Avoid creating an empty identifer in Symbol::to_ident_string.

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() }
     }
 }
 

tests/ui/extern/extern-empty-string-issue-140884.rs

@@ -0,0 +1,3 @@
+extern "" {} //~ ERROR invalid ABI: found ``
+
+fn main() {}

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`.