Introduce default_field_values feature

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

Support default fields in enum struct variant

Allow default values in an enum struct variant definition:

```rust
pub enum Bar {
    Foo {
        bar: S = S,
        baz: i32 = 42 + 3,
    }
}
```

Allow using `..` without a base on an enum struct variant

```rust
Bar::Foo { .. }
```

`#[derive(Default)]` doesn't account for these as it is still gating `#[default]` only being allowed on unit variants.

Support `#[derive(Default)]` on enum struct variants with all defaulted fields

```rust
pub enum Bar {
    #[default]
    Foo {
        bar: S = S,
        baz: i32 = 42 + 3,
    }
}
```

Check for missing fields in typeck instead of mir_build.

Expand test with `const` param case (needs `generic_const_exprs` enabled).

Properly instantiate MIR const

The following works:

```rust
struct S<A> {
    a: Vec<A> = Vec::new(),
}
S::<i32> { .. }
```

Author: estebank

compiler/rustc_ast/src/ast.rs

@@ -2995,6 +2995,7 @@ 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

@@ -1048,12 +1048,13 @@ pub fn walk_flat_map_field_def<T: MutVisitor>(
     visitor: &mut T,
     mut fd: FieldDef,
 ) -> SmallVec<[FieldDef; 1]> {
-    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _ } = &mut fd;
+    let FieldDef { span, ident, vis, id, ty, attrs, is_placeholder: _, default } = &mut fd;
     visitor.visit_id(id);
     visit_attrs(visitor, attrs);
     visitor.visit_vis(vis);
     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);
     smallvec![fd]
 }

compiler/rustc_ast/src/visit.rs

@@ -934,11 +934,12 @@ 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: _ } = field;
+    let FieldDef { attrs, id: _, span: _, vis, ident, ty, is_placeholder: _, 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/src/expr.rs

@@ -316,12 +316,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ),
                 ExprKind::Struct(se) => {
                     let rest = match &se.rest {
-                        StructRest::Base(e) => Some(self.lower_expr(e)),
+                        StructRest::Base(e) => hir::Rest::Base(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)))
+                            if !self.tcx.features().default_field_values {
+                                self.dcx().emit_err(BaseExpressionDoubleDot { span: *sp });
+                            }
+                            hir::Rest::DefaultFields(*sp)
                         }
-                        StructRest::None => None,
+                        StructRest::None => hir::Rest::None,
                     };
                     hir::ExprKind::Struct(
                         self.arena.alloc(self.lower_qpath(
@@ -1460,7 +1462,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         hir::ExprKind::Struct(
             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span))),
             fields,
-            None,
+            hir::Rest::None,
         )
     }
 

compiler/rustc_ast_lowering/src/item.rs

@@ -772,6 +772,7 @@ 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,
         }
     }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -541,6 +541,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
+    gate_all!(default_field_values, "default values on `struct` fields aren't supported");
     gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(postfix_match, "postfix match is experimental");

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1146,7 +1146,9 @@ 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::Rest::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 };
@@ -1234,7 +1236,7 @@ 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::Rest::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/src/deriving/decodable.rs

@@ -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)
                 })

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -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)
         }
@@ -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)),
     };
@@ -156,7 +196,12 @@ fn extract_default_variant<'a>(
         }
     };
 
-    if !matches!(variant.data, VariantData::Unit(..)) {
+    if cx.ecfg.features.default_field_values
+        && let VariantData::Struct { fields, .. } = &variant.data
+        && fields.iter().all(|f| f.default.is_some())
+    {
+        // Allowed
+    } else if !matches!(variant.data, VariantData::Unit(..)) {
         let guar = cx.dcx().emit_err(errors::NonUnitDefault { span: variant.ident.span });
         return Err(guar);
     }

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -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};
@@ -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.
@@ -1443,7 +1443,7 @@ 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),
             }
         }