rustdoc-search: show impl Trait inline when unhighlighted

While normal generics can be skipped in this case, no-names need
something to show here.

Before: `TyCtxt, , Symbol -> bool`

After: `TyCtxt, Into<DefId>, Symbol -> bool`

src/librustdoc/html/static/js/search.js

@@ -2592,7 +2592,12 @@ class DocSearch {
             const writeFn = (fnType, result) => {
                 if (fnType.id < 0) {
                     if (fnParamNames[-1 - fnType.id] === "") {
-                        for (const nested of fnType.generics) {
+                        // Normally, there's no need to shown an unhighlighted
+                        // where clause, but if it's impl Trait, then we do.
+                        const generics = fnType.generics.length > 0 ?
+                            fnType.generics :
+                            obj.type.where_clause[-1 - fnType.id];
+                        for (const nested of generics) {
                             writeFn(nested, result);
                         }
                         return;

tests/rustdoc-js/impl-trait-inlining.js

@@ -0,0 +1,25 @@
+// exact-check
+// ignore-order
+
+const EXPECTED = [
+    {
+        'query': 'tyctxt, symbol -> bool',
+        'others': [
+            {
+                'path': 'foo::TyCtxt',
+                'name': 'has_attr',
+                'displayType': "`TyCtxt`, Into<DefId>, `Symbol` -> `bool`",
+            },
+        ],
+    },
+    {
+        'query': 'tyctxt, into<defid>, symbol -> bool',
+        'others': [
+            {
+                'path': 'foo::TyCtxt',
+                'name': 'has_attr',
+                'displayType': "`TyCtxt`, `Into`<`DefId`>, `Symbol` -> `bool`",
+            },
+        ],
+    },
+];

tests/rustdoc-js/impl-trait-inlining.rs

@@ -0,0 +1,11 @@
+#![crate_name="foo"]
+
+pub struct TyCtxt;
+pub struct DefId;
+pub struct Symbol;
+
+impl TyCtxt {
+    pub fn has_attr(self, _did: impl Into<DefId>, _attr: Symbol) -> bool {
+        unimplemented!();
+    }
+}