add regression test for #61216

Fixes #61216.

src/test/run-make-fulldeps/rustdoc-determinism/Makefile

@@ -0,0 +1,16 @@
+-include ../tools.mk
+
+# Assert that the search index is generated deterministically, regardless of the
+# order that crates are documented in.
+
+# ignore-windows
+# Uses `diff`.
+
+all:
+	$(RUSTDOC) foo.rs -o $(TMPDIR)/foo_first
+	$(RUSTDOC) bar.rs -o $(TMPDIR)/foo_first
+
+	$(RUSTDOC) bar.rs -o $(TMPDIR)/bar_first
+	$(RUSTDOC) foo.rs -o $(TMPDIR)/bar_first
+
+	diff $(TMPDIR)/foo_first/search-index.js $(TMPDIR)/bar_first/search-index.js

src/test/run-make-fulldeps/rustdoc-determinism/bar.rs

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

src/test/run-make-fulldeps/rustdoc-determinism/foo.rs

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