fix: doctest searches native libs in build script outputs

HACK: `rustdoc --test` not only compiles but executes doctests.
Ideally only execution phase should have search paths appended,
so the executions can find native libs just like other tests.
However, there is no way to separate these two phase, so this
hack is added for both phases.

src/cargo/core/compiler/compilation.rs

@@ -293,6 +293,18 @@ impl<'gctx> Compilation<'gctx> {
     ) -> CargoResult<ProcessBuilder> {
         let mut search_path = Vec::new();
         if tool_kind.is_rustc_tool() {
+            if matches!(tool_kind, ToolKind::Rustdoc) {
+                // HACK: `rustdoc --test` not only compiles but executes doctests.
+                // Ideally only execution phase should have search paths appended,
+                // so the executions can find native libs just like other tests.
+                // However, there is no way to separate these two phase, so this
+                // hack is added for both phases.
+                // TODO: handle doctest-xcompile
+                search_path.extend(super::filter_dynamic_search_path(
+                    self.native_dirs.iter(),
+                    &self.root_output[&CompileKind::Host],
+                ));
+            }
             search_path.push(self.deps_output[&CompileKind::Host].clone());
         } else {
             search_path.extend(super::filter_dynamic_search_path(

tests/testsuite/test.rs

@@ -2801,8 +2801,8 @@ fn doctest_with_library_paths() {
                     pub fn assert_search_path() {{
                         let search_path = std::env::var_os("{}").unwrap();
                         let paths = std::env::split_paths(&search_path).collect::<Vec<_>>();
-                        assert!(!paths.contains(&r#"{}"#.into()));
-                        assert!(!paths.contains(&r#"{}"#.into()));
+                        assert!(paths.contains(&r#"{}"#.into()));
+                        assert!(paths.contains(&r#"{}"#.into()));
                     }}
                 "##,
                 dylib_path_envvar(),