rustdoc-search: avoid infinite where clause unbox

Fixes #118242

tests/rustdoc-js/assoc-type-loop.js

@@ -0,0 +1,9 @@
+// Crash reduction of
+// https://github.com/rust-lang/rust/issues/118242
+
+const EXPECTED = [
+    {
+        'query': 't',
+        'correction': null,
+    },
+];

tests/rustdoc-js/assoc-type-loop.rs

@@ -0,0 +1,35 @@
+#![crate_name="foo"]
+
+// reduced from sqlx 0.7.3
+use std::future::Future;
+use std::pin::Pin;
+use std::ops::{Deref, DerefMut};
+pub enum Error {}
+pub trait Acquire<'c> {
+    type Database: Database;
+    type Connection: Deref<Target = <Self::Database as Database>::Connection> + DerefMut + Send;
+}
+pub trait Database {
+    type Connection: Connection<Database = Self>;
+}
+pub trait Connection {
+    type Database: Database;
+    type Options: ConnectionOptions<Connection = Self>;
+    fn begin(
+        &mut self
+    ) -> Pin<Box<dyn Future<Output = Result<Transaction<'_, Self::Database>, Error>> + Send + '_>>
+    where
+        Self: Sized;
+}
+pub trait ConnectionOptions {
+    type Connection: Connection;
+}
+pub struct Transaction<'c, DB: Database> {
+    _db: &'c DB,
+}
+impl<'t, 'c, DB: Database> Acquire<'t> for &'t mut Transaction<'c, DB>
+    where <DB as Database>::Connection: Send
+{
+    type Database = DB;
+    type Connection = &'t mut <DB as Database>::Connection;
+}