Nits

compiler/rustc_infer/src/traits/engine.rs

@@ -64,6 +64,7 @@ fn register_predicate_obligations(
         }
     }
 
+    #[must_use]
     fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
         let errors = self.select_where_possible(infcx);
         if !errors.is_empty() {

compiler/rustc_macros/src/extension.rs

@@ -68,7 +68,14 @@ impl #generics #trait_ for #self_ty {
 
 /// Only keep `#[doc]` attrs.
 fn scrub_attrs(attrs: &[Attribute]) -> Vec<Attribute> {
-    attrs.into_iter().cloned().filter(|attr| attr.path().segments[0].ident == "doc").collect()
+    attrs
+        .into_iter()
+        .cloned()
+        .filter(|attr| {
+            let ident = &attr.path().segments[0].ident;
+            ident == "doc" || ident == "must_use"
+        })
+        .collect()
 }
 
 /// Scrub arguments so that they're valid for trait signatures.

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -37,6 +37,12 @@ impl<'cx, 'tcx> At<'cx, 'tcx> {
     /// normalizing, but for now should be used only when we actually
     /// know that normalization will succeed, since error reporting
     /// and other details are still "under development".
+    ///
+    /// This normalization should *only* be used when the projection does not
+    /// have possible ambiguity or may not be well-formed.
+    ///
+    /// After codegen, when lifetimes do not matter, it is preferable to instead
+    /// use [`TyCtxt::normalize_erasing_regions`], which wraps this procedure.
     fn query_normalize<T>(self, value: T) -> Result<Normalized<'tcx, T>, NoSolution>
     where
         T: TypeFoldable<TyCtxt<'tcx>>,