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Merged
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Jun 17, 2025
Merged

Reject union default field values #142570

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32c0cb0
Add union with default field values case test
jieyouxu Jun 16, 2025
2dd9cc1
Reject union default field values
jieyouxu Jun 16, 2025
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2 changes: 2 additions & 0 deletions compiler/rustc_ast_lowering/messages.ftl
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Original file line number Diff line number Diff line change
Expand Up @@ -179,6 +179,8 @@ ast_lowering_underscore_expr_lhs_assign =
in expressions, `_` can only be used on the left-hand side of an assignment
.label = `_` not allowed here
ast_lowering_union_default_field_values = unions cannot have default field values
ast_lowering_unstable_inline_assembly = inline assembly is not stable yet on this architecture
ast_lowering_unstable_inline_assembly_label_operand_with_outputs =
using both label and output operands for inline assembly is unstable
Expand Down
7 changes: 7 additions & 0 deletions compiler/rustc_ast_lowering/src/errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -475,3 +475,10 @@ pub(crate) struct UseConstGenericArg {
#[suggestion_part(code = "{other_args}")]
pub call_args: Span,
}

#[derive(Diagnostic)]
#[diag(ast_lowering_union_default_field_values)]
pub(crate) struct UnionWithDefault {
#[primary_span]
pub span: Span,
}
42 changes: 32 additions & 10 deletions compiler/rustc_ast_lowering/src/item.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@ use tracing::instrument;

use super::errors::{
InvalidAbi, InvalidAbiSuggestion, MisplacedRelaxTraitBound, TupleStructWithDefault,
UnionWithDefault,
};
use super::stability::{enabled_names, gate_unstable_abi};
use super::{
Expand Down Expand Up @@ -316,7 +317,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
|this| {
this.arena.alloc_from_iter(
enum_definition.variants.iter().map(|x| this.lower_variant(x)),
enum_definition.variants.iter().map(|x| this.lower_variant(i, x)),
)
},
);
Expand All @@ -328,7 +329,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
generics,
id,
ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
|this| this.lower_variant_data(hir_id, struct_def),
|this| this.lower_variant_data(hir_id, i, struct_def),
);
hir::ItemKind::Struct(ident, generics, struct_def)
}
Expand All @@ -338,7 +339,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
generics,
id,
ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
|this| this.lower_variant_data(hir_id, vdata),
|this| this.lower_variant_data(hir_id, i, vdata),
);
hir::ItemKind::Union(ident, generics, vdata)
}
Expand Down Expand Up @@ -714,13 +715,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
}
}

fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
fn lower_variant(&mut self, item_kind: &ItemKind, v: &Variant) -> hir::Variant<'hir> {
let hir_id = self.lower_node_id(v.id);
self.lower_attrs(hir_id, &v.attrs, v.span);
hir::Variant {
hir_id,
def_id: self.local_def_id(v.id),
data: self.lower_variant_data(hir_id, &v.data),
data: self.lower_variant_data(hir_id, item_kind, &v.data),
disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const_to_anon_const(e)),
ident: self.lower_ident(v.ident),
span: self.lower_span(v.span),
Expand All @@ -730,15 +731,36 @@ impl<'hir> LoweringContext<'_, 'hir> {
fn lower_variant_data(
&mut self,
parent_id: hir::HirId,
item_kind: &ItemKind,
vdata: &VariantData,
) -> hir::VariantData<'hir> {
match vdata {
VariantData::Struct { fields, recovered } => hir::VariantData::Struct {
fields: self
VariantData::Struct { fields, recovered } => {
let fields = self
.arena
.alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
recovered: *recovered,
},
.alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f)));

if let ItemKind::Union(..) = item_kind {
for field in &fields[..] {
if let Some(default) = field.default {
// Unions cannot derive `Default`, and it's not clear how to use default
// field values of unions if that was supported. Therefore, blanket reject
// trying to use field values with unions.
if self.tcx.features().default_field_values() {
self.dcx().emit_err(UnionWithDefault { span: default.span });
} else {
let _ = self.dcx().span_delayed_bug(
default.span,
"expected union default field values feature gate error but none \
was produced",
);
}
}
}
}

hir::VariantData::Struct { fields, recovered: *recovered }
}
VariantData::Tuple(fields, id) => {
let ctor_id = self.lower_node_id(*id);
self.alias_attrs(ctor_id, parent_id);
Expand Down
10 changes: 10 additions & 0 deletions tests/ui/feature-gates/feature-gate-default-field-values.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,16 @@ pub enum OptEnum {
}
}

// Default field values may not be used on `union`s (at least, this is not described in the accepted
// RFC, and it's not currently clear how to extend the design to do so). We emit a feature gate
// error when the feature is not enabled, but syntactically reject default field values when used
// with unions when the feature is enabled. This can be adjusted if there's an acceptable design
// extension, or just unconditionally reject always.
union U {
x: i32 = 0, //~ ERROR default values on fields are experimental
y: f32 = 0.0, //~ ERROR default values on fields are experimental
}

fn main () {
let x = Foo { .. }; //~ ERROR base expression required after `..`
let y = Foo::default();
Expand Down
54 changes: 37 additions & 17 deletions tests/ui/feature-gates/feature-gate-default-field-values.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -124,8 +124,28 @@ LL | optional: () = (),
= help: add `#![feature(default_field_values)]` to the crate attributes to enable
= note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error[E0658]: default values on fields are experimental
--> $DIR/feature-gate-default-field-values.rs:67:11
|
LL | x: i32 = 0,
| ^^^^
|
= note: see issue #132162 <https://github.com/rust-lang/rust/issues/132162> for more information
= help: add `#![feature(default_field_values)]` to the crate attributes to enable
= note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error[E0658]: default values on fields are experimental
--> $DIR/feature-gate-default-field-values.rs:68:11
|
LL | y: f32 = 0.0,
| ^^^^^^
|
= note: see issue #132162 <https://github.com/rust-lang/rust/issues/132162> for more information
= help: add `#![feature(default_field_values)]` to the crate attributes to enable
= note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:62:21
--> $DIR/feature-gate-default-field-values.rs:72:21
|
LL | let x = Foo { .. };
| ^
Expand All @@ -140,7 +160,7 @@ LL | let x = Foo { ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:64:29
--> $DIR/feature-gate-default-field-values.rs:74:29
|
LL | let z = Foo { baz: 1, .. };
| ^
Expand All @@ -155,7 +175,7 @@ LL | let z = Foo { baz: 1, ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:70:26
--> $DIR/feature-gate-default-field-values.rs:80:26
|
LL | let x = Bar::Foo { .. };
| ^
Expand All @@ -170,7 +190,7 @@ LL | let x = Bar::Foo { ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:72:34
--> $DIR/feature-gate-default-field-values.rs:82:34
|
LL | let z = Bar::Foo { baz: 1, .. };
| ^
Expand All @@ -185,7 +205,7 @@ LL | let z = Bar::Foo { baz: 1, ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:78:31
--> $DIR/feature-gate-default-field-values.rs:88:31
|
LL | let x = Qux::<i32, 4> { .. };
| ^
Expand All @@ -200,7 +220,7 @@ LL | let x = Qux::<i32, 4> { ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:79:73
--> $DIR/feature-gate-default-field-values.rs:89:73
|
LL | assert!(matches!(Qux::<i32, 4> { bar: S, baz: 42, bat: 2, bay: 4, .. }, x));
| ^
Expand All @@ -215,7 +235,7 @@ LL | assert!(matches!(Qux::<i32, 4> { bar: S, baz: 42, bat: 2, bay: 4, ../*
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:82:38
--> $DIR/feature-gate-default-field-values.rs:92:38
|
LL | let y = Opt { mandatory: None, .. };
| ^
Expand All @@ -230,7 +250,7 @@ LL | let y = Opt { mandatory: None, ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:86:47
--> $DIR/feature-gate-default-field-values.rs:96:47
|
LL | assert!(matches!(Opt { mandatory: None, .. }, z));
| ^
Expand All @@ -245,7 +265,7 @@ LL | assert!(matches!(Opt { mandatory: None, ../* expr */ }, z));
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:88:30
--> $DIR/feature-gate-default-field-values.rs:98:30
|
LL | assert!(matches!(Opt { .. }, z));
| ^
Expand All @@ -256,7 +276,7 @@ LL | assert!(matches!(Opt { ../* expr */ }, z));
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:90:44
--> $DIR/feature-gate-default-field-values.rs:100:44
|
LL | assert!(matches!(Opt { optional: (), .. }, z));
| ^
Expand All @@ -267,7 +287,7 @@ LL | assert!(matches!(Opt { optional: (), ../* expr */ }, z));
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:92:61
--> $DIR/feature-gate-default-field-values.rs:102:61
|
LL | assert!(matches!(Opt { optional: (), mandatory: None, .. }, z));
| ^
Expand All @@ -279,7 +299,7 @@ LL + assert!(matches!(Opt { optional: (), mandatory: None, }, z));
|

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:94:51
--> $DIR/feature-gate-default-field-values.rs:104:51
|
LL | let y = OptEnum::Variant { mandatory: None, .. };
| ^
Expand All @@ -294,7 +314,7 @@ LL | let y = OptEnum::Variant { mandatory: None, ../* expr */ };
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:98:60
--> $DIR/feature-gate-default-field-values.rs:108:60
|
LL | assert!(matches!(OptEnum::Variant { mandatory: None, .. }, z));
| ^
Expand All @@ -309,7 +329,7 @@ LL | assert!(matches!(OptEnum::Variant { mandatory: None, ../* expr */ }, z)
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:100:43
--> $DIR/feature-gate-default-field-values.rs:110:43
|
LL | assert!(matches!(OptEnum::Variant { .. }, z));
| ^
Expand All @@ -320,7 +340,7 @@ LL | assert!(matches!(OptEnum::Variant { ../* expr */ }, z));
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:102:57
--> $DIR/feature-gate-default-field-values.rs:112:57
|
LL | assert!(matches!(OptEnum::Variant { optional: (), .. }, z));
| ^
Expand All @@ -331,7 +351,7 @@ LL | assert!(matches!(OptEnum::Variant { optional: (), ../* expr */ }, z));
| ++++++++++

error[E0797]: base expression required after `..`
--> $DIR/feature-gate-default-field-values.rs:104:74
--> $DIR/feature-gate-default-field-values.rs:114:74
|
LL | assert!(matches!(OptEnum::Variant { optional: (), mandatory: None, .. }, z));
| ^
Expand All @@ -342,7 +362,7 @@ LL - assert!(matches!(OptEnum::Variant { optional: (), mandatory: None, .. }
LL + assert!(matches!(OptEnum::Variant { optional: (), mandatory: None, }, z));
|

error: aborting due to 29 previous errors
error: aborting due to 31 previous errors

Some errors have detailed explanations: E0658, E0797.
For more information about an error, try `rustc --explain E0658`.
6 changes: 6 additions & 0 deletions tests/ui/structs/default-field-values/failures.rs
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Expand Up @@ -49,6 +49,12 @@ enum E {
Variant {} //~ ERROR the `#[default]` attribute may only be used on unit enum variants
}

union U
{
x: i32 = 1, //~ ERROR unions cannot have default field values
y: f32 = 2., //~ ERROR unions cannot have default field values
}

fn main () {
let _ = Foo { .. }; // ok
let _ = Foo::default(); // ok
Expand Down
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