Hello Rust community,
I've been implementing a builder pattern with complex generic types and encountered a puzzling compiler behavior that I can't explain. I've created a minimal reproducible example that demonstrates the issue.
The Problem
I have a builder pattern where chaining two methods (build_b followed by build_last) works perfectly. However, when I try to encapsulate this chain in a single method (build), I get trait bound errors. What's particularly confusing is that creating a new instance and chaining methods works in a helper function, but using the passed-in instance fails with the same error.
Complete Example
Here's a minimal example demonstrating the issue:
pub trait Call<Input> {
type Output;
fn docall(&self, input: Input) -> Self::Output;
}
struct ToAddCall;
impl<T> Call<T> for ToAddCall {
type Output = AddCall;
fn docall(&self, _input: T) -> Self::Output {
AddCall(1)
}
}
struct AddCall(i32);
impl Call<i32> for AddCall {
type Output = i32;
fn docall(&self, input: i32) -> Self::Output {
input + self.0
}
}
pub struct A<Data1>(Data1);
fn build<C1, C2, C3>(this: A<C1>, input: i32) -> C2::Output
where
C1: Call<C2, Output = C3>,
C2: Call<i32, Output = i32>,
C3: Call<i32, Output = i32>,
{
// This doesn't work:
// this.build_b().build_last(input) // <-- the trait bound is not satisfied)
// But creating a new instance and chaining works!
drop(this);
let this = A(ToAddCall);
this.build_b().build_last(input)
}
impl<C1> A<C1> {
pub fn build<C2, C3>(self, input: i32) -> i32
where
C1: Call<C2, Output = C3>,
C2: Call<i32, Output = i32>,
C3: Call<i32, Output = i32>,
{
// This doesn't work:
// self.build_b().build_last(input) // <-- the trait bound is not satisfied
// Delegating to a helper function works, but it doesn't use the passed instance
build::<C1, C2, C3>(self, input)
}
pub fn build_b(self) -> B<C1, impl Call<i32, Output = i32>> {
B(self.0, AddCall(1))
}
}
pub struct B<C1, C2>(C1, C2);
impl<C1, C2> B<C1, C2> {
pub fn build_last<C3, Input2>(self, input: Input2) -> C3::Output
where
C1: Call<C2, Output = C3>,
C3: Call<Input2>,
{
self.0.docall(self.1).docall(input)
}
}
#[test]
fn test() {
// Direct chaining works perfectly
let result = A(ToAddCall).build_b().build_last(10);
assert_eq!(result, 11);
// Without type annotations doesn't work
// let result = A(ToAddCall).build(10); // <-- type annotation needed
// With type annotations works
let result = A(ToAddCall).build::<AddCall, AddCall>(10);
assert_eq!(result, 11);
}
Key Observations
-
Direct method chaining works perfectly:
A(ToAddCall).build_b().build_last(10)compiles and runs without issue
-
Inside helper function
build:- Using the passed instance with
this.build_b().build_last(input)fails - Creating a new instance
A(ToAddCall).build_b().build_last(input)works
- Using the passed instance with
-
Inside
A::buildmethod:- Chaining directly with
self.build_b().build_last(input)fails - The error is:
the trait bound 'C1: Call<impl Call<i32, Output = i32>>' is not satisfied - But the generic constraints is matched:
C1: Call<C2, Output=C3>, C2: Call<i32, Ouput=i32>
- Chaining directly with
-
Using the
buildmethod requires explicit type annotations:A(ToAddCall).build(10)fails with type inference errorsA(ToAddCall).build::<AddCall, AddCall>(10)works correctly
Real World Use Case
In my actual codebase, I have a more complex builder pattern where users currently need to chain build_b().build_last(). I want to provide a cleaner API by offering a single build() method that encapsulates this chain, hiding implementation details.
The problem is that I can't get this to work without requiring explicit type annotations, which defeats the purpose of API simplification. The real types are much more complex, so I can't just replace impl Trait with concrete types.
Questions
- Why does method chaining directly work, but attempting to encapsulate the chain in a method fails?
- Why does a helper function work when creating a new instance, but fail when using the passed instance?
- What's happening with the
impl Traitthat prevents it from working with generic constraints in this scenario? - Is there a way to encapsulate this chain in a single method without requiring explicit type annotations?
I've been puzzled by this behavior for days and would greatly appreciate any insights into what's going on here!
You can find the complete code in this Rust Playground: Rust Playground
.