feat: Emit an error if #[pavex::methods] is used on an impl block with no Pavex-annotated methods

Author: Duckaet

CONTRIBUTORS.md

@@ -20,3 +20,4 @@ We contributors to Pavex:
 - Oliver Barnes (@oliverbarnes)
 - Joe Hasson (@joehasson)
 - Peter Wischer (@peddermaster2)
+- Paras Sharma (@duckaet)

runtime/pavex_macros/src/methods.rs

@@ -20,6 +20,8 @@ pub fn methods(_metadata: TokenStream, input: TokenStream) -> Result<TokenStream
     let mut impl_: syn::ItemImpl = syn::parse(input).map_err(|e| e.into_compile_error())?;
     let mut new_items: Vec<proc_macro2::TokenStream> = Vec::new();
 
+    let mut no_pavex_method = true;
+
     for item in impl_.items.iter_mut() {
         let ImplItem::Fn(method) = item else {
             continue;
@@ -31,6 +33,7 @@ pub fn methods(_metadata: TokenStream, input: TokenStream) -> Result<TokenStream
         else {
             continue;
         };
+        no_pavex_method = false;
         let attr = method.attrs.remove(attr_index);
         let metadata = match attr.meta {
             // No arguments, just the path—e.g. `#[error_handler]`.
@@ -74,6 +77,18 @@ pub fn methods(_metadata: TokenStream, input: TokenStream) -> Result<TokenStream
         method.attrs.extend(new_attributes);
     }
 
+    if no_pavex_method {
+        return Err(syn::Error::new(
+            proc_macro2::Span::call_site(),
+            "`#[pavex::methods]` is used to provide context for Pavex attributes on methods \
+            (e.g., `#[pavex::get]`, `#[pavex::post]`, `#[pavex::error_handler]`, etc.).\n\
+            This `impl` block contains no methods with Pavex attributes, so `#[pavex::methods]` \
+            is unnecessary.",
+        )
+        .into_compile_error()
+        .into());
+    }
+
     PxStripper.visit_item_impl_mut(&mut impl_);
 
     Ok(quote! {

runtime/pavex_macros/tests/methods/fail/empty_impl.rs

@@ -0,0 +1,7 @@
+pub struct A;
+
+#[pavex::methods]
+impl A {
+}
+
+fn main() {}

runtime/pavex_macros/tests/methods/fail/empty_impl.stderr

@@ -0,0 +1,8 @@
+error: `#[pavex::methods]` is used to provide context for Pavex attributes on methods (e.g., `#[pavex::get]`, `#[pavex::post]`, `#[pavex::error_handler]`, etc.).
+       This `impl` block contains no methods with Pavex attributes, so `#[pavex::methods]` is unnecessary.
+ --> tests/methods/fail/empty_impl.rs:3:1
+  |
+3 | #[pavex::methods]
+  | ^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `pavex::methods` (in Nightly builds, run with -Z macro-backtrace for more info)

runtime/pavex_macros/tests/methods/fail/no_pavex_methods.rs

@@ -0,0 +1,14 @@
+pub struct A;
+
+#[pavex::methods]
+impl A {
+    pub fn new() -> Self {
+        A
+    }
+    
+    pub fn regular_method(&self) -> i32 {
+        42
+    }
+}
+
+fn main() {}

runtime/pavex_macros/tests/methods/fail/no_pavex_methods.stderr

@@ -0,0 +1,8 @@
+error: `#[pavex::methods]` is used to provide context for Pavex attributes on methods (e.g., `#[pavex::get]`, `#[pavex::post]`, `#[pavex::error_handler]`, etc.).
+       This `impl` block contains no methods with Pavex attributes, so `#[pavex::methods]` is unnecessary.
+ --> tests/methods/fail/no_pavex_methods.rs:3:1
+  |
+3 | #[pavex::methods]
+  | ^^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in the attribute macro `pavex::methods` (in Nightly builds, run with -Z macro-backtrace for more info)

runtime/pavex_macros/tests/methods/success/with_pavex_methods.rs

@@ -0,0 +1,15 @@
+pub struct A;
+
+#[pavex::methods]
+impl A {
+    #[pavex::get(path = "/test")]
+    pub fn handler(&self) -> String {
+        "Hello".to_string()
+    }
+    
+    pub fn regular_method(&self) -> i32 {
+        42
+    }
+}
+
+fn main() {}