fix(derive-encode): keep struct's generic parameters while deriving EncodeLabelSet

Signed-off-by: ADD-SP <[email protected]>

Author: ADD-SP

CHANGELOG.md

@@ -14,14 +14,22 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `Family::get_or_create_owned` can access a metric in a labeled family. This
    method avoids the risk of runtime deadlocks at the expense of creating an
    owned type. See [PR 244].
- 
+
 [PR 244]: https://github.com/prometheus/client_rust/pull/244
 [PR 257]: https://github.com/prometheus/client_rust/pull/257
 
 ### Changed
 
 - `EncodeLabelSet::encode()` now accepts a mutable reference to its encoder parameter.
 
+### Fixed
+
+- Fixed an issue where derive macros `EncodeLabelSet` didn't work
+  when the `struct` has generic parameters (like `'a`).
+  See [PR 266].
+
+[PR 266]: https://github.com/prometheus/client_rust/pull/266
+
 ## [0.23.1]
 
 ### Changed

derive-encode/src/lib.rs

@@ -70,8 +70,9 @@ pub fn derive_encode_label_set(input: TokenStream) -> TokenStream {
         syn::Data::Union(_) => panic!("Can not derive Encode for union."),
     };
 
+    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
     let gen = quote! {
-        impl ::prometheus_client::encoding::EncodeLabelSet for #name {
+        impl #impl_generics ::prometheus_client::encoding::EncodeLabelSet for #name #ty_generics #where_clause {
             fn encode(&self, encoder: &mut ::prometheus_client::encoding::LabelSetEncoder) -> ::core::result::Result<(), ::core::fmt::Error> {
                 use ::prometheus_client::encoding::EncodeLabel;
                 use ::prometheus_client::encoding::EncodeLabelKey;

derive-encode/tests/build/keep-impl-generics.rs

@@ -0,0 +1,18 @@
+use prometheus_client::encoding::EncodeLabelSet;
+use prometheus_client::encoding::EncodeLabelValue;
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelValue)]
+enum CpuUsageLabelMode {
+    User,
+    System,
+    Irq,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
+struct CpuUsageLabelSet<'a> { // <-- `'a` lifetime is used in the struct,
+                              // this should be preserved in the impl block
+    mode: CpuUsageLabelMode,
+    service: &'a str
+}
+
+fn main() {}

derive-encode/tests/lib.rs

@@ -209,5 +209,6 @@ fn flatten() {
 #[test]
 fn build() {
     let t = trybuild::TestCases::new();
-    t.pass("tests/build/redefine-prelude-symbols.rs")
+    t.pass("tests/build/redefine-prelude-symbols.rs");
+    t.pass("tests/build/keep-impl-generics.rs");
 }