From 1cc0e38fdc4d2ec8c9c130d7ffc9c30d100885a2 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <n.nethercote@gmail.com>
Date: Wed, 21 May 2025 13:28:08 +1000
Subject: [PATCH] Avoid creating an empty identifer in
 `Symbol::to_ident_string`.

Because that causes an assertion failure in debug builds.

Fixes #140884.
---
 compiler/rustc_span/src/symbol.rs                 |  3 ++-
 tests/crashes/140884.rs                           |  6 ------
 .../ui/extern/extern-empty-string-issue-140884.rs |  3 +++
 .../extern-empty-string-issue-140884.stderr       | 15 +++++++++++++++
 4 files changed, 20 insertions(+), 7 deletions(-)
 delete mode 100644 tests/crashes/140884.rs
 create mode 100644 tests/ui/extern/extern-empty-string-issue-140884.rs
 create mode 100644 tests/ui/extern/extern-empty-string-issue-140884.stderr

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index efae6250b0720..fbe3b4ca6f5f6 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -2592,7 +2592,8 @@ impl Symbol {
     /// (`token_to_string`, `Ident::to_string`), except that symbols don't keep the rawness flag
     /// or edition, so we have to guess the rawness using the global edition.
     pub fn to_ident_string(self) -> String {
-        Ident::with_dummy_span(self).to_string()
+        // Avoid creating an empty identifier, because that asserts in debug builds.
+        if self == kw::Empty { String::new() } else { Ident::with_dummy_span(self).to_string() }
     }
 }
 
diff --git a/tests/crashes/140884.rs b/tests/crashes/140884.rs
deleted file mode 100644
index 6840760933a39..0000000000000
--- a/tests/crashes/140884.rs
+++ /dev/null
@@ -1,6 +0,0 @@
-//@ known-bug: #140884
-//@ needs-rustc-debug-assertions
-
-fn a() {
-    extern "" {}
-}
diff --git a/tests/ui/extern/extern-empty-string-issue-140884.rs b/tests/ui/extern/extern-empty-string-issue-140884.rs
new file mode 100644
index 0000000000000..10291513d34d2
--- /dev/null
+++ b/tests/ui/extern/extern-empty-string-issue-140884.rs
@@ -0,0 +1,3 @@
+extern "" {} //~ ERROR invalid ABI: found ``
+
+fn main() {}
diff --git a/tests/ui/extern/extern-empty-string-issue-140884.stderr b/tests/ui/extern/extern-empty-string-issue-140884.stderr
new file mode 100644
index 0000000000000..accae0c0f7c73
--- /dev/null
+++ b/tests/ui/extern/extern-empty-string-issue-140884.stderr
@@ -0,0 +1,15 @@
+error[E0703]: invalid ABI: found ``
+  --> $DIR/extern-empty-string-issue-140884.rs:1:8
+   |
+LL | extern "" {}
+   |        ^^ invalid ABI
+   |
+   = note: invoke `rustc --print=calling-conventions` for a full list of supported calling conventions
+help: there's a similarly named valid ABI `C`
+   |
+LL | extern "C" {}
+   |         +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0703`.