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

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);
}
```

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -3119,6 +3119,7 @@ pub struct FieldDef {
     pub ident: Option<Ident>,
 
     pub ty: P<Ty>,
+    pub default: Option<AnonConst>,
     pub is_placeholder: bool,
 }
 

compiler/rustc_ast/src/mut_visit.rs

@@ -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);
 }
 

compiler/rustc_ast/src/visit.rs

@@ -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()
 }
 

compiler/rustc_ast_lowering/messages.ftl

@@ -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
 
@@ -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
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -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 {
@@ -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 {

compiler/rustc_ast_lowering/src/expr.rs

@@ -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,
@@ -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(
@@ -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,
         )
     }
 

compiler/rustc_ast_lowering/src/item.rs

@@ -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,
@@ -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);
@@ -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),
         }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -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");

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -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 };
@@ -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`

compiler/rustc_builtin_macros/messages.ftl

@@ -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