Veetaha's followup for #[builder(generics(setters))] (#368)

Veetaha · web-flow · commit 32e18382da3e · 2026-02-12T05:17:38.000+02:00
Followup for #364. A bunch of assorted changes described in commit messages
diff --git a/bon-macros/src/builder/builder_gen/generic_setters.rs b/bon-macros/src/builder/builder_gen/generic_setters.rs
@@ -1,8 +1,10 @@
 use super::models::BuilderGenCtx;
 use crate::parsing::ItemSigConfig;
 use crate::util::prelude::*;
+use std::collections::BTreeSet;
 use syn::punctuated::Punctuated;
 use syn::token::Where;
+use syn::visit::Visit;
 
 pub(super) struct GenericSettersCtx<'a> {
     base: &'a BuilderGenCtx,
@@ -28,19 +30,24 @@ impl<'a> GenericSettersCtx<'a> {
         // Check for interdependent type parameters in generic bounds
         for param in generics {
             if let syn::GenericParam::Type(type_param) = param {
-                let params_in_bounds =
-                    find_type_params_in_bounds(&type_param.bounds, &type_param_idents);
-                if params_in_bounds.len() > 1
-                    || (params_in_bounds.len() == 1
-                        && params_in_bounds.first() != Some(&&type_param.ident))
-                {
-                    let params_str = params_in_bounds
+                let mut params = TypeParamFinder::new(&type_param_idents);
+
+                for bound in &type_param.bounds {
+                    params.visit_type_param_bound(bound);
+                }
+
+                // Self-referential type params are fine
+                params.found.remove(&type_param.ident);
+
+                if let Some(first_param) = params.found.iter().next() {
+                    let params_str = params
+                        .found
                         .iter()
                         .map(|p| format!("`{p}`"))
                         .collect::<Vec<_>>()
                         .join(", ");
                     bail!(
-                        &type_param.bounds,
+                        first_param,
                         "generic conversion methods cannot be generated for interdependent type parameters; \
                          the bounds on generic parameter `{}` reference other type parameters: {}\n\
                          \n\
@@ -55,10 +62,11 @@ impl<'a> GenericSettersCtx<'a> {
         // Check for interdependent type parameters in where clauses
         if let Some(where_clause) = &self.base.generics.where_clause {
             for predicate in &where_clause.predicates {
-                let params_in_predicate =
-                    find_type_params_in_predicate(predicate, &type_param_idents);
-                if params_in_predicate.len() > 1 {
-                    let params_str = params_in_predicate
+                let mut params = TypeParamFinder::new(&type_param_idents);
+                params.visit_where_predicate(predicate);
+                if params.found.len() > 1 {
+                    let params_str = params
+                        .found
                         .iter()
                         .map(|p| format!("`{p}`"))
                         .collect::<Vec<_>>()
@@ -85,8 +93,9 @@ impl<'a> GenericSettersCtx<'a> {
                 syn::GenericParam::Const(const_param) => {
                     bail!(
                         &const_param.ident,
-                        "const generic parameters are not supported in `generics(setters(...))`; \
-                         only type parameters can be converted"
+                        "const generic parameters are not yet supported with `generics(setters(...))`; \
+                         only type parameters can be overridden, feel free to open an issue if you need \
+                         this feature"
                     );
                 }
                 syn::GenericParam::Lifetime(_) => {
@@ -122,8 +131,12 @@ impl<'a> GenericSettersCtx<'a> {
         let docs = self.method_docs(param_ident);
 
         // Build the generic arguments for the output type, where the current parameter
-        // is replaced with a new type variable
-        let new_type_var = self.base.namespace.unique_ident(param_ident.to_string());
+        // is replaced with a new type variable. Even though the `GenericsNamespace`
+        let new_type_var = self
+            .base
+            .namespace
+            // Add `New` prefix to make the type variable more readable in the docs and IDE hints
+            .unique_ident(format!("New{param_ident}"));
 
         // Copy the bounds from the original type parameter to the new one
         let bounds = &type_param.bounds;
@@ -168,7 +181,10 @@ impl<'a> GenericSettersCtx<'a> {
 
                     // Add runtime assert that this field is None
                     let field_ident = &member.name.orig;
-                    let message = format!("BUG: field `{field_ident}` should be None when converting generic parameter `{param_ident}`");
+                    let message = format!(
+                        "BUG: field `{field_ident}` should be None \
+                        when converting generic parameter `{param_ident}`"
+                    );
                     runtime_asserts.push(quote! {
                         ::core::assert!(named.#index.is_none(), #message);
                     });
@@ -211,11 +227,7 @@ impl<'a> GenericSettersCtx<'a> {
                 clause.predicates.push(syn::parse_quote!(#bound));
             }
 
-            if clause.predicates.is_empty() {
-                None
-            } else {
-                Some(clause)
-            }
+            (!clause.predicates.is_empty()).then(|| clause)
         };
 
         quote! {
@@ -280,82 +292,34 @@ impl<'a> GenericSettersCtx<'a> {
     }
 }
 
-fn find_type_params_in_bounds<'b>(
-    bounds: &Punctuated<syn::TypeParamBound, syn::token::Plus>,
-    type_params: &'b [&'b syn::Ident],
-) -> Vec<&'b syn::Ident> {
-    use syn::visit::Visit;
+struct TypeParamFinder<'ty, 'ast> {
+    type_params: &'ty [&'ty syn::Ident],
 
-    struct TypeParamFinder<'a> {
-        type_params: &'a [&'a syn::Ident],
-        found: std::collections::HashSet<&'a syn::Ident>,
-    }
+    // Use a `BTreeSet` for deterministic ordering
+    found: BTreeSet<&'ast syn::Ident>,
+}
 
-    impl<'ast> Visit<'ast> for TypeParamFinder<'_> {
-        fn visit_path(&mut self, path: &'ast syn::Path) {
-            // Check if this path is one of our type parameters
-            for &param in self.type_params {
-                if path.is_ident(param) {
-                    self.found.insert(param);
-                }
-            }
-            // Continue visiting nested paths
-            syn::visit::visit_path(self, path);
+impl<'ty> TypeParamFinder<'ty, '_> {
+    fn new(type_params: &'ty [&'ty syn::Ident]) -> Self {
+        Self {
+            type_params,
+            found: BTreeSet::new(),
         }
     }
-
-    let mut finder = TypeParamFinder {
-        type_params,
-        found: std::collections::HashSet::new(),
-    };
-
-    for bound in bounds {
-        finder.visit_type_param_bound(bound);
-    }
-
-    // Preserve the original order of type parameters for deterministic output
-    type_params
-        .iter()
-        .filter(|param| finder.found.contains(*param))
-        .copied()
-        .collect()
 }
 
-fn find_type_params_in_predicate<'b>(
-    predicate: &syn::WherePredicate,
-    type_params: &'b [&'b syn::Ident],
-) -> Vec<&'b syn::Ident> {
-    use syn::visit::Visit;
-
-    struct TypeParamFinder<'a> {
-        type_params: &'a [&'a syn::Ident],
-        found: std::collections::HashSet<&'a syn::Ident>,
-    }
-
-    impl<'ast> Visit<'ast> for TypeParamFinder<'_> {
-        fn visit_path(&mut self, path: &'ast syn::Path) {
-            // Check if this path is one of our type parameters
-            for &param in self.type_params {
-                if path.is_ident(param) {
-                    self.found.insert(param);
-                }
+impl<'ast> Visit<'ast> for TypeParamFinder<'_, 'ast> {
+    fn visit_path(&mut self, path: &'ast syn::Path) {
+        // Check if this path is one of our type parameters
+        if let Some(param) = path.get_ident() {
+            if self.type_params.contains(&param) {
+                self.found.insert(param);
             }
-            // Continue visiting nested paths
-            syn::visit::visit_path(self, path);
         }
-    }
 
-    let mut finder = TypeParamFinder {
-        type_params,
-        found: std::collections::HashSet::new(),
-    };
-    finder.visit_where_predicate(predicate);
-    // Preserve the original order of type parameters for deterministic output
-    type_params
-        .iter()
-        .filter(|param| finder.found.contains(*param))
-        .copied()
-        .collect()
+        // Continue visiting nested paths
+        syn::visit::visit_path(self, path);
+    }
 }
 
 fn replace_type_param_in_predicate(
@@ -406,8 +370,6 @@ fn member_uses_generic_param(member: &super::NamedMember, param_ident: &syn::Ide
 
 /// Recursively check if a type uses a specific generic parameter
 fn type_uses_generic_param(ty: &syn::Type, param_ident: &syn::Ident) -> bool {
-    use syn::visit::Visit;
-
     struct GenericParamVisitor<'a> {
         param_ident: &'a syn::Ident,
         found: bool,
diff --git a/bon-macros/src/builder/builder_gen/top_level_config/mod.rs b/bon-macros/src/builder/builder_gen/top_level_config/mod.rs
@@ -183,6 +183,19 @@ impl TopLevelConfig {
             ..Self::from_list(&configs)?
         };
 
+        if let Some(generics) = &me.generics {
+            if generics.setters.is_some() {
+                if let Some(const_) = &me.const_ {
+                    bail!(
+                        const_,
+                        "`generics(setters(...))` cannot be used together with `const` \
+                         functions; if you have a use case for this, consider opening an \
+                         issue to discuss it!"
+                    );
+                }
+            }
+        }
+
         if let Some(on) = me.on.iter().skip(1).find(|on| on.required.is_present()) {
             bail!(
                 &on.required.span(),
diff --git a/bon/tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs b/bon/tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs
@@ -5,14 +5,35 @@ use bon::Builder;
 
 #[derive(Builder)]
 #[builder(generics(setters(name = "conv_{}")))]
-struct FnPointerField<T> {
+struct FnPointerFieldInOut<T> {
     value: fn(T) -> T,
 }
 
+#[derive(Builder)]
+#[builder(generics(setters(name = "conv_{}")))]
+struct FnPointerFieldIn<T> {
+    value: fn(T),
+}
+
+#[derive(Builder)]
+#[builder(generics(setters(name = "conv_{}")))]
+struct FnPointerFieldOut<T> {
+    value: fn() -> T,
+}
+
 fn main() {
-    // Test fn(T) -> T - can't change type after setting field
-    FnPointerField::<()>::builder()
-        .value(|x| x)
+    FnPointerFieldInOut::<()>::builder()
+        .value(|()| ())
+        .conv_t::<bool>()
+        .build();
+
+    FnPointerFieldIn::<()>::builder()
+        .value(|()| ())
+        .conv_t::<bool>()
+        .build();
+
+    FnPointerFieldOut::<()>::builder()
+        .value(|| ())
         .conv_t::<bool>()
         .build();
 }
diff --git a/bon/tests/integration/ui/compile_fail/generics_setters/fn_pointer.stderr b/bon/tests/integration/ui/compile_fail/generics_setters/fn_pointer.stderr
@@ -1,16 +1,50 @@
-error[E0277]: the member `Set<value>` was already set, but this method requires it to be unset
-  --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:16:10
+error[E0277]: the member `Set<fn_pointer_field_in_out_builder::members::value>` was already set, but this method requires it to be unset
+  --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:27:10
    |
-16 |         .conv_t::<bool>()
-   |          ^^^^^^ the member `Set<value>` was already set, but this method requires it to be unset
+27 |         .conv_t::<bool>()
+   |          ^^^^^^ the member `Set<fn_pointer_field_in_out_builder::members::value>` was already set, but this method requires it to be unset
    |
-   = help: the trait `IsUnset` is not implemented for `Set<value>`
-note: required by a bound in `FnPointerFieldBuilder::<T, S>::conv_t`
+   = help: the trait `IsUnset` is not implemented for `Set<fn_pointer_field_in_out_builder::members::value>`
+note: required by a bound in `FnPointerFieldInOutBuilder::<T, S>::conv_t`
   --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:6:10
    |
  6 | #[derive(Builder)]
-   |          ^^^^^^^ required by this bound in `FnPointerFieldBuilder::<T, S>::conv_t`
+   |          ^^^^^^^ required by this bound in `FnPointerFieldInOutBuilder::<T, S>::conv_t`
  7 | #[builder(generics(setters(name = "conv_{}")))]
- 8 | struct FnPointerField<T> {
-   |                       - required by a bound in this associated function
+ 8 | struct FnPointerFieldInOut<T> {
+   |                            - required by a bound in this associated function
+   = note: this error originates in the derive macro `Builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the member `Set<fn_pointer_field_in_builder::members::value>` was already set, but this method requires it to be unset
+  --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:32:10
+   |
+32 |         .conv_t::<bool>()
+   |          ^^^^^^ the member `Set<fn_pointer_field_in_builder::members::value>` was already set, but this method requires it to be unset
+   |
+   = help: the trait `IsUnset` is not implemented for `Set<fn_pointer_field_in_builder::members::value>`
+note: required by a bound in `FnPointerFieldInBuilder::<T, S>::conv_t`
+  --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:12:10
+   |
+12 | #[derive(Builder)]
+   |          ^^^^^^^ required by this bound in `FnPointerFieldInBuilder::<T, S>::conv_t`
+13 | #[builder(generics(setters(name = "conv_{}")))]
+14 | struct FnPointerFieldIn<T> {
+   |                         - required by a bound in this associated function
+   = note: this error originates in the derive macro `Builder` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the member `Set<fn_pointer_field_out_builder::members::value>` was already set, but this method requires it to be unset
+  --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:37:10
+   |
+37 |         .conv_t::<bool>()
+   |          ^^^^^^ the member `Set<fn_pointer_field_out_builder::members::value>` was already set, but this method requires it to be unset
+   |
+   = help: the trait `IsUnset` is not implemented for `Set<fn_pointer_field_out_builder::members::value>`
+note: required by a bound in `FnPointerFieldOutBuilder::<T, S>::conv_t`
+  --> tests/integration/ui/compile_fail/generics_setters/fn_pointer.rs:18:10
+   |
+18 | #[derive(Builder)]
+   |          ^^^^^^^ required by this bound in `FnPointerFieldOutBuilder::<T, S>::conv_t`
+19 | #[builder(generics(setters(name = "conv_{}")))]
+20 | struct FnPointerFieldOut<T> {
+   |                          - required by a bound in this associated function
    = note: this error originates in the derive macro `Builder` (in Nightly builds, run with -Z macro-backtrace for more info)
diff --git a/bon/tests/integration/ui/compile_fail/generics_setters/interdependent_param_bounds.stderr b/bon/tests/integration/ui/compile_fail/generics_setters/interdependent_param_bounds.stderr
@@ -1,7 +1,7 @@
 error: generic conversion methods cannot be generated for interdependent type parameters; the bounds on generic parameter `Iter` reference other type parameters: `Item`
 
        Consider removing `generics(setters(...))` or restructuring your types to avoid interdependencies
- --> tests/integration/ui/compile_fail/generics_setters/interdependent_param_bounds.rs:7:18
+ --> tests/integration/ui/compile_fail/generics_setters/interdependent_param_bounds.rs:7:34
   |
 7 | struct Sut<Iter: Iterator<Item = Item>, Item> {
-  |                  ^^^^^^^^
+  |                                  ^^^^
diff --git a/scripts/test-msrv.sh b/scripts/test-msrv.sh
@@ -35,6 +35,7 @@ with_log cd bon
 step echo '[workspace]' >> Cargo.toml
 
 step cargo update --precise 0.21.3  -p darling
+step cargo update --precise 1.0.22  -p unicode-ident
 step cargo update --precise 1.0.15  -p itoa
 step cargo update --precise 1.0.101 -p proc-macro2
 step cargo update --precise 1.0.40  -p quote

Original file line number	Diff line number	Diff line change
`@@ -1,7 +1,7 @@`
`1`	`1`	error: generic conversion methods cannot be generated for interdependent type parameters; the bounds on generic parameter `Iter` reference other type parameters: `Item`
`2`	`2`
`3`	`3`	Consider removing `generics(setters(...))` or restructuring your types to avoid interdependencies
`4`		`- --> tests/integration/ui/compile_fail/generics_setters/interdependent_param_bounds.rs:7:18`
	`4`	`+ --> tests/integration/ui/compile_fail/generics_setters/interdependent_param_bounds.rs:7:34`
`5`	`5`	`\|`
`6`	`6`	`7 \| struct Sut<Iter: Iterator<Item = Item>, Item> {`
`7`		`- \| ^^^^^^^^`
	`7`	`+ \| ^^^^`