From cfb5df7050afe08ba5c50805cc9d8532a3935988 Mon Sep 17 00:00:00 2001
From: David Pedersen <david.pdrsn@gmail.com>
Date: Wed, 22 Mar 2023 14:48:27 +0100
Subject: [PATCH] Give better error if generics are used with
 #[derive(FromRef)] (#1874)

---
 axum-macros/CHANGELOG.md                        |  2 ++
 axum-macros/src/from_ref.rs                     | 16 +++++++++++++---
 axum-macros/src/lib.rs                          |  2 +-
 axum-macros/tests/from_ref/fail/generics.rs     |  8 ++++++++
 axum-macros/tests/from_ref/fail/generics.stderr |  5 +++++
 5 files changed, 29 insertions(+), 4 deletions(-)
 create mode 100644 axum-macros/tests/from_ref/fail/generics.rs
 create mode 100644 axum-macros/tests/from_ref/fail/generics.stderr

diff --git a/axum-macros/CHANGELOG.md b/axum-macros/CHANGELOG.md
index 4bb7e6e7..abc8be40 100644
--- a/axum-macros/CHANGELOG.md
+++ b/axum-macros/CHANGELOG.md
@@ -8,8 +8,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 # Unreleased
 
 - **change:** Update to syn 2.0 ([#1862])
+- **fixed:** Give better error if generics are used with `#[derive(FromRef)]` ([#1874])
 
 [#1862]: https://github.com/tokio-rs/axum/pull/1862
+[#1874]: https://github.com/tokio-rs/axum/pull/1874
 
 # 0.3.6 (13. March, 2023)
 
diff --git a/axum-macros/src/from_ref.rs b/axum-macros/src/from_ref.rs
index 56628b9c..2ab69eb5 100644
--- a/axum-macros/src/from_ref.rs
+++ b/axum-macros/src/from_ref.rs
@@ -8,12 +8,22 @@ use syn::{
 
 use crate::attr_parsing::{combine_unary_attribute, parse_attrs, Combine};
 
-pub(crate) fn expand(item: ItemStruct) -> TokenStream {
-    item.fields
+pub(crate) fn expand(item: ItemStruct) -> syn::Result<TokenStream> {
+    if !item.generics.params.is_empty() {
+        return Err(syn::Error::new_spanned(
+            item.generics,
+            "`#[derive(FromRef)]` doesn't support generics",
+        ));
+    }
+
+    let tokens = item
+        .fields
         .iter()
         .enumerate()
         .map(|(idx, field)| expand_field(&item.ident, idx, field))
-        .collect()
+        .collect();
+
+    Ok(tokens)
 }
 
 fn expand_field(state: &Ident, idx: usize, field: &Field) -> TokenStream {
diff --git a/axum-macros/src/lib.rs b/axum-macros/src/lib.rs
index 2fe5de10..584d8315 100644
--- a/axum-macros/src/lib.rs
+++ b/axum-macros/src/lib.rs
@@ -655,7 +655,7 @@ pub fn derive_typed_path(input: TokenStream) -> TokenStream {
 /// [`FromRef`]: https://docs.rs/axum/latest/axum/extract/trait.FromRef.html
 #[proc_macro_derive(FromRef, attributes(from_ref))]
 pub fn derive_from_ref(item: TokenStream) -> TokenStream {
-    expand_with(item, |item| Ok(from_ref::expand(item)))
+    expand_with(item, from_ref::expand)
 }
 
 fn expand_with<F, I, K>(input: TokenStream, f: F) -> TokenStream
diff --git a/axum-macros/tests/from_ref/fail/generics.rs b/axum-macros/tests/from_ref/fail/generics.rs
new file mode 100644
index 00000000..2f9f2bb8
--- /dev/null
+++ b/axum-macros/tests/from_ref/fail/generics.rs
@@ -0,0 +1,8 @@
+use axum::extract::FromRef;
+
+#[derive(Clone, FromRef)]
+struct AppState<T> {
+    foo: T,
+}
+
+fn main() {}
diff --git a/axum-macros/tests/from_ref/fail/generics.stderr b/axum-macros/tests/from_ref/fail/generics.stderr
new file mode 100644
index 00000000..bfe26643
--- /dev/null
+++ b/axum-macros/tests/from_ref/fail/generics.stderr
@@ -0,0 +1,5 @@
+error: `#[derive(FromRef)]` doesn't support generics
+ --> tests/from_ref/fail/generics.rs:4:16
+  |
+4 | struct AppState<T> {
+  |                ^^^