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Introduce default_field_values feature #129514

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1 change: 1 addition & 0 deletions compiler/rustc_ast/src/ast.rs
Original file line number Diff line number Diff line change
Expand Up @@ -3119,6 +3119,7 @@ pub struct FieldDef {
pub ident: Option<Ident>,

pub ty: P<Ty>,
pub default: Option<AnonConst>,
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pub is_placeholder: bool,
}

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3 changes: 2 additions & 1 deletion compiler/rustc_ast/src/mut_visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1120,13 +1120,14 @@ fn walk_poly_trait_ref<T: MutVisitor>(vis: &mut T, p: &mut PolyTraitRef) {
}

pub fn walk_field_def<T: MutVisitor>(visitor: &mut T, fd: &mut FieldDef) {
let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _, safety } = fd;
let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _, safety, default } = fd;
visitor.visit_id(id);
visit_attrs(visitor, attrs);
visitor.visit_vis(vis);
visit_safety(visitor, safety);
visit_opt(ident, |ident| visitor.visit_ident(ident));
visitor.visit_ty(ty);
visit_opt(default, |default| visitor.visit_anon_const(default));
visitor.visit_span(span);
}

Expand Down
4 changes: 3 additions & 1 deletion compiler/rustc_ast/src/visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -975,11 +975,13 @@ pub fn walk_struct_def<'a, V: Visitor<'a>>(
}

pub fn walk_field_def<'a, V: Visitor<'a>>(visitor: &mut V, field: &'a FieldDef) -> V::Result {
let FieldDef { attrs, id: _, span: _, vis, ident, ty, is_placeholder: _, safety: _ } = field;
let FieldDef { attrs, id: _, span: _, vis, ident, ty, is_placeholder: _, safety: _, default } =
field;
walk_list!(visitor, visit_attribute, attrs);
try_visit!(visitor.visit_vis(vis));
visit_opt!(visitor, visit_ident, ident);
try_visit!(visitor.visit_ty(ty));
visit_opt!(visitor, visit_anon_const, &*default);
V::Result::output()
}

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7 changes: 3 additions & 4 deletions compiler/rustc_ast_lowering/messages.ftl
Original file line number Diff line number Diff line change
Expand Up @@ -45,10 +45,6 @@ ast_lowering_bad_return_type_notation_output =

ast_lowering_bad_return_type_notation_position = return type notation not allowed in this position yet

ast_lowering_base_expression_double_dot =
base expression required after `..`
.suggestion = add a base expression here

ast_lowering_clobber_abi_not_supported =
`clobber_abi` is not supported on this target

Expand All @@ -57,6 +53,9 @@ ast_lowering_closure_cannot_be_static = closures cannot be static
ast_lowering_coroutine_too_many_parameters =
too many parameters for a coroutine (expected 0 or 1 parameters)

ast_lowering_default_field_in_tuple = default fields are not supported in tuple structs
.label = default fields are only supported on structs

ast_lowering_does_not_support_modifiers =
the `{$class_name}` register class does not support template modifiers

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16 changes: 8 additions & 8 deletions compiler/rustc_ast_lowering/src/errors.rs
Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,14 @@ pub(crate) struct InvalidAbi {
pub suggestion: Option<InvalidAbiSuggestion>,
}

#[derive(Diagnostic)]
#[diag(ast_lowering_default_field_in_tuple)]
pub(crate) struct TupleStructWithDefault {
#[primary_span]
#[label]
pub span: Span,
}

pub(crate) struct InvalidAbiReason(pub &'static str);

impl Subdiagnostic for InvalidAbiReason {
Expand Down Expand Up @@ -114,14 +122,6 @@ pub(crate) struct UnderscoreExprLhsAssign {
pub span: Span,
}

#[derive(Diagnostic)]
#[diag(ast_lowering_base_expression_double_dot, code = E0797)]
pub(crate) struct BaseExpressionDoubleDot {
#[primary_span]
#[suggestion(code = "/* expr */", applicability = "has-placeholders", style = "verbose")]
pub span: Span,
}

#[derive(Diagnostic)]
#[diag(ast_lowering_await_only_in_async_fn_and_blocks, code = E0728)]
pub(crate) struct AwaitOnlyInAsyncFnAndBlocks {
Expand Down
19 changes: 8 additions & 11 deletions compiler/rustc_ast_lowering/src/expr.rs
Original file line number Diff line number Diff line change
Expand Up @@ -19,10 +19,10 @@ use thin_vec::{ThinVec, thin_vec};
use visit::{Visitor, walk_expr};

use super::errors::{
AsyncCoroutinesNotSupported, AwaitOnlyInAsyncFnAndBlocks, BaseExpressionDoubleDot,
ClosureCannotBeStatic, CoroutineTooManyParameters,
FunctionalRecordUpdateDestructuringAssignment, InclusiveRangeWithNoEnd, MatchArmWithNoBody,
NeverPatternWithBody, NeverPatternWithGuard, UnderscoreExprLhsAssign,
AsyncCoroutinesNotSupported, AwaitOnlyInAsyncFnAndBlocks, ClosureCannotBeStatic,
CoroutineTooManyParameters, FunctionalRecordUpdateDestructuringAssignment,
InclusiveRangeWithNoEnd, MatchArmWithNoBody, NeverPatternWithBody, NeverPatternWithGuard,
UnderscoreExprLhsAssign,
};
use super::{
GenericArgsMode, ImplTraitContext, LoweringContext, ParamMode, ResolverAstLoweringExt,
Expand Down Expand Up @@ -357,12 +357,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
),
ExprKind::Struct(se) => {
let rest = match &se.rest {
StructRest::Base(e) => Some(self.lower_expr(e)),
StructRest::Rest(sp) => {
let guar = self.dcx().emit_err(BaseExpressionDoubleDot { span: *sp });
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This is now gated in hir_typeck so that we can look at the struct for default fields. If there are any, then we mention enabling the feature. Otherwise we emit the same error we emitted here.

Some(&*self.arena.alloc(self.expr_err(*sp, guar)))
}
StructRest::None => None,
StructRest::Base(e) => hir::StructTailExpr::Base(self.lower_expr(e)),
StructRest::Rest(sp) => hir::StructTailExpr::DefaultFields(*sp),
StructRest::None => hir::StructTailExpr::None,
};
hir::ExprKind::Struct(
self.arena.alloc(self.lower_qpath(
Expand Down Expand Up @@ -1526,7 +1523,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
hir::ExprKind::Struct(
self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span))),
fields,
None,
hir::StructTailExpr::None,
)
}

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34 changes: 26 additions & 8 deletions compiler/rustc_ast_lowering/src/item.rs
Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,10 @@ use smallvec::{SmallVec, smallvec};
use thin_vec::ThinVec;
use tracing::instrument;

use super::errors::{InvalidAbi, InvalidAbiReason, InvalidAbiSuggestion, MisplacedRelaxTraitBound};
use super::errors::{
InvalidAbi, InvalidAbiReason, InvalidAbiSuggestion, MisplacedRelaxTraitBound,
TupleStructWithDefault,
};
use super::{
AstOwner, FnDeclKind, ImplTraitContext, ImplTraitPosition, LoweringContext, ParamMode,
ResolverAstLoweringExt,
Expand Down Expand Up @@ -690,13 +693,27 @@ impl<'hir> LoweringContext<'_, 'hir> {
VariantData::Tuple(fields, id) => {
let ctor_id = self.lower_node_id(*id);
self.alias_attrs(ctor_id, parent_id);
hir::VariantData::Tuple(
self.arena.alloc_from_iter(
fields.iter().enumerate().map(|f| self.lower_field_def(f)),
),
ctor_id,
self.local_def_id(*id),
)
let fields = self
.arena
.alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f)));
for field in &fields[..] {
if let Some(default) = field.default {
// Default values in tuple struct and tuple variants are not allowed by the
// RFC due to concerns about the syntax, both in the item definition and the
// expression. We could in the future allow `struct S(i32 = 0);` and force
// users to construct the value with `let _ = S { .. };`.
if self.tcx.features().default_field_values() {
self.dcx().emit_err(TupleStructWithDefault { span: default.span });
} else {
let _ = self.dcx().span_delayed_bug(
default.span,
"expected `default values on `struct` fields aren't supported` \
feature-gate error but none was produced",
);
}
}
}
hir::VariantData::Tuple(fields, ctor_id, self.local_def_id(*id))
}
VariantData::Unit(id) => {
let ctor_id = self.lower_node_id(*id);
Expand All @@ -723,6 +740,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
None => Ident::new(sym::integer(index), self.lower_span(f.span)),
},
vis_span: self.lower_span(f.vis.span),
default: f.default.as_ref().map(|v| self.lower_anon_const_to_anon_const(v)),
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ty,
safety: self.lower_safety(f.safety, hir::Safety::Safe),
}
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_ast_passes/src/feature_gate.rs
Original file line number Diff line number Diff line change
Expand Up @@ -557,6 +557,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
gate_all!(explicit_tail_calls, "`become` expression is experimental");
gate_all!(generic_const_items, "generic const items are experimental");
gate_all!(guard_patterns, "guard patterns are experimental", "consider using match arm guards");
gate_all!(default_field_values, "default values on fields are experimental");
gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
gate_all!(postfix_match, "postfix match is experimental");
gate_all!(mut_ref, "mutable by-reference bindings are experimental");
Expand Down
8 changes: 6 additions & 2 deletions compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1151,7 +1151,11 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
expr: &hir::Expr<'_>,
) {
let typeck_results = self.infcx.tcx.typeck(self.mir_def_id());
let hir::ExprKind::Struct(struct_qpath, fields, Some(base)) = expr.kind else { return };
let hir::ExprKind::Struct(struct_qpath, fields, hir::StructTailExpr::Base(base)) =
expr.kind
else {
return;
};
let hir::QPath::Resolved(_, path) = struct_qpath else { return };
let hir::def::Res::Def(_, def_id) = path.res else { return };
let Some(expr_ty) = typeck_results.node_type_opt(expr.hir_id) else { return };
Expand Down Expand Up @@ -1239,7 +1243,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
expr: &'tcx hir::Expr<'tcx>,
use_spans: Option<UseSpans<'tcx>>,
) {
if let hir::ExprKind::Struct(_, _, Some(_)) = expr.kind {
if let hir::ExprKind::Struct(_, _, hir::StructTailExpr::Base(_)) = expr.kind {
// We have `S { foo: val, ..base }`. In `check_aggregate_rvalue` we have a single
// `Location` that covers both the `S { ... }` literal, all of its fields and the
// `base`. If the move happens because of `S { foo: val, bar: base.bar }` the `expr`
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_builtin_macros/messages.ftl
Original file line number Diff line number Diff line change
Expand Up @@ -261,7 +261,7 @@ builtin_macros_non_exhaustive_default = default variant must be exhaustive

builtin_macros_non_generic_pointee = the `#[pointee]` attribute may only be used on generic parameters

builtin_macros_non_unit_default = the `#[default]` attribute may only be used on unit enum variants
builtin_macros_non_unit_default = the `#[default]` attribute may only be used on unit enum variants{$post}
.help = consider a manual implementation of `Default`

builtin_macros_only_one_argument = {$name} takes 1 argument
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_builtin_macros/src/deriving/decodable.rs
Original file line number Diff line number Diff line change
Expand Up @@ -205,7 +205,7 @@ where
let fields = fields
.iter()
.enumerate()
.map(|(i, &(ident, span))| {
.map(|(i, &(ident, span, _))| {
let arg = getarg(cx, span, ident.name, i);
cx.field_imm(span, ident, arg)
})
Expand Down
83 changes: 70 additions & 13 deletions compiler/rustc_builtin_macros/src/deriving/default.rs
Original file line number Diff line number Diff line change
Expand Up @@ -54,26 +54,38 @@ pub(crate) fn expand_deriving_default(
trait_def.expand(cx, mitem, item, push)
}

fn default_call(cx: &ExtCtxt<'_>, span: Span) -> ast::ptr::P<ast::Expr> {
// Note that `kw::Default` is "default" and `sym::Default` is "Default"!
let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
cx.expr_call_global(span, default_ident, ThinVec::new())
}

fn default_struct_substructure(
cx: &ExtCtxt<'_>,
trait_span: Span,
substr: &Substructure<'_>,
summary: &StaticFields,
) -> BlockOrExpr {
// Note that `kw::Default` is "default" and `sym::Default` is "Default"!
let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
let default_call = |span| cx.expr_call_global(span, default_ident.clone(), ThinVec::new());

let expr = match summary {
Unnamed(_, IsTuple::No) => cx.expr_ident(trait_span, substr.type_ident),
Unnamed(fields, IsTuple::Yes) => {
let exprs = fields.iter().map(|sp| default_call(*sp)).collect();
let exprs = fields.iter().map(|sp| default_call(cx, *sp)).collect();
cx.expr_call_ident(trait_span, substr.type_ident, exprs)
}
Named(fields) => {
let default_fields = fields
.iter()
.map(|&(ident, span)| cx.field_imm(span, ident, default_call(span)))
.map(|(ident, span, default_val)| {
let value = match default_val {
// We use `Default::default()`.
None => default_call(cx, *span),
// We use the field default const expression.
Some(val) => {
cx.expr(val.value.span, ast::ExprKind::ConstBlock(val.clone()))
}
};
cx.field_imm(*span, *ident, value)
})
.collect();
cx.expr_struct_ident(trait_span, substr.type_ident, default_fields)
}
Expand All @@ -93,10 +105,38 @@ fn default_enum_substructure(
} {
Ok(default_variant) => {
// We now know there is exactly one unit variant with exactly one `#[default]` attribute.
cx.expr_path(cx.path(default_variant.span, vec![
Ident::new(kw::SelfUpper, default_variant.span),
default_variant.ident,
]))
match &default_variant.data {
VariantData::Unit(_) => cx.expr_path(cx.path(default_variant.span, vec![
Ident::new(kw::SelfUpper, default_variant.span),
default_variant.ident,
])),
VariantData::Struct { fields, .. } => {
// This only happens if `#![feature(default_field_values)]`. We have validated
// all fields have default values in the definition.
let default_fields = fields
.iter()
.map(|field| {
cx.field_imm(field.span, field.ident.unwrap(), match &field.default {
// We use `Default::default()`.
None => default_call(cx, field.span),
// We use the field default const expression.
Some(val) => {
cx.expr(val.value.span, ast::ExprKind::ConstBlock(val.clone()))
}
})
})
.collect();
let path = cx.path(default_variant.span, vec![
Ident::new(kw::SelfUpper, default_variant.span),
default_variant.ident,
]);
cx.expr_struct(default_variant.span, path, default_fields)
}
// Logic error in `extract_default_variant`.
VariantData::Tuple(..) => {
cx.dcx().bug("encountered tuple variant annotated with `#[default]`")
}
}
}
Err(guar) => DummyResult::raw_expr(trait_span, Some(guar)),
};
Expand Down Expand Up @@ -156,8 +196,20 @@ fn extract_default_variant<'a>(
}
};

if !matches!(variant.data, VariantData::Unit(..)) {
let guar = cx.dcx().emit_err(errors::NonUnitDefault { span: variant.ident.span });
if cx.ecfg.features.default_field_values()
&& let VariantData::Struct { fields, .. } = &variant.data
&& fields.iter().all(|f| f.default.is_some())
// Disallow `#[default] Variant {}`
&& !fields.is_empty()
{
// Allowed
} else if !matches!(variant.data, VariantData::Unit(..)) {
let post = if cx.ecfg.features.default_field_values() {
" or variants where every field has a default value"
} else {
""
};
let guar = cx.dcx().emit_err(errors::NonUnitDefault { span: variant.ident.span, post });
return Err(guar);
}

Expand Down Expand Up @@ -216,7 +268,12 @@ struct DetectNonVariantDefaultAttr<'a, 'b> {
impl<'a, 'b> rustc_ast::visit::Visitor<'a> for DetectNonVariantDefaultAttr<'a, 'b> {
fn visit_attribute(&mut self, attr: &'a rustc_ast::Attribute) {
if attr.has_name(kw::Default) {
self.cx.dcx().emit_err(errors::NonUnitDefault { span: attr.span });
let post = if self.cx.ecfg.features.default_field_values() {
" or variants where every field has a default value"
} else {
""
};
self.cx.dcx().emit_err(errors::NonUnitDefault { span: attr.span, post });
}

rustc_ast::visit::walk_attribute(self, attr);
Expand Down
8 changes: 4 additions & 4 deletions compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -182,8 +182,8 @@ pub(crate) use StaticFields::*;
pub(crate) use SubstructureFields::*;
use rustc_ast::ptr::P;
use rustc_ast::{
self as ast, BindingMode, ByRef, EnumDef, Expr, GenericArg, GenericParamKind, Generics,
Mutability, PatKind, VariantData,
self as ast, AnonConst, BindingMode, ByRef, EnumDef, Expr, GenericArg, GenericParamKind,
Generics, Mutability, PatKind, VariantData,
};
use rustc_attr as attr;
use rustc_expand::base::{Annotatable, ExtCtxt};
Expand Down Expand Up @@ -296,7 +296,7 @@ pub(crate) enum StaticFields {
/// Tuple and unit structs/enum variants like this.
Unnamed(Vec<Span>, IsTuple),
/// Normal structs/struct variants.
Named(Vec<(Ident, Span)>),
Named(Vec<(Ident, Span, Option<AnonConst>)>),
}

/// A summary of the possible sets of fields.
Expand Down Expand Up @@ -1435,7 +1435,7 @@ impl<'a> TraitDef<'a> {
for field in struct_def.fields() {
let sp = field.span.with_ctxt(self.span.ctxt());
match field.ident {
Some(ident) => named_idents.push((ident, sp)),
Some(ident) => named_idents.push((ident, sp, field.default.clone())),
_ => just_spans.push(sp),
}
}
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_builtin_macros/src/errors.rs
Original file line number Diff line number Diff line change
Expand Up @@ -424,6 +424,7 @@ pub(crate) struct MultipleDefaultsSugg {
pub(crate) struct NonUnitDefault {
#[primary_span]
pub(crate) span: Span,
pub(crate) post: &'static str,
}

#[derive(Diagnostic)]
Expand Down
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