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feat(derive): Add #[command(defer)] for lazy subcommand initialization #6256

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057aef7
refactor(derive): Extract top-level methods
r-near Feb 10, 2026
e6e635d
refactor(derive): Unify Kind::Command codegen
r-near Feb 10, 2026
33ab745
test(derive): Add baseline defer tests
r-near Feb 13, 2026
2f7917c
test(derive): Add build() call in help test
r-near Feb 10, 2026
531fdd5
feat(derive): Add #[command(defer)] attribute
r-near Feb 13, 2026
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19 changes: 19 additions & 0 deletions clap_derive/src/attr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,23 @@ impl ClapAttr {
}
}
}

pub(crate) fn lit_bool_or_abort(&self) -> Result<bool, syn::Error> {
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let value = self.value_or_abort()?;
match value {
AttrValue::Expr(Expr::Lit(syn::ExprLit {
lit: syn::Lit::Bool(lit),
..
})) => Ok(lit.value()),
_ => {
abort!(
self.name,
"attribute `{}` can only accept bool literals",
self.name
)
}
}
}
}

impl Parse for ClapAttr {
Expand Down Expand Up @@ -102,6 +119,7 @@ impl Parse for ClapAttr {
"long_help" => Some(MagicAttrName::LongHelp),
"author" => Some(MagicAttrName::Author),
"version" => Some(MagicAttrName::Version),
"defer" => Some(MagicAttrName::Defer),
_ => None,
};

Expand Down Expand Up @@ -168,6 +186,7 @@ pub(crate) enum MagicAttrName {
DefaultValuesOsT,
NextDisplayOrder,
NextHelpHeading,
Defer,
}

#[derive(Clone)]
Expand Down
15 changes: 9 additions & 6 deletions clap_derive/src/derives/args.rs
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Expand Up @@ -77,6 +77,8 @@ pub(crate) fn gen_for_struct(
let app_var = Ident::new("__clap_app", Span::call_site());
let augmentation = gen_augment(fields, &app_var, item, false)?;
let augmentation_update = gen_augment(fields, &app_var, item, true)?;
let initial_app_methods = item.initial_top_level_methods();
let final_app_methods = item.final_top_level_methods();
Comment on lines +80 to +81
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@epage epage Feb 10, 2026

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The intention would be that we would do this consistently across all derives

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@r-near r-near Feb 10, 2026

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Ok both callers now apply initial/final uniformly. Added here 560fef5 (this PR)


let group_id = if item.skip_group() {
quote!(None)
Expand Down Expand Up @@ -155,10 +157,14 @@ pub(crate) fn gen_for_struct(
#group_id
}
fn augment_args<'b>(#app_var: clap::Command) -> clap::Command {
#augmentation
let #app_var = #app_var #initial_app_methods;
let #app_var = #augmentation;
#app_var #final_app_methods
}
fn augment_args_for_update<'b>(#app_var: clap::Command) -> clap::Command {
#augmentation_update
let #app_var = #app_var #initial_app_methods;
let #app_var = #augmentation_update;
#app_var #final_app_methods
}
}
})
Expand Down Expand Up @@ -380,8 +386,6 @@ pub(crate) fn gen_augment(
} else {
quote!()
};
let initial_app_methods = parent_item.initial_top_level_methods();
let final_app_methods = parent_item.final_top_level_methods();
let group_app_methods = if parent_item.skip_group() {
quote!()
} else {
Expand Down Expand Up @@ -432,11 +436,10 @@ pub(crate) fn gen_augment(
Ok(quote! {{
#deprecations
let #app_var = #app_var
#initial_app_methods
#group_app_methods
;
#( #args )*
#app_var #final_app_methods
#app_var
}})
}

Expand Down
71 changes: 41 additions & 30 deletions clap_derive/src/derives/subcommand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,8 @@ pub(crate) fn gen_for_enum(
let augmentation = gen_augment(variants, item, false)?;
let augmentation_update = gen_augment(variants, item, true)?;
let has_subcommand = gen_has_subcommand(variants)?;
let initial_app_methods = item.initial_top_level_methods();
let final_app_methods = item.final_top_level_methods();

Ok(quote! {
#[allow(
Expand Down Expand Up @@ -123,10 +125,14 @@ pub(crate) fn gen_for_enum(
#[automatically_derived]
impl #impl_generics clap::Subcommand for #item_name #ty_generics #where_clause {
fn augment_subcommands <'b>(__clap_app: clap::Command) -> clap::Command {
#augmentation
let __clap_app = __clap_app #initial_app_methods;
let __clap_app = #augmentation;
__clap_app #final_app_methods
}
fn augment_subcommands_for_update <'b>(__clap_app: clap::Command) -> clap::Command {
#augmentation_update
let __clap_app = __clap_app #initial_app_methods;
let __clap_app = #augmentation_update;
__clap_app #final_app_methods
}
fn has_subcommand(__clap_name: &str) -> bool {
#has_subcommand
Expand Down Expand Up @@ -275,22 +281,22 @@ fn gen_augment(

Kind::Command(_) => {
let subcommand_var = Ident::new("__clap_subcommand", Span::call_site());
let sub_augment = match variant.fields {
let (sub_augment, initial_app_methods, final_from_attrs) = match variant.fields {
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Named(ref fields) => {
// Defer to `gen_augment` for adding cmd methods
let fields = collect_args_fields(item, fields)?;
args::gen_augment(&fields, &subcommand_var, item, override_required)?
}
Unit => {
let arg_block = quote!( #subcommand_var );
let initial_app_methods = item.initial_top_level_methods();
let final_from_attrs = item.final_top_level_methods();
quote! {
let #subcommand_var = #subcommand_var #initial_app_methods;
let #subcommand_var = #arg_block;
#subcommand_var #final_from_attrs
}
let sub_augment =
args::gen_augment(&fields, &subcommand_var, item, override_required)?;
(
sub_augment,
item.initial_top_level_methods(),
item.final_top_level_methods(),
)
}
Unit => (
quote! { #subcommand_var },
item.initial_top_level_methods(),
item.final_top_level_methods(),
),
Unnamed(FieldsUnnamed { ref unnamed, .. }) if unnamed.len() == 1 => {
let ty = &unnamed[0].ty;
let arg_block = if override_required {
Expand All @@ -306,19 +312,25 @@ fn gen_augment(
}
}
};
let initial_app_methods = item.initial_top_level_methods();
let final_from_attrs = item.final_top_level_methods();
quote! {
let #subcommand_var = #subcommand_var #initial_app_methods;
let #subcommand_var = #arg_block;
#subcommand_var #final_from_attrs
}
(
arg_block,
item.initial_top_level_methods(),
item.final_top_level_methods(),
)
}
Unnamed(..) => {
abort!(variant, "invalid variant for `#[command(subcommand)]`, expected a newtype variant")
}
};

let sub_augment = if parent_item.defer() {
quote! {
#subcommand_var.defer(|#subcommand_var| { #sub_augment })
}
} else {
sub_augment
};
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Comment on lines +326 to +332
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@epage epage Feb 12, 2026

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We don't seem to do this for Args. We're missing a test case for this.

Looking back at #6225, I see deferring of arguments discussed at #6225 (comment)

This is causing me to look back at #4959 where new trait methods are suggested and I'm wondering why I suggested those originally and if there is some missing functionality, e.g. around flatten.

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@r-near r-near Feb 13, 2026

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Added a test case for this.

So for the current scope, I think flatten is covered under deferred subcommands, right?

The extra trait-method ideas from #4959 seem like broader defer semantics. IDK if you wanna add that into this PR?


let deprecations = if !override_required {
item.deprecations()
} else {
Expand All @@ -328,8 +340,10 @@ fn gen_augment(
let subcommand = quote! {
let #app_var = #app_var.subcommand({
#deprecations
let #subcommand_var = clap::Command::new(#name);
#sub_augment
let #subcommand_var = clap::Command::new(#name)
#initial_app_methods;
let #subcommand_var = #sub_augment;
#subcommand_var #final_from_attrs
});
};
Some(subcommand)
Expand All @@ -343,14 +357,11 @@ fn gen_augment(
} else {
quote!()
};
let initial_app_methods = parent_item.initial_top_level_methods();
let final_app_methods = parent_item.final_top_level_methods();
Ok(quote! {
Ok(quote! {{
#deprecations;
let #app_var = #app_var #initial_app_methods;
#( #subcommands )*;
#app_var #final_app_methods
})
#app_var
}})
}

fn gen_has_subcommand(variants: &[(&Variant, Item)]) -> Result<TokenStream, syn::Error> {
Expand Down
11 changes: 11 additions & 0 deletions clap_derive/src/item.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,7 @@ pub(crate) struct Item {
group_id: Name,
group_methods: Vec<Method>,
kind: Sp<Kind>,
defer: bool,
}

impl Item {
Expand Down Expand Up @@ -279,6 +280,7 @@ impl Item {
group_id,
group_methods: vec![],
kind,
defer: cfg!(feature = "unstable-v5"),
}
}

Expand Down Expand Up @@ -835,6 +837,11 @@ impl Item {
self.skip_group = true;
}

Some(MagicAttrName::Defer) => {
assert_attr_kind(attr, &[AttrKind::Command])?;
self.defer = attr.lit_bool_or_abort()?;
}

None
// Magic only for the default, otherwise just forward to the builder
| Some(MagicAttrName::Short)
Expand Down Expand Up @@ -1090,6 +1097,10 @@ impl Item {
pub(crate) fn skip_group(&self) -> bool {
self.skip_group
}

pub(crate) fn defer(&self) -> bool {
self.defer
}
}

#[derive(Clone)]
Expand Down
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