concat { .. } | MetaVarExpr::Index(..) | MetaVarExpr::Len(..) => None,
+    pub(crate) fn idents<A>(&self, mut aux: A, mut cb: impl FnMut(A, &Ident) -> A) -> A {
+        match self {
+            MetaVarExpr::Concat(elems) => {
+                for elem in elems {
+                    if let MetaVarExprConcatElem::Var(ident) = elem {
+                        aux = cb(aux, ident)
+                    }
+                }
+                aux
+            }
+            MetaVarExpr::Count(ident, _) | MetaVarExpr::Ignore(ident) => cb(aux, ident),
+            MetaVarExpr::Index(..) | MetaVarExpr::Len(..) => aux,
         }
     }
 }

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -556,17 +556,13 @@ fn lockstep_iter_size(
             }
         }
         TokenTree::MetaVarExpr(_, expr) => {
-            let default_rslt = LockstepIterSize::Unconstrained;
-            let Some(ident) = expr.ident() else {
-                return default_rslt;
-            };
-            let name = MacroRulesNormalizedIdent::new(ident);
-            match lookup_cur_matched(name, interpolations, repeats) {
-                Some(MatchedSeq(ads)) => {
-                    default_rslt.with(LockstepIterSize::Constraint(ads.len(), name))
+            expr.idents(LockstepIterSize::Unconstrained, |mut lis, ident| {
+                let name = MacroRulesNormalizedIdent::new(*ident);
+                if let Some(MatchedSeq(ads)) = lookup_cur_matched(name, interpolations, repeats) {
+                    lis = lis.with(LockstepIterSize::Constraint(ads.len(), name));
                 }
-                _ => default_rslt,
-            }
+                lis
+            })
         }
         TokenTree::Token(..) => LockstepIterSize::Unconstrained,
     }

tests/ui/macros/macro-metavar-expr-concat/repetitions.rs

@@ -0,0 +1,14 @@
+#![feature(macro_metavar_expr_concat)]
+
+macro_rules! many_idents_multi_metavar {
+    ($($a:ident),*) => {
+        $(
+            const ${concat($a, B)}: i32 = 3;
+            //~^ ERROR: `${concat(..)}` currently only accepts identifiers or meta-variables as parameters
+        )*
+    };
+}
+
+fn main() {
+    many_idents_multi_metavar!(A, B, C)
+}

tests/ui/macros/macro-metavar-expr-concat/repetitions.stderr

@@ -0,0 +1,8 @@
+error: `${concat(..)}` currently only accepts identifiers or meta-variables as parameters
+  --> $DIR/repetitions.rs:6:29
+   |
+LL |             const ${concat($a, B)}: i32 = 3;
+   |                             ^
+
+error: aborting due to 1 previous error
+