Adding support for build_from and build_from_clone builder methods.

bon-macros/src/builder/builder_gen/build_from.rs

@@ -0,0 +1,133 @@
+use crate::builder::builder_gen::{BuilderGenCtx, member::Member};
+use crate::util::prelude::*;
+use proc_macro2::Span;
+use quote::quote;
+use syn::{Type, spanned::Spanned};
+
+pub(super) fn emit(ctx: &BuilderGenCtx, target_ty: &Type) -> Result<TokenStream> {
+    let mut tokens = TokenStream::new();
+
+    let field_vars: Vec<_> = ctx
+        .members
+        .iter()
+        .map(|member| {
+            let ident = member.orig_ident();
+            let ty = member.norm_ty();
+            let default_expr = quote! { ::core::default::Default::default() };
+
+            match member {
+                Member::Field(_) | Member::StartFn(_) => quote! {
+                    let #ident: #ty = self.#ident;
+                },
+                Member::Named(member) => {
+                    let index = &member.index;
+                    quote! {
+                        let #ident: #ty = match self.__unsafe_private_named.#index {
+                            Some(value) => value,
+                            None => from.#ident.clone(),
+                        };
+                    }
+                }
+                Member::FinishFn(_) => quote! {
+                    let #ident: #ty = from.#ident.clone();
+                },
+                Member::Skip(_) => quote! {
+                    let #ident: #ty = #default_expr;
+                },
+            }
+        })
+        .collect();
+
+    let ctor_args: Vec<_> = ctx
+        .members
+        .iter()
+        .map(|m| {
+            let ident = m.orig_ident();
+            quote! { #ident }
+        })
+        .collect();
+
+    if ctx.build_from {
+        tokens.extend(emit_build_from_method(
+            false,
+            target_ty,
+            &field_vars,
+            &ctor_args,
+        ));
+    }
+
+    if ctx.build_from_clone {
+        tokens.extend(emit_build_from_method(
+            true,
+            target_ty,
+            &field_vars,
+            &ctor_args,
+        )?);
+    }
+
+    Ok(tokens)
+}
+
+fn emit_build_from_method(
+    clone: bool,
+    target_ty: &Type,
+    field_vars: &[TokenStream],
+    ctor_args: &[TokenStream],
+) -> Result<TokenStream> {
+    let doc = if clone {
+        "Fills unset builder fields from an owned value of the target type and builds it."
+    } else {
+        "Fills unset builder fields from a reference to the target type and builds it."
+    };
+
+    let method_name = if clone {
+        quote!(build_from_clone)
+    } else {
+        quote!(build_from)
+    };
+
+    let arg_type = if clone {
+        quote!(#target_ty)
+    } else {
+        quote!(&#target_ty)
+    };
+
+    let arg_pat = if clone {
+        quote!(mut from)
+    } else {
+        quote!(from)
+    };
+
+    let ctor_path = extract_ctor_ident_path(target_ty, target_ty.span())?;
+
+    Ok(quote! {
+        #[inline(always)]
+        #[doc = #doc]
+        pub fn #method_name(self, #arg_pat: #arg_type) -> #target_ty {
+            #( #field_vars )*
+            #ctor_path {
+                #( #ctor_args, )*
+            }
+        }
+    })
+}
+
+pub(crate) fn extract_ctor_ident_path(ty: &Type, span: Span) -> Result<TokenStream> {
+    use quote::quote_spanned;
+
+    let path = ty.as_path_no_qself().ok_or_else(|| {
+        err!(
+            &span,
+            "expected a concrete type path (like `MyStruct`) for constructor"
+        )
+    })?;
+
+    let ident = path
+        .segments
+        .last()
+        .ok_or_else(|| err!(&span, "expected a named type, but found an empty path"))?
+        .ident
+        .clone();
+
+    Ok(quote_spanned! { span => #ident })
+}

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

@@ -412,6 +412,8 @@ impl<'a> FnInputCtx<'a> {
             state_mod: self.config.state_mod,
             start_fn: self.start_fn,
             finish_fn,
+            build_from: self.config.build_from,
+            build_from_clone: self.config.build_from_clone,
         })
     }
 }

bon-macros/src/builder/builder_gen/input_struct.rs

@@ -238,6 +238,8 @@ impl StructInputCtx {
             state_mod: self.config.state_mod,
             start_fn,
             finish_fn,
+            build_from: self.config.build_from,
+            build_from_clone: self.config.build_from_clone,
         })
     }
 }

bon-macros/src/builder/builder_gen/mod.rs

@@ -1,3 +1,4 @@
+mod build_from;
 mod builder_decl;
 mod builder_derives;
 mod finish_fn;
@@ -134,6 +135,11 @@ impl BuilderGenCtx {
 
         let allows = allow_warnings_on_member_types();
 
+        let build_froms = match &self.finish_fn.output {
+            syn::ReturnType::Type(_, ty) => build_from::emit(self, ty)?,
+            syn::ReturnType::Default => quote! {},
+        };
+
         Ok(quote! {
             #allows
             #[automatically_derived]
@@ -145,6 +151,7 @@ impl BuilderGenCtx {
             #where_clause
             {
                 #finish_fn
+                #build_froms
                 #(#accessor_methods)*
             }
         })

bon-macros/src/builder/builder_gen/models.rs

@@ -173,6 +173,8 @@ pub(crate) struct BuilderGenCtx {
     pub(super) state_mod: StateMod,
     pub(super) start_fn: StartFn,
     pub(super) finish_fn: FinishFn,
+    pub(super) build_from: bool,
+    pub(super) build_from_clone: bool,
 }
 
 pub(super) struct BuilderGenCtxParams<'a> {
@@ -201,6 +203,8 @@ pub(super) struct BuilderGenCtxParams<'a> {
     pub(super) state_mod: ItemSigConfig,
     pub(super) start_fn: StartFnParams,
     pub(super) finish_fn: FinishFnParams,
+    pub(super) build_from: bool,
+    pub(super) build_from_clone: bool,
 }
 
 impl BuilderGenCtx {
@@ -219,6 +223,8 @@ impl BuilderGenCtx {
             state_mod,
             start_fn,
             finish_fn,
+            build_from,
+            build_from_clone,
         } = params;
 
         let builder_type = BuilderType {
@@ -370,6 +376,8 @@ impl BuilderGenCtx {
             state_mod,
             start_fn,
             finish_fn,
+            build_from,
+            build_from_clone,
         })
     }
 }

bon-macros/src/builder/builder_gen/top_level_config/mod.rs

@@ -1,14 +1,14 @@
 mod on;
 
 pub(crate) use on::OnConfig;
 
 use crate::parsing::{BonCratePath, ItemSigConfig, ItemSigConfigParsing, SpannedKey};
 use crate::util::prelude::*;
-use darling::ast::NestedMeta;
 use darling::FromMeta;
+use darling::ast::NestedMeta;
+use syn::ItemFn;
 use syn::parse::Parser;
 use syn::punctuated::Punctuated;
-use syn::ItemFn;
 
 fn parse_finish_fn(meta: &syn::Meta) -> Result<ItemSigConfig> {
     ItemSigConfigParsing {
@@ -75,6 +75,12 @@
     /// Specifies the derives to apply to the builder.
     #[darling(default, with = crate::parsing::parse_non_empty_paren_meta_list)]
     pub(crate) derive: DerivesConfig,
+
+    #[darling(default)]
+    pub(crate) build_from: bool,
+
+    #[darling(default)]
+    pub(crate) build_from_clone: bool,
 }
 
 impl TopLevelConfig {

bon-macros/src/builder/builder_gen/top_level_config/on.rs

@@ -1,6 +1,6 @@
 use crate::util::prelude::*;
-use darling::util::Flag;
 use darling::FromMeta;
+use darling::util::Flag;
 use syn::parse::Parse;
 use syn::spanned::Spanned;
 use syn::visit::Visit;
@@ -79,7 +79,7 @@
 
         impl Visit<'_> for FindAttr {
             fn visit_attribute(&mut self, attr: &'_ syn::Attribute) {
-                self.attr.get_or_insert(attr.span());
+                self.attr.get_or_insert_with(|| attr.span());
             }
         }
 

bon/src/__/ide.rs

@@ -101,6 +101,40 @@ pub mod builder_top_level {
         pub use core::convert::Into;
     }
 
+    /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from>
+    pub const build_from: Option<Identifier> = None;
+
+    /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from>
+    pub mod build_from {
+        use super::*;
+
+        /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from#name>
+        pub const name: Identifier = Identifier;
+
+        /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from#vis>
+        pub const vis: VisibilityString = VisibilityString;
+
+        /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from#doc>
+        pub const doc: DocComments = DocComments;
+    }
+
+    /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from_clone>
+    pub const build_from_clone: Option<Identifier> = None;
+
+    /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from_clone>
+    pub mod build_from_clone {
+        use super::*;
+
+        /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from_clone#name>
+        pub const name: Identifier = Identifier;
+
+        /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from_clone#vis>
+        pub const vis: VisibilityString = VisibilityString;
+
+        /// See the docs at <https://bon-rs.com/reference/builder/top-level/build_from_clone#doc>
+        pub const doc: DocComments = DocComments;
+    }
+
     /// The real name of this parameter is `crate` (without the underscore).
     /// It's hinted with an underscore due to the limitations of the current
     /// completions limitation. This will be fixed in the future.

bon/tests/integration/builder/build_from.rs

@@ -0,0 +1,26 @@
+use crate::prelude::*;
+
+#[derive(Builder, Clone)]
+#[builder(build_from, build_from_clone)]
+struct User {
+    name: String,
+    age: u8,
+}
+
+#[test]
+fn test_build_from_works() {
+    let jon = User::builder().name("Jon".into()).age(25).build();
+    let alice = User::builder().name("Alice".into()).build_from(&jon);
+    assert_eq!(alice.age, jon.age);
+    assert_eq!(alice.name, "Alice");
+}
+
+#[test]
+fn test_build_from_clone_works() {
+    let jon = User::builder().name("Jon".into()).age(25).build();
+    let alice = User::builder()
+        .name("Alice".into())
+        .build_from_clone(jon.clone());
+    assert_eq!(alice.age, jon.age);
+    assert_eq!(alice.name, "Alice");
+}

bon/tests/integration/builder/mod.rs

@@ -14,6 +14,7 @@ mod attr_setters;
 mod attr_skip;
 mod attr_top_level_start_fn;
 mod attr_with;
+mod build_from;
 mod cfgs;
 mod generics;
 mod init_order;