Add compiler error when trying to use concat metavar expr in repetitions

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -556,7 +556,12 @@ fn metavar_expr_concat<'tx>(
                         };
                         match &named_matches[*curr_idx] {
                             // FIXME(c410-f3r) Nested repetitions are unimplemented
-                            MatchedSeq(_) => unimplemented!(),
+                            MatchedSeq(_) => {
+                                return Err(dcx.struct_span_err(
+                                    ident.span,
+                                    "nested repetitions with `${concat(...)}` metavariable expressions are not yet supported",
+                                ));
+                            }
                             MatchedSingle(pnr) => extract_symbol_from_pnr(dcx, pnr, ident.span)?,
                         }
                     }

tests/ui/macros/macro-metavar-expr-concat/in-repetition.rs

@@ -0,0 +1,21 @@
+// issue: <https://github.com/rust-lang/rust/issues/140479>
+// Ensure a proper compiler error, instead of an ICE occurs.
+// FIXME(macro_metavar_expr_concat): this error message could be improved
+#![feature(macro_metavar_expr_concat)]
+
+macro_rules! InRepetition {
+    (
+        $(
+            $($arg:ident),+
+        )+
+     ) => {
+        $(
+            $(
+                ${concat(_, $arg)} //~ ERROR nested repetitions with `${concat(...)}` metavariable expressions are not yet supported
+            )*
+        )*
+    };
+}
+InRepetition!(other);
+
+fn main() {}

tests/ui/macros/macro-metavar-expr-concat/in-repetition.stderr

@@ -0,0 +1,8 @@
+error: nested repetitions with `${concat(...)}` metavariable expressions are not yet supported
+  --> $DIR/in-repetition.rs:14:30
+   |
+LL |                 ${concat(_, $arg)}
+   |                              ^^^
+
+error: aborting due to 1 previous error
+