allow type-based search on foreign functions

fixes https://github.com/rust-lang/rust/issues/131804

src/librustdoc/html/render/search_index.rs

@@ -759,7 +759,10 @@ pub(crate) fn get_function_type_for_search<'tcx>(
         }
     });
     let (mut inputs, mut output, where_clause) = match item.kind {
-        clean::FunctionItem(ref f) | clean::MethodItem(ref f, _) | clean::TyMethodItem(ref f) => {
+        clean::ForeignFunctionItem(ref f, _)
+        | clean::FunctionItem(ref f)
+        | clean::MethodItem(ref f, _)
+        | clean::TyMethodItem(ref f) => {
             get_fn_inputs_and_outputs(f, tcx, impl_or_trait_generics, cache)
         }
         _ => return None,

tests/rustdoc-js/extern-func.js

@@ -0,0 +1,8 @@
+const EXPECTED = [
+    {
+        'query': 'c_float -> c_float',
+        'others': [
+            { 'path': 'extern_func', 'name': 'sqrt' }
+        ],
+    },
+];

tests/rustdoc-js/extern-func.rs

@@ -0,0 +1,5 @@
+use std::ffi::c_float;
+
+extern "C" {
+    pub fn sqrt(x: c_float) -> c_float;
+}