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Auto merge of #129514 - estebank:default-field-values, r=compiler-errors
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Introduce `default_field_values` feature

Initial implementation of `#[feature(default_field_values]`, proposed in rust-lang/rfcs#3681.

We now parse const expressions after a `=` in a field definition, to specify a `struct` field default value.

We now allow `Struct { field, .. }` where there's no base after `..`.

`#[derive(Default)]` now uses the default value if present, continuing to use `Default::default()` if not.

```rust
#[derive(Debug)]
pub struct S;

#[derive(Debug, Default)]
pub struct Foo {
    pub bar: S = S,
    pub baz: i32 = 42 + 3,
}

fn main () {
    let x = Foo { .. };
    let y = Foo::default();
    let z = Foo { baz: 1, .. };

    assert_eq!(45, x.baz);
    assert_eq!(45, y.baz);
    assert_eq!(1, z.baz);
}
```
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bors committed Dec 10, 2024
2 parents a224f38 + fa331f4 commit 974ccc1
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Showing 77 changed files with 1,569 additions and 394 deletions.
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>,
pub is_placeholder: bool,
}

Expand Down
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);
}

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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
Expand Down
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 });
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,
)
}

Expand Down
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)),
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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