Fix clippy::return_self_not_must_use (TraceMachina#1435)

.bazelrc

@@ -45,7 +45,7 @@ build --aspects=@rules_rust//rust:defs.bzl%rustfmt_aspect
 build --aspects=@rules_rust//rust:defs.bzl%rust_clippy_aspect
 
 # TODO(aaronmondal): Extend these flags until we can run with clippy::pedantic.
-build --@rules_rust//:clippy_flags=-Dwarnings,-Dclippy::uninlined_format_args,-Dclippy::manual_string_new,-Dclippy::manual_let_else,-Dclippy::single_match_else,-Dclippy::redundant_closure_for_method_calls,-Dclippy::semicolon_if_nothing_returned,-Dclippy::unreadable_literal,-Dclippy::range_plus_one,-Dclippy::inconsistent_struct_constructor,-Dclippy::match_wildcard_for_single_variants,-Dclippy::implicit_clone,-Dclippy::needless_pass_by_value,-Dclippy::explicit_deref_methods,-Dclippy::trivially_copy_pass_by_ref,-Dclippy::unnecessary_wraps,-Dclippy::cast_lossless,-Dclippy::map_unwrap_or,-Dclippy::ref_as_ptr,-Dclippy::inline_always,-Dclippy::redundant_else
+build --@rules_rust//:clippy_flags=-Dwarnings,-Dclippy::uninlined_format_args,-Dclippy::manual_string_new,-Dclippy::manual_let_else,-Dclippy::single_match_else,-Dclippy::redundant_closure_for_method_calls,-Dclippy::semicolon_if_nothing_returned,-Dclippy::unreadable_literal,-Dclippy::range_plus_one,-Dclippy::inconsistent_struct_constructor,-Dclippy::match_wildcard_for_single_variants,-Dclippy::implicit_clone,-Dclippy::needless_pass_by_value,-Dclippy::explicit_deref_methods,-Dclippy::trivially_copy_pass_by_ref,-Dclippy::unnecessary_wraps,-Dclippy::cast_lossless,-Dclippy::map_unwrap_or,-Dclippy::ref_as_ptr,-Dclippy::inline_always,-Dclippy::redundant_else,-Dclippy::return_self_not_must_use
 build --@rules_rust//:clippy.toml=//:clippy.toml
 
 test --@rules_rust//:rustfmt.toml=//:.rustfmt.toml

nativelink-util/src/health_utils.rs

@@ -141,6 +141,7 @@ impl HealthRegistryBuilder {
     }
 
     /// Create a sub builder for a namespace.
+    #[must_use]
     pub fn sub_builder(&mut self, namespace: &str) -> HealthRegistryBuilder {
         HealthRegistryBuilder {
             namespace: format!("{}/{}", self.namespace, namespace).into(),

nativelink-util/src/store_trait.rs

@@ -167,6 +167,7 @@ impl<'a> StoreKey<'a> {
     /// Returns a shallow clone of the key.
     /// This is extremely cheap and should be used when clone
     /// is needed but the key is not going to be modified.
+    #[must_use]
     pub fn borrow(&'a self) -> StoreKey<'a> {
         match self {
             StoreKey::Str(Cow::Owned(s)) => StoreKey::Str(Cow::Borrowed(s)),