rustdoc: add test case for alias bug

Reproduces
<https://github.com/rust-lang/cargo/pull/16309#discussion_r2586792981>

tests/run-make/rustdoc-merge-directory-alias/dep1.rs

@@ -0,0 +1,10 @@
+pub struct Dep1;
+pub struct Dep2;
+pub struct Dep3;
+pub struct Dep4;
+
+//@ hasraw crates.js 'dep1'
+//@ hasraw search.index/name/*.js 'Dep1'
+//@ has dep1/index.html
+#[doc(alias="dep1_missing")]
+pub struct Dep5;

tests/run-make/rustdoc-merge-directory-alias/dep2.rs

@@ -0,0 +1,4 @@
+//@ hasraw crates.js 'dep2'
+//@ hasraw search.index/name/*.js 'Second'
+//@ has dep2/index.html
+pub struct Second;

tests/run-make/rustdoc-merge-directory-alias/rmake.rs

@@ -0,0 +1,66 @@
+// 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::{htmldocck, path, rustdoc};
+
+fn main() {
+    let out_dir = path("out");
+    let merged_dir = path("merged");
+    let parts_out_dir = path("parts");
+
+    rustdoc()
+        .input("dep1.rs")
+        .out_dir(&out_dir)
+        .arg("-Zunstable-options")
+        .arg(format!("--parts-out-dir={}", parts_out_dir.display()))
+        .arg("--merge=none")
+        .run();
+    assert!(parts_out_dir.join("dep1.json").exists());
+
+    let output = rustdoc()
+        .arg("-Zunstable-options")
+        .out_dir(&out_dir)
+        .arg(format!("--include-parts-dir={}", parts_out_dir.display()))
+        .arg("--merge=finalize")
+        .run();
+    output.assert_stderr_not_contains("error: the compiler unexpectedly panicked. this is a bug.");
+
+    rustdoc()
+        .input("dep2.rs")
+        .out_dir(&out_dir)
+        .arg("-Zunstable-options")
+        .arg(format!("--parts-out-dir={}", parts_out_dir.display()))
+        .arg("--merge=none")
+        .run();
+    assert!(parts_out_dir.join("dep2.json").exists());
+
+    let output2 = rustdoc()
+        .arg("-Zunstable-options")
+        .out_dir(&out_dir)
+        .arg(format!("--include-parts-dir={}", parts_out_dir.display()))
+        .arg("--merge=finalize")
+        .run();
+    output2.assert_stderr_not_contains("error: the compiler unexpectedly panicked. this is a bug.");
+
+    rustdoc()
+        .input("dep1.rs")
+        .out_dir(&out_dir)
+        .arg("-Zunstable-options")
+        .arg(format!("--parts-out-dir={}", parts_out_dir.display()))
+        .arg("--merge=none")
+        .run();
+    assert!(parts_out_dir.join("dep1.json").exists());
+
+    let output3 = rustdoc()
+        .arg("-Zunstable-options")
+        .out_dir(&out_dir)
+        .arg(format!("--include-parts-dir={}", parts_out_dir.display()))
+        .arg("--merge=finalize")
+        .run();
+    output3.assert_stderr_not_contains("error: the compiler unexpectedly panicked. this is a bug.");
+
+    htmldocck().arg(&out_dir).arg("dep1.rs").run();
+    htmldocck().arg(&out_dir).arg("dep2.rs").run();
+}