Bug 1945218 - [1/1] silence unused HasFloat warning r=glandium

This will go away once we update to bindgen 0.70 or greater. For now,
though, slap an `#[allow(dead_code)]` on it to get rid of the warning.

Differential Revision: https://phabricator.services.mozilla.com/D236239

Cargo.toml

@@ -153,6 +153,7 @@ backtrace = { path = "build/rust/backtrace" }
 # Patch bindgen 0.63 to 0.69
 bindgen_0_63 = { package = "bindgen", path = "build/rust/bindgen-0.63" }
 # Locally patch bindgen for https://github.com/rust-lang/rust-bindgen/pull/2824
+# and for bug 1945218
 bindgen = { path = "third_party/rust/bindgen" }
 
 # Patch nix 0.26 to 0.29

third_party/rust/bindgen/ir/item.rs

@@ -85,6 +85,7 @@ pub(crate) trait HasTypeParamInArray {
     fn has_type_param_in_array(&self, ctx: &BindgenContext) -> bool;
 }
 
+#[allow(dead_code)]
 /// A trait for determining if some IR thing has float or not.
 pub(crate) trait HasFloat {
     /// Returns `true` if the thing has float, and `false` otherwise.