Work around issue 106930.

Author: m-ou-se

compiler/rustc_ast/src/format.rs

@@ -67,6 +67,12 @@ pub struct FormatArguments {
     names: FxHashMap<Symbol, usize>,
 }
 
+// FIXME: Rustdoc has trouble proving Send/Sync for this. See #106930.
+#[cfg(parallel_compiler)]
+unsafe impl Sync for FormatArguments {}
+#[cfg(parallel_compiler)]
+unsafe impl Send for FormatArguments {}
+
 impl FormatArguments {
     pub fn new() -> Self {
         Self {