regression test for issue 111888.

(cherry picked from commit 7a0e2ee)

Author: pnkfelix

tests/ui/proc-macro/auxiliary/exports_no_mangle.rs

@@ -0,0 +1,9 @@
+#![crate_type="lib"]
+
+// Issue 111888: this crate (1.) is imported by a proc-macro crate and (2.)
+// exports a no_mangle function; that combination of acts was broken for some
+// period of time. See further discussion in the test file that imports this
+// crate.
+
+#[no_mangle]
+pub fn some_no_mangle_function() { }

tests/ui/proc-macro/no-mangle-in-proc-macro-issue-111888.rs

@@ -0,0 +1,20 @@
+// run-pass
+// aux-build:exports_no_mangle.rs
+#![crate_type = "proc-macro"]
+
+// Issue #111888: this proc-macro crate imports another crate that itself
+// exports a no_mangle function.
+//
+// That combination was broken for a period of time, because:
+//
+// In PR #99944 we *stopped* exporting no_mangle symbols from
+// proc-macro crates. The constructed linker version script still referred
+// to them, but resolving that discrepancy was left as a FIXME in the code.
+//
+// In PR #108017 we started telling the linker to check (via the
+// `--no-undefined-version` linker invocation flag) that every symbol referenced
+// in the "linker version script" is actually present in the linker input. So
+// the unresolved discrepancy from #99944 started surfacing as a compile-time
+// error.
+
+extern crate exports_no_mangle;