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> { .. }
```

Add lint for default fields that will always fail const-eval

We *allow* this to happen for API writers that might want to rely on users'
getting a compile error when using the default field, different to the error
that they would get when the field isn't default. We could change this to
*always* error instead of being a lint, if we wanted.

This will *not* catch errors for partially evaluated consts, like when the
expression relies on a const parameter.

Suggestions when encountering `Foo { .. }` without `#[feature(default_field_values)]`:

 - Suggest adding a base expression if there are missing fields.
 - Suggest enabling the feature if all the missing fields have optional values.
 - Suggest removing `..` if there are no missing fields.

Author: estebank

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
 

compiler/rustc_ast_lowering/src/errors.rs

@@ -114,14 +114,6 @@ 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

@@ -723,6 +723,7 @@ 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 `struct` fields aren't supported");
     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/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)
                 })