refactor(derive-encode): emit better compilation error message

Signed-off-by: ADD-SP <add_sp@outlook.com>

Author: ADD-SP

CHANGELOG.md

@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - `EncodeLabelSet::encode()` now accepts a mutable reference to its encoder parameter.
+- Emit better compilation error message when deriving marcos `EncodeLabelSet` and `EncodeLabelValue`.
+  See [PR 267].
 
 ### Fixed
 

derive-encode/src/lib.rs

@@ -8,7 +8,7 @@
 
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
-use quote::quote;
+use quote::{quote, ToTokens};
 use syn::DeriveInput;
 
 /// Derive `prometheus_client::encoding::EncodeLabelSet`.
@@ -60,14 +60,36 @@ pub fn derive_encode_label_set(input: TokenStream) -> TokenStream {
                 })
                 .collect(),
             syn::Fields::Unnamed(_) => {
-                panic!("Can not derive Encode for struct with unnamed fields.")
+                return syn::Error::new(
+                    name.span(),
+                    "Can not derive `EncodeLabelSet` for struct with unnamed fields.",
+                )
+                .to_compile_error()
+                .to_token_stream()
+                .into();
+            }
+            syn::Fields::Unit => {
+                return syn::Error::new(
+                    name.span(),
+                    "Can not derive `EncodeLabelSet` for unit struct.",
+                )
+                .to_compile_error()
+                .to_token_stream()
+                .into();
             }
-            syn::Fields::Unit => panic!("Can not derive Encode for struct with unit field."),
         },
         syn::Data::Enum(syn::DataEnum { .. }) => {
-            panic!("Can not derive Encode for enum.")
+            return syn::Error::new(name.span(), "Can not derive `EncodeLabelSet` for enum.")
+                .to_compile_error()
+                .to_token_stream()
+                .into();
+        }
+        syn::Data::Union(_) => {
+            return syn::Error::new(name.span(), "Can not derive `EncodeLabelSet` for union.")
+                .to_compile_error()
+                .to_token_stream()
+                .into()
         }
-        syn::Data::Union(_) => panic!("Can not derive Encode for union."),
     };
 
     let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();
@@ -96,7 +118,10 @@ pub fn derive_encode_label_value(input: TokenStream) -> TokenStream {
 
     let body = match ast.clone().data {
         syn::Data::Struct(_) => {
-            panic!("Can not derive EncodeLabel for struct.")
+            return syn::Error::new(name.span(), "Can not derive `EncodeLabelValue` for struct.")
+                .to_compile_error()
+                .to_token_stream()
+                .into();
         }
         syn::Data::Enum(syn::DataEnum { variants, .. }) => {
             let match_arms: TokenStream2 = variants
@@ -115,7 +140,12 @@ pub fn derive_encode_label_value(input: TokenStream) -> TokenStream {
                 }
             }
         }
-        syn::Data::Union(_) => panic!("Can not derive Encode for union."),
+        syn::Data::Union(_) => {
+            return syn::Error::new(name.span(), "Can not derive `EncodeLabelValue` for union.")
+                .to_compile_error()
+                .to_token_stream()
+                .into()
+        }
     };
 
     let gen = quote! {

derive-encode/tests/build/friendly-compilation-error-msg.rs

@@ -0,0 +1,50 @@
+use prometheus_client::encoding::EncodeLabelSet;
+use prometheus_client::encoding::EncodeLabelValue;
+
+mod A {
+    use super::*;
+
+    #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
+    struct Unnamed(String);
+
+    #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
+    struct Unit;
+
+    #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
+    enum Enum {
+        A,
+        B,
+    }
+
+    #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
+    enum DataEnum {
+        A,
+        B(String),
+    }
+
+    #[derive(Clone, Copy, EncodeLabelSet)]
+    #[repr(C)]
+    union Union {
+        a: u32,
+        b: u64,
+    }
+}
+
+mod B {
+    use super::*;
+
+    #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelValue)]
+    struct Struct {
+        a: String,
+        b: String,
+    }
+
+    #[derive(Clone, Copy, EncodeLabelValue)]
+    #[repr(C)]
+    union Union {
+        a: u32,
+        b: u64,
+    }
+}
+
+fn main() {}

derive-encode/tests/build/friendly-compilation-error-msg.stderr

@@ -0,0 +1,41 @@
+error: Can not derive `EncodeLabelSet` for struct with unnamed fields.
+ --> tests/build/friendly-compilation-error-msg.rs:8:12
+  |
+8 |     struct Unnamed(String);
+  |            ^^^^^^^
+
+error: Can not derive `EncodeLabelSet` for unit struct.
+  --> tests/build/friendly-compilation-error-msg.rs:11:12
+   |
+11 |     struct Unit;
+   |            ^^^^
+
+error: Can not derive `EncodeLabelSet` for enum.
+  --> tests/build/friendly-compilation-error-msg.rs:14:10
+   |
+14 |     enum Enum {
+   |          ^^^^
+
+error: Can not derive `EncodeLabelSet` for enum.
+  --> tests/build/friendly-compilation-error-msg.rs:20:10
+   |
+20 |     enum DataEnum {
+   |          ^^^^^^^^
+
+error: Can not derive `EncodeLabelSet` for union.
+  --> tests/build/friendly-compilation-error-msg.rs:27:11
+   |
+27 |     union Union {
+   |           ^^^^^
+
+error: Can not derive `EncodeLabelValue` for struct.
+  --> tests/build/friendly-compilation-error-msg.rs:37:12
+   |
+37 |     struct Struct {
+   |            ^^^^^^
+
+error: Can not derive `EncodeLabelValue` for union.
+  --> tests/build/friendly-compilation-error-msg.rs:44:11
+   |
+44 |     union Union {
+   |           ^^^^^

derive-encode/tests/lib.rs

@@ -211,4 +211,5 @@ fn build() {
     let t = trybuild::TestCases::new();
     t.pass("tests/build/redefine-prelude-symbols.rs");
     t.pass("tests/build/keep-impl-generics.rs");
+    t.compile_fail("tests/build/friendly-compilation-error-msg.rs");
 }