Improve error reporting for using the #[bon::builder] fully qualified attribute inside of impl blocks

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions-rust-lang/setup-rust-toolchain@v1
 
-      - run: sudo apt-get install -y valgrind
+      - run: sudo apt-get update && sudo apt-get install -y valgrind
       - run: cd ./benchmarks/runtime && ./run.sh ${{ matrix.benchmark }}
 
   compilation-benchmarks:

bon-macros/src/builder/builder_gen/input_fn/validation.rs

@@ -5,15 +5,18 @@ impl super::FnInputCtx<'_> {
     pub(super) fn validate(&self) -> Result {
         if self.impl_ctx.is_none() {
             let explanation = "\
-                but #[bon] attribute is absent on top of the impl block; this \
-                additional #[bon] attribute on the impl block is required for \
-                the macro to see the type of `Self` and properly generate \
-                the builder struct definition adjacently to the impl block.";
+                which likely means the builder attribute was used inside of an \
+                impl block; the impl block needs to be annotated with the #[bon] \
+                attribute and the builder attribute must be spelled as #[builder] \
+                without any additional path prefix, since it's used as a simple \
+                inert config attribute for #[bon] in impl blocks; more info on \
+                inert vs active attributes: \
+                https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes";
 
             if let Some(receiver) = &self.fn_item.orig.sig.receiver() {
                 bail!(
                     &receiver.self_token,
-                    "function contains a `self` parameter {explanation}"
+                    "this function contains a `self` parameter {explanation}"
                 );
             }
 
@@ -22,7 +25,7 @@ impl super::FnInputCtx<'_> {
             if let Some(self_span) = ctx.self_span {
                 bail!(
                     &self_span,
-                    "function contains a `Self` type reference {explanation}"
+                    "this function contains a `Self` type reference {explanation}"
                 );
             }
         }

bon-macros/src/builder/item_impl.rs

@@ -43,11 +43,7 @@ pub(crate) fn generate(
         });
 
     if builder_fns.is_empty() {
-        bail!(
-            &Span::call_site(),
-            "there are no #[builder] functions in the impl block, so there is no \
-            need for a #[bon] attribute here"
-        );
+        return Err(no_builder_attrs_error(&other_items));
     }
 
     orig_impl_block.items = builder_fns;
@@ -188,3 +184,42 @@ fn conv_impl_item_fn_into_fn_item(func: syn::ImplItemFn) -> Result<syn::ItemFn>
         block: Box::new(block),
     })
 }
+
+fn no_builder_attrs_error(other_items: &[syn::ImplItem]) -> Error {
+    let builder_like_err = other_items.iter().find_map(|item| {
+        let item = match item {
+            syn::ImplItem::Fn(fn_item) => fn_item,
+            _ => return None,
+        };
+
+        let builder_like = item
+            .attrs
+            .iter()
+            .find(|attr| attr.path().ends_with_segment("builder"))?;
+
+        let builder_like_str = darling::util::path_to_string(builder_like.path());
+        let builder_like_prefix = builder_like_str
+            .strip_suffix("builder")
+            .unwrap_or(&builder_like_str);
+
+        Some(err!(
+            &builder_like.path(),
+            "#[bon] macro found no #[builder] attributes in the impl block, but \
+            it looks like this attribute was meant for #[bon]; note that #[bon] \
+            expects a bare #[builder] attribute without the `{builder_like_prefix}` \
+            prefix; #[builder] acts as a simple config attribute for the active \
+            #[bon] attribute in impl blocks; more info on inert vs active attributes: \
+            https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes"
+        ))
+    });
+
+    if let Some(err) = builder_like_err {
+        return err;
+    }
+
+    err!(
+        &Span::call_site(),
+        "there are no #[builder] functions in the impl block, so there is no \
+        need for a #[bon] attribute here"
+    )
+}

bon-macros/src/util/path.rs

@@ -2,6 +2,7 @@ use crate::util::prelude::*;
 
 pub(crate) trait PathExt {
     fn starts_with_segment(&self, desired_segment: &str) -> bool;
+    fn ends_with_segment(&self, desired_segment: &str) -> bool;
 
     /// Returns an error if this path has some generic arguments.
     fn require_mod_style(&self) -> Result;
@@ -15,6 +16,13 @@ impl PathExt for syn::Path {
             .unwrap_or(false)
     }
 
+    fn ends_with_segment(&self, desired_segment: &str) -> bool {
+        self.segments
+            .last()
+            .map(|first| first.ident == desired_segment)
+            .unwrap_or(false)
+    }
+
     fn require_mod_style(&self) -> Result {
         if self
             .segments

bon/tests/integration/ui/compile_fail/attr_bon.rs

@@ -26,5 +26,34 @@ impl NoBuilderMethods {
     const NOT_BUILDER: () = ();
 }
 
+struct BuilderLikeMethodAttribute;
+
+#[bon]
+impl BuilderLikeMethodAttribute {
+    fn not_builder() {}
+
+    #[::foo::builder]
+    fn builder_like() {}
+}
+
+struct ActiveBuilderInsideImplWithSelf;
+
+#[bon]
+impl ActiveBuilderInsideImplWithSelf {
+    #[bon::builder]
+    fn active_bon_with_self(&self) {}
+}
+
+struct ActiveBuilderInsideImplClueless;
+
+#[bon]
+impl ActiveBuilderInsideImplClueless {
+    // The `builder` attribute is "clueless" here because nothing tells it that
+    // it is inside of an impl block, so it silently "succeeds" to generate
+    // a builder method, but it produces a bunch of errors that items can't
+    // be defined inside of an impl block.
+    #[bon::builder]
+    fn active_bon_clueless() {}
+}
 
 fn main() {}

bon/tests/integration/ui/compile_fail/attr_bon.stderr

@@ -17,3 +17,92 @@ error: there are no #[builder] functions in the impl block, so there is no need
    | ^^^^^^
    |
    = note: this error originates in the attribute macro `bon` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: #[bon] macro found no #[builder] attributes in the impl block, but it looks like this attribute was meant for #[bon]; note that #[bon] expects a bare #[builder] attribute without the `foo::` prefix; #[builder] acts as a simple config attribute for the active #[bon] attribute in impl blocks; more info on inert vs active attributes: https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes
+  --> tests/integration/ui/compile_fail/attr_bon.rs:35:7
+   |
+35 |     #[::foo::builder]
+   |       ^
+
+error: #[bon] macro found no #[builder] attributes in the impl block, but it looks like this attribute was meant for #[bon]; note that #[bon] expects a bare #[builder] attribute without the `bon::` prefix; #[builder] acts as a simple config attribute for the active #[bon] attribute in impl blocks; more info on inert vs active attributes: https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes
+  --> tests/integration/ui/compile_fail/attr_bon.rs:43:7
+   |
+43 |     #[bon::builder]
+   |       ^^^
+
+error: this function contains a `self` parameter which likely means the builder attribute was used inside of an impl block; the impl block needs to be annotated with the #[bon] attribute and the builder attribute must be spelled as #[builder] without any additional path prefix, since it's used as a simple inert config attribute for #[bon] in impl blocks; more info on inert vs active attributes: https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes
+  --> tests/integration/ui/compile_fail/attr_bon.rs:44:30
+   |
+44 |     fn active_bon_with_self(&self) {}
+   |                              ^^^^
+
+error: #[bon] macro found no #[builder] attributes in the impl block, but it looks like this attribute was meant for #[bon]; note that #[bon] expects a bare #[builder] attribute without the `bon::` prefix; #[builder] acts as a simple config attribute for the active #[bon] attribute in impl blocks; more info on inert vs active attributes: https://doc.rust-lang.org/reference/attributes.html#active-and-inert-attributes
+  --> tests/integration/ui/compile_fail/attr_bon.rs:55:7
+   |
+55 |     #[bon::builder]
+   |       ^^^
+
+error: struct is not supported in `trait`s or `impl`s
+  --> tests/integration/ui/compile_fail/attr_bon.rs:49:1
+   |
+49 | #[bon]
+   | ^^^^^^
+   |
+   = help: consider moving the struct out to a nearby module scope
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: implementation is not supported in `trait`s or `impl`s
+  --> tests/integration/ui/compile_fail/attr_bon.rs:49:1
+   |
+49 | #[bon]
+   | ^^^^^^
+   |
+   = help: consider moving the implementation out to a nearby module scope
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: module is not supported in `trait`s or `impl`s
+  --> tests/integration/ui/compile_fail/attr_bon.rs:49:1
+   |
+49 | #[bon]
+   | ^^^^^^
+   |
+   = help: consider moving the module out to a nearby module scope
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `const` items in this context need a name
+  --> tests/integration/ui/compile_fail/attr_bon.rs:41:1
+   |
+41 | #[bon]
+   | ^^^^^^ `_` is not a valid name for this `const` item
+   |
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `const` items in this context need a name
+  --> tests/integration/ui/compile_fail/attr_bon.rs:49:1
+   |
+49 | #[bon]
+   | ^^^^^^ `_` is not a valid name for this `const` item
+   |
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0433]: failed to resolve: could not find `foo` in the list of imported crates
+  --> tests/integration/ui/compile_fail/attr_bon.rs:35:9
+   |
+35 |     #[::foo::builder]
+   |         ^^^ could not find `foo` in the list of imported crates
+
+error[E0412]: cannot find type `ActiveBonCluelessBuilder` in this scope
+  --> tests/integration/ui/compile_fail/attr_bon.rs:49:1
+   |
+49 | #[bon]
+   | ^^^^^^ not found in this scope
+   |
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0422]: cannot find struct, variant or union type `ActiveBonCluelessBuilder` in this scope
+  --> tests/integration/ui/compile_fail/attr_bon.rs:49:1
+   |
+49 | #[bon]
+   | ^^^^^^ not found in this scope
+   |
+   = note: this error originates in the attribute macro `bon::builder` (in Nightly builds, run with -Z macro-backtrace for more info)