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[BernardIgiri]

Add build_from and build_from_clone builder methods

Closes #310

This PR is a fresh continuation of the original #313 , which I accidentally closed when rebasing my master branch. I have since transplanted my work on here:

Original PR Overview

This PR adds support for two new optional top-level builder attributes:

• #[builder(build_from)] – Adds a .build_from(T) / .call_from(T) method (takes T by value).
• #[builder(build_from_clone)] – Adds a .build_from_clone(&T) / .call_from_clone(&T) method (takes &T and clones fields).

These methods allow partially configured builders to fill in missing fields from an existing instance of the target type before finalizing the build. This is useful when:

1. You want to override only a few fields but reuse most of the existing data.
2. You're working with types that aren't Default, making field reuse non-trivial.

Example

Rust

#[derive(Builder, Clone)]
#[builder(build_from, build_from_clone)]
struct User {
    name: String,
    age: u8,
}

let jon = User::builder().name("Jon".into()).age(25).build();
let alice = User::builder().name("Alice".into()).build_from_clone(&jon);

assert_eq!(alice.name, "Alice");
assert_eq!(alice.age, 25);

Implementation Notes

• The code generation engine lives in builder_gen/build_from.rs and is conditionally compiled/emitted via the experimental-build-from feature flag.
• Integrating ItemSigConfig directly into darling::FromMeta allows native attribute parsing without structural wrapper types.
• Each field is handled according to its member kind:
  • Named fields: Fallback to the instance data if not explicitly set in the builder state.
  • FinishFn fields: Defer to the instance data.
  • Skip fields: Drop back to Default::default().

WIP Status & Next Steps

This PR is currently open as a Draft / WIP. Before marking it ready for full review, I am working on polishing a few key items:

• Inherit Attributes: Ensure that custom visibility (vis) and documentation (doc) passed inside #[builder(build_from(...))] are properly inherited by the generated methods.
• Function/Method Builders: Refactor code generation to correctly support bon when applied to functions or associated methods (ensuring we invoke the execution block rather than blindly emitting a structural literal path).
• Expanded Integration Tests: Adding test coverage for complex namespaces, turbofish paths, and custom vis/doc combinations.