From f6e7c81061c24a2210149ff94670d8672bd0224a Mon Sep 17 00:00:00 2001
From: jullang <jullanggit@proton.me>
Date: Sun, 31 Aug 2025 19:27:03 +0000
Subject: [PATCH] Add compiler error when trying to use concat metavar expr in
 repetitions

Replace unimplemented()! with a more helpful compiler error.
---
 compiler/rustc_expand/src/mbe/transcribe.rs   |  7 ++++++-
 tests/crashes/140479.rs                       |  5 -----
 .../in-repetition.rs                          | 21 +++++++++++++++++++
 .../in-repetition.stderr                      |  8 +++++++
 4 files changed, 35 insertions(+), 6 deletions(-)
 delete mode 100644 tests/crashes/140479.rs
 create mode 100644 tests/ui/macros/macro-metavar-expr-concat/in-repetition.rs
 create mode 100644 tests/ui/macros/macro-metavar-expr-concat/in-repetition.stderr

diff --git a/compiler/rustc_expand/src/mbe/transcribe.rs b/compiler/rustc_expand/src/mbe/transcribe.rs
index 174844d6ad639..e4e4866b64cdf 100644
--- a/compiler/rustc_expand/src/mbe/transcribe.rs
+++ b/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)?,
                         }
                     }
diff --git a/tests/crashes/140479.rs b/tests/crashes/140479.rs
deleted file mode 100644
index ed3ca887546f9..0000000000000
--- a/tests/crashes/140479.rs
+++ /dev/null
@@ -1,5 +0,0 @@
-//@ known-bug: #140479
-macro_rules! a { ( $( { $ [ $b:c ] } )) => ( $(${ concat(d, $b)} ))}
-fn e() {
-    a!({})
-}
diff --git a/tests/ui/macros/macro-metavar-expr-concat/in-repetition.rs b/tests/ui/macros/macro-metavar-expr-concat/in-repetition.rs
new file mode 100644
index 0000000000000..d2bd31b06d606
--- /dev/null
+++ b/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() {}
diff --git a/tests/ui/macros/macro-metavar-expr-concat/in-repetition.stderr b/tests/ui/macros/macro-metavar-expr-concat/in-repetition.stderr
new file mode 100644
index 0000000000000..ec39ca799e198
--- /dev/null
+++ b/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
+