v3.1.0

Added

• Add #[builder(field)] to define custom private fields on the builder type. See Custom Fields guide for details (#207)

Other

• Internal refactoring of naming in bons test suite (#215)

v3.0.2

Fixed

• Fix unexpected_cfgs lint coming from #[cfg(rust_analyzer)] on the latest nightly (#212)

v3.0.1

Fixed

• Fix handling of lifetimes not used in fn param types (#208)

v3.0.0

See the most interesting changes described in Bon 3.0 Release blog post.

All the breaking changes are very unlikely to actually break your code that was written against the v2 version of bon. 99% of users should be able to update without any migration.

Changed

• 🎉🎉 Stabilize the builder's typestate API allowing for custom builder extensions. This is the main theme of this release. This new API brings the flexibility to a whole new level 🚀 🚀 (#145)

• Improve rustdoc output. See the rustoc examples and comparison in the Alternatives section (#145)

  • Add info that the member is required or optional.

  • For members with default values show the default value in the docs.

  • For optional members provide links to {member}(T) and maybe_{member}(Option<T>) setters.

  • Remove __ prefixes for generic types and lifetimes from internal symbols. Instead, the prefixes added only if the macro detects a name collision.

• ⚠️ Breaking. Reject unnecessary empty attributes e.g. #[builder()] or #[builder] with no parameters on a member (#145)

• ⚠️ Breaking. Reject square brackets and curly braces delimiters for builder_type, finish_fn, start_fn and on attributes syntax. Only parentheses are accepted e.g. #[builder(finish_fn(...))] or #[builder(on(...))]. This no longer works: #[builder(finish_fn[...])] or #[builder(on{...})] (#145)

• ⚠️ Breaking. Reject non-consecutive on(...) clauses. For example, the following now generates a compile error: #[builder(on(String, into), finish_fn = build, on(Vec<_>, into))], because there is a finish_fn = ... between on(...) clauses. (#155)

• ⚠️ Breaking. #[builder(derive(Clone, Debug))] now generates impl blocks that follow the behaviour of standard Clone and Debug derives in that it conservatively adds Clone/Debug trait bounds for all the generic types declared on the original item (struct or function). Previously no additional bounds were required on Clone and Debug impls. See the Added section for details on the way to override these bounds with #[builder(derive(Clone/Debug(bounds(...))))] (#145)

• ⚠️ Breaking. The name of the builder struct generated for methods named builder changed from TBuilderBuilder to just TBuilder making methods named builder work the same as methods named new. (#145)

• ⚠️ Breaking. The type of the builder is now dependent on the order of the setters' invocation. This may only break code like the following:

  let builder = if condition {
      Foo::builder().a(1).b(2)
  } else {
      Foo::builder().b(1).a(2)
  };
  
  builder.build();

  This is because the types of the builders returned from the branches are the following:

  • FooBuilder<SetB<SetA>> (if branch)
  • FooBuilder<SetA<SetB>> (else branch)

  We believe such code should generally be very rare and even if it breaks, it's easy to fix it by reordering the setter method calls. This compromise was accepted as a design tradeoff such that the builder's type signature becomes simpler, the generated documentation becomes much less noisy, it removes an annoying special case for the builder of just one member, and it improves the type-checking performance considerably compared to the previous approach that used tuples to represent the type state. (#145)

Removed

• ⚠️ Breaking. Remove support for #[bon::builder] proc-macro attribute on top of a struct. Use #[derive(bon::Builder)] for that instead. This syntax has been deprecated since 2.1 and it is now removed as part of a major version cleanup (#145)

• ⚠️ Breaking. Remove #[builder(expose_positional_fn = positional_fn_name)] attribute. Use #[builder(start_fn = builder_fn_name)] instead, since this attribute works additively keeping the function with positional arguments under the attribute unchanged. (#153)

Added

• ⚠️ Breaking. Builder macros now generate additional mod builder_name {} where builder_name is the snake_case version of the name of the builder struct. This new module contains the type state API of the builder. There is a low probability that this new module name may conflict with existing symbols in your scope, so this change is marked as breaking (#145)

• Add #[builder(builder_type(vis = "...", doc { ... }))] that allows overriding the visibility and docs of the builder struct (#145)

• Add #[builder(finish_fn(vis = "...", doc { ... } ))] that allows overriding the visibility and docs of the finishing function (#145)

• Add #[builder(start_fn(doc { ... }))] that allows overriding the docs of the starting function (#145)

• Add #[builder(with = closure)] syntax to customize setters with a closure. If the closure returns a Result<_, E> the setters become fallible (#145)

• Add #[builder(with = Some)], #[builder(with = FromIterator::from_iter)], #[builder(with = <_>::from_iter)] syntax support for two well-known functions that will probably be used frequently (#157)

• Add #[builder(required)] for Option fields to opt out from their special handling which makes bon treat them as regular required fields. It's also available at the top-level via #[builder(on(_, required))] (#145, #155)

• Add #[builder(crate = path::to::bon)] and #[bon(crate = path::to::bon)] to allow overriding the path to bon crate used in the generated code, which is useful for the cases when bon macros are wrapped by other macros (#153)

• Add #[builder(state_mod)] to configure the builder's type state API module name, visibility and docs (#145)

• 🔬 Experimental. Add #[builder(overwritable)] and #[builder(on(..., overwritable)] to make it possible to call setters multiple times for the same member. This attribute is available under the cargo feature "experimental-overwritable". The fate of this feature depends on your feedback in the tracking issue #149. Please, let us know if you have a use case for this attribute! (#145)

• Add #[builder(setters)] to fine-tune the setters names, visibility and docs (#145)

• Add #[builder(derive(Clone/Debug(bounds(...))] to allow overriding trait bounds on the Clone/Debug impl block of the builder (#145)

• Add inheritance of #[allow()] and #[expect()] lint attributes to all generated items. This is useful to suppress any lints coming from the generated code. Although, lints coming from the generated code are generally considered defects in bon and should be reported via a Github issue, but this provides an easy temporary workaround for the problem (#145)

Fixed

• Fix false-positive unused_mut lints coming from #[builder] on a method that takes mut self (#197)
• Fix #[cfg/cfg_attr()] not being expanded when used on function arguments with doc comments or other attributes (#145)
• Fix raw identifiers in optional/default members (#175)

Other

• Add graceful internal panic handling. If some bon macro panics due to an internal bug, the macro will try to generate a fallback for IDEs to still provide intellisense (#145)
• Switch from elastio.github.io/bon to a custom domain bon-rs.com (#158)
• Add anonymous stats with umami for the docs website (#158)

Docs

• Refactor the README.md and all pages in the Guide Book by simplifying them and removing redundancies (#170)

• Add new pages to the Guide Book:

  • Custom Conversions (#170)

  • [T...

v3.0.0-rc

All the breaking changes are very unlikely to actually break your code that was written against the v2 version of bon. 99% of users should be able to update without any migration.

Changed

• 🎉🎉 Stabilize the builder's typestate API allowing for custom builder extensions. This is the main theme of this release. This new API brings the flexibility to a whole new level 🚀 🚀 (#145)

• Improve rustdoc output. See the rustoc examples and comparison in the Alternatives section (#145)

  • Add info that the member is required or optional.

  • For members with default values show the default value in the docs.

  • For optional members provide links to {member}(T) and maybe_{member}(Option<T>) setters.

  • Remove __ prefixes for generic types and lifetimes from internal symbols. Instead, the prefixes added only if the macro detects a name collision.

• ⚠️ Breaking. Reject unnecessary empty attributes e.g. #[builder()] or #[builder] with no parameters on a member (#145)

• ⚠️ Breaking. Reject square brackets and curly braces delimiters for builder_type, finish_fn, start_fn and on attributes syntax. Only parentheses are accepted e.g. #[builder(finish_fn(...))] or #[builder(on(...))]. This no longer works: #[builder(finish_fn[...])] or #[builder(on{...})] (#145)

• ⚠️ Breaking. Reject non-consecutive on(...) clauses. For example, the following now generates a compile error: #[builder(on(String, into), finish_fn = build, on(Vec<_>, into))], because there is a finish_fn = ... between on(...) clauses. (#155)

• ⚠️ Breaking. #[builder(derive(Clone, Debug))] now generates impl blocks that follow the behaviour of standard Clone and Debug derives in that it conservatively adds Clone/Debug trait bounds for all the generic types declared on the original item (struct or function). Previously no additional bounds were required on Clone and Debug impls. See the Added section for details on the way to override these bounds with #[builder(derive(Clone/Debug(bounds(...))))] (#145)

• ⚠️ Breaking. The name of the builder struct generated for methods named builder changed from TBuilderBuilder to just TBuilder making methods named builder work the same as methods named new. (#145)

• ⚠️ Breaking. The type of the builder is now dependent on the order of the setters' invocation. This may only break code like the following:

  let builder = if condition {
      Foo::builder().a(1).b(2)
  } else {
      Foo::builder().b(1).a(2)
  };
  
  builder.build();

  This is because the types of the builders returned from the branches are the following:

  • FooBuilder<SetB<SetA>> (if branch)
  • FooBuilder<SetA<SetB>> (else branch)

  We believe such code should generally be very rare and even if it breaks, it's easy to fix it by reordering the setter method calls. This compromise was accepted as a design tradeoff such that the builder's type signature becomes simpler, the generated documentation becomes much less noisy, it removes an annoying special case for the builder of just one member, and it improves the type-checking performance considerably compared to the previous approach that used tuples to represent the type state. (#145)

Removed

• ⚠️ Breaking. Remove support for #[bon::builder] proc-macro attribute on top of a struct. Use #[derive(bon::Builder)] for that instead. This syntax has been deprecated since 2.1 and it is now removed as part of a major version cleanup (#145)

• ⚠️ Breaking. Remove #[builder(expose_positional_fn = positional_fn_name)] attribute. Use #[builder(start_fn = builder_fn_name)] instead, since this attribute works additively keeping the function with positional arguments under the attribute unchanged. (#153)

Added

• ⚠️ Breaking. Builder macros now generate additional mod builder_name {} where builder_name is the snake_case version of the name of the builder struct. This new module contains the type state API of the builder. There is a low probability that this new module name may conflict with existing symbols in your scope, so this change is marked as breaking (#145)

• Add #[builder(builder_type(vis = "...", doc { ... }))] that allows overriding the visibility and docs of the builder struct (#145)

• Add #[builder(finish_fn(vis = "...", doc { ... } ))] that allows overriding the visibility and docs of the finishing function (#145)

• Add #[builder(start_fn(doc { ... }))] that allows overriding the docs of the starting function (#145)

• Add #[builder(with = closure)] syntax to customize setters with a closure. If the closure returns a Result<_, E> the setters become fallible (#145)

• Add #[builder(with = Some)], #[builder(with = FromIterator::from_iter)], #[builder(with = <_>::from_iter)] syntax support for two well-known functions that will probably be used frequently (#157)

• Add #[builder(required)] for Option fields to opt out from their special handling which makes bon treat them as regular required fields. It's also available at the top-level via #[builder(on(_, required))] (#145, #155)

• Add #[builder(crate = path::to::bon)] and #[bon(crate = path::to::bon)] to allow overriding the path to bon crate used in the generated code, which is useful for the cases when bon macros are wrapped by other macros (#153)

• Add #[builder(state_mod)] to configure the builder's type state API module name, visibility and docs (#145)

• 🔬 Experimental. Add #[builder(overwritable)] and #[builder(on(..., overwritable)] to make it possible to call setters multiple times for the same member. This attribute is available under the cargo feature "experimental-overwritable". The fate of this feature depends on your feedback in the tracking issue #149. Please, let us know if you have a use case for this attribute! (#145)

• Add #[builder(setters)] to fine-tune the setters names, visibility and docs (#145)

• Add #[builder(derive(Clone/Debug(bounds(...))] to allow overriding trait bounds on the Clone/Debug impl block of the builder (#145)

• Add inheritance of #[allow()] and #[expect()] lint attributes to all generated items. This is useful to suppress any lints coming from the generated code. Although, lints coming from the generated code are generally considered defects in bon and should be reported via a Github issue, but this provides an easy temporary workaround for the problem (#145)

Fixed

• Fix #[cfg/cfg_attr()] not being expanded when used on function arguments with doc comments or other attributes.
• Fix raw identifiers in optional/default members (#175)

Other

• Add graceful internal panic handling. If some bon macro panics due to an internal bug, the macro will try to generate a fallback for IDEs to still provide intellisense (#145)
• Switch from elastio.github.io/bon to a custom domain bon-rs.com (#158)
• Add anonymous stats with umami for the docs website (#158)

Docs

• Refactor the README.md and all pages in the Guide Book by simplifying them and removing redundancies (#170)

• Add new pages to the Guide Book:

  • Custom Conversions (#170)

  • Typestate API (#170)

  • Builder's Type Signature (#170)

  • [Custom Methods](https://bon-rs.com/guide/typestate-api/...

v2.3.0

See the blog post for this release that describes some of the most notable changes in detail.

Added

• Add support for positional params in start_fn and finish_fn (#125)

Fixed

• Forward lint suppression from #[allow()/expect()] attributes written by the user on the top-level to the generated items (#125)
• Suppress the false-positive (clippy issue) clippy::future_not_send lint (#125)
• Fix the cases where #[builder(derive(Debug, Clone))] didn't validate for all members to implement Clone/Debug if these members were of reference or generic types (#125)

v2.2.1

Changed

• Lower MSRV from 1.70.0 to 1.59.0 (#120)

v2.2.0

See the blog post for this release that describes some of the most notable changes in detail.

Changed

• The #[bon::builder] attribute was deprecated on structs. The new #[derive(bon::Builder)] should be used to derive a builder from a struct. Starting with bon 2.3 (next minor release) all usages of #[bon::builder] on structs will generate deprecation warnings. (#99).

  There is a CLI to assist in migrating to the new syntax. See the release blog post for details about that.

Added

• Add the top-level #[builder(derive(...))] attribute to be able to derive Clone and Debug for the builder type itself (#113)

• Add support for conditional compilation with cfg/cfg_attr (#99)

Fixed

• Fix developer experience in Rust Rover. The new #[derive(Builder)] syntax should now be easier for Rust Rover to analyze (#99)
• Fix a bug where a member of opaque Option type (i.e. the Option type that was renamed to make the builder macro not detect it as Option) was still optional. (#99)
• Fix code generation for structs with default values for generic parameters (#108)

v2.1.1

Added

• Set MSRV to 1.70.0. Note that we plan to set an even lower MSRV. This is just an initial attempt to define the MSRV that should be good enough in the meantime while we work on lowering it even more (#101)

Fixed

• Fix lints triggered by generated code such as private_bounds, clippy::missing_const_for_fn (#101)
• Add more context to the messages such that it's clear what member isn't set in Rust Analyzer error messages (#98)

v2.1.0

See the blog post for this release that describes some of the most notable changes in detail. This post is available on the platform of your choice:

• Reddit
• X (Twitter)

Added

• #[must_use] on the build() method for structs and call() for functions (if the original function has #[must_use]) (#82). Thanks @EdJoPaTo for the contribution!

Changed

• Optimize bon's generated code type-checking performance and improve error messages (#84)
• Improve builder() method docs (#76). Thanks @EdJoPaTo for the contribution!

Fixed

• Don't warn on clippy::impl_trait_in_params (#80). Thanks @EdJoPaTo for the contribution!
• Fix typos in messages and code comments (#79). Thanks @EdJoPaTo for the contribution!

Other

• Add more tests for #[must_use] (#87)