Add regression test

Author: el-ev

tests/run-make/rustdoc-merge-no-input-finalize/rmake.rs

@@ -0,0 +1,22 @@
+// Running --merge=finalize without an input crate root should not trigger ICE.
+// Issue: https://github.com/rust-lang/rust/issues/146646
+
+//@ needs-target-std
+
+use run_make_support::rustdoc;
+
+fn main() {
+    rustdoc()
+        .input("sierra.rs")
+        .arg("-Zunstable-options")
+        .arg("--parts-out-dir=parts")
+        .arg("--merge=none")
+        .run();
+
+    rustdoc()
+        .arg("-Zunstable-options")
+        .arg("--include-parts-dir=parts")
+        .arg("--merge=finalize")
+        .out_dir("out")
+        .run();
+}

tests/run-make/rustdoc-merge-no-input-finalize/sierra.rs

@@ -0,0 +1 @@
+pub struct Sierra;