Introduce `default_field_values` feature

Initial implementation of `#[feature(default_field_values]`, proposed in https://github.com/rust-lang/rfcs/pull/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.

13 files changed, 44 insertions, 29 deletions

diff --git a/src/tools/clippy/clippy_lints/src/default.rs b/src/tools/clippy/clippy_lints/src/default.rs
index de775b64795..ffdd946aadb 100644
--- a/src/tools/clippy/clippy_lints/src/default.rs
+++ b/src/tools/clippy/clippy_lints/src/default.rs
@@ -5,7 +5,7 @@ use clippy_utils::{contains_name, get_parent_expr, in_automatically_derived, is_
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::Applicability;
 use rustc_hir::def::Res;
-use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind};
+use rustc_hir::{Block, Expr, ExprKind, PatKind, QPath, Stmt, StmtKind, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
@@ -285,7 +285,7 @@ fn field_reassigned_by_stmt<'tcx>(this: &Stmt<'tcx>, binding_name: Symbol) -> Op
 /// Returns whether `expr` is the update syntax base: `Foo { a: 1, .. base }`
 fn is_update_syntax_base<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) -> bool {
     if let Some(parent) = get_parent_expr(cx, expr)
-        && let ExprKind::Struct(_, _, Some(base)) = parent.kind
+        && let ExprKind::Struct(_, _, StructTailExpr::Base(base)) = parent.kind
     {
         base.hir_id == expr.hir_id
     } else {
diff --git a/src/tools/clippy/clippy_lints/src/default_numeric_fallback.rs b/src/tools/clippy/clippy_lints/src/default_numeric_fallback.rs
index ef6b141920d..6819ad547f8 100644
--- a/src/tools/clippy/clippy_lints/src/default_numeric_fallback.rs
+++ b/src/tools/clippy/clippy_lints/src/default_numeric_fallback.rs
@@ -4,7 +4,7 @@ use clippy_utils::source::snippet_opt;
 use rustc_ast::ast::{LitFloatType, LitIntType, LitKind};
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::{Visitor, walk_expr, walk_stmt};
-use rustc_hir::{Block, Body, ConstContext, Expr, ExprKind, FnRetTy, HirId, Lit, Stmt, StmtKind};
+use rustc_hir::{Block, Body, ConstContext, Expr, ExprKind, FnRetTy, HirId, Lit, Stmt, StmtKind, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::{self, FloatTy, IntTy, PolyFnSig, Ty};
@@ -197,7 +197,7 @@ impl<'tcx> Visitor<'tcx> for NumericFallbackVisitor<'_, 'tcx> {
                     }
 
                     // Visit base with no bound.
-                    if let Some(base) = base {
+                    if let StructTailExpr::Base(base) = base {
                         self.ty_bounds.push(ExplicitTyBound(false));
                         self.visit_expr(base);
                         self.ty_bounds.pop();
diff --git a/src/tools/clippy/clippy_lints/src/inconsistent_struct_constructor.rs b/src/tools/clippy/clippy_lints/src/inconsistent_struct_constructor.rs
index d386bfca6ba..4fcd2abb769 100644
--- a/src/tools/clippy/clippy_lints/src/inconsistent_struct_constructor.rs
+++ b/src/tools/clippy/clippy_lints/src/inconsistent_struct_constructor.rs
@@ -3,7 +3,7 @@ use clippy_utils::fulfill_or_allowed;
 use clippy_utils::source::snippet;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::Applicability;
-use rustc_hir::{self as hir, ExprKind};
+use rustc_hir::{self as hir, ExprKind, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 use rustc_span::symbol::Symbol;
@@ -95,7 +95,7 @@ impl<'tcx> LateLintPass<'tcx> for InconsistentStructConstructor {
             }
             fields_snippet.push_str(&last_ident.to_string());
 
-            let base_snippet = if let Some(base) = base {
+            let base_snippet = if let StructTailExpr::Base(base) = base {
                 format!(", ..{}", snippet(cx, base.span, ".."))
             } else {
                 String::new()
diff --git a/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs b/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs
index 7f183bb601e..7a14bbfb9e8 100644
--- a/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs
+++ b/src/tools/clippy/clippy_lints/src/init_numbered_fields.rs
@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
 use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::{Expr, ExprKind};
+use rustc_hir::{Expr, ExprKind, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 use rustc_span::SyntaxContext;
@@ -43,7 +43,7 @@ declare_lint_pass!(NumberedFields => [INIT_NUMBERED_FIELDS]);
 
 impl<'tcx> LateLintPass<'tcx> for NumberedFields {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
-        if let ExprKind::Struct(path, fields @ [field, ..], None) = e.kind
+        if let ExprKind::Struct(path, fields @ [field, ..], StructTailExpr::None) = e.kind
             // If the first character of any field is a digit it has to be a tuple.
             && field.ident.as_str().as_bytes().first().is_some_and(u8::is_ascii_digit)
             // Type aliases can't be used as functions.
diff --git a/src/tools/clippy/clippy_lints/src/loops/never_loop.rs b/src/tools/clippy/clippy_lints/src/loops/never_loop.rs
index 1c55e3e22e8..ed9879de13b 100644
--- a/src/tools/clippy/clippy_lints/src/loops/never_loop.rs
+++ b/src/tools/clippy/clippy_lints/src/loops/never_loop.rs
@@ -5,7 +5,7 @@ use clippy_utils::higher::ForLoop;
 use clippy_utils::macros::root_macro_call_first_node;
 use clippy_utils::source::snippet;
 use rustc_errors::Applicability;
-use rustc_hir::{Block, Destination, Expr, ExprKind, HirId, InlineAsmOperand, Pat, Stmt, StmtKind};
+use rustc_hir::{Block, Destination, Expr, ExprKind, HirId, InlineAsmOperand, Pat, Stmt, StmtKind, StructTailExpr};
 use rustc_lint::LateContext;
 use rustc_span::{Span, sym};
 use std::iter::once;
@@ -164,7 +164,7 @@ fn never_loop_expr<'tcx>(
         },
         ExprKind::Struct(_, fields, base) => {
             let fields = never_loop_expr_all(cx, fields.iter().map(|f| f.expr), local_labels, main_loop_id);
-            if let Some(base) = base {
+            if let StructTailExpr::Base(base) = base {
                 combine_seq(fields, || never_loop_expr(cx, base, local_labels, main_loop_id))
             } else {
                 fields
diff --git a/src/tools/clippy/clippy_lints/src/needless_update.rs b/src/tools/clippy/clippy_lints/src/needless_update.rs
index 6a2893cefbd..0cba72bd2c6 100644
--- a/src/tools/clippy/clippy_lints/src/needless_update.rs
+++ b/src/tools/clippy/clippy_lints/src/needless_update.rs
@@ -1,5 +1,5 @@
 use clippy_utils::diagnostics::span_lint;
-use rustc_hir::{Expr, ExprKind};
+use rustc_hir::{Expr, ExprKind, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_session::declare_lint_pass;
@@ -51,7 +51,7 @@ declare_lint_pass!(NeedlessUpdate => [NEEDLESS_UPDATE]);
 
 impl<'tcx> LateLintPass<'tcx> for NeedlessUpdate {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if let ExprKind::Struct(_, fields, Some(base)) = expr.kind {
+        if let ExprKind::Struct(_, fields, StructTailExpr::Base(base)) = expr.kind {
             let ty = cx.typeck_results().expr_ty(expr);
             if let ty::Adt(def, _) = ty.kind() {
                 if fields.len() == def.non_enum_variant().fields.len()
diff --git a/src/tools/clippy/clippy_lints/src/no_effect.rs b/src/tools/clippy/clippy_lints/src/no_effect.rs
index 8ecff9c3f9b..9e44bb02c56 100644
--- a/src/tools/clippy/clippy_lints/src/no_effect.rs
+++ b/src/tools/clippy/clippy_lints/src/no_effect.rs
@@ -8,7 +8,7 @@ use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{
     BinOpKind, BlockCheckMode, Expr, ExprKind, HirId, HirIdMap, ItemKind, LocalSource, Node, PatKind, Stmt, StmtKind,
-    UnsafeSource, is_range_literal,
+    UnsafeSource, StructTailExpr, is_range_literal,
 };
 use rustc_infer::infer::TyCtxtInferExt as _;
 use rustc_lint::{LateContext, LateLintPass, LintContext};
@@ -238,7 +238,10 @@ fn has_no_effect(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
         ExprKind::Struct(_, fields, ref base) => {
             !has_drop(cx, cx.typeck_results().expr_ty(expr))
                 && fields.iter().all(|field| has_no_effect(cx, field.expr))
-                && base.as_ref().is_none_or(|base| has_no_effect(cx, base))
+                && match &base {
+                    StructTailExpr::None | StructTailExpr::DefaultFields(_) => true,
+                    StructTailExpr::Base(base) => has_no_effect(cx, base),
+                }
         },
         ExprKind::Call(callee, args) => {
             if let ExprKind::Path(ref qpath) = callee.kind {
@@ -342,6 +345,10 @@ fn reduce_expression<'a>(cx: &LateContext<'_>, expr: &'a Expr<'a>) -> Option<Vec
             if has_drop(cx, cx.typeck_results().expr_ty(expr)) {
                 None
             } else {
+                let base = match base {
+                    StructTailExpr::Base(base) => Some(base),
+                    StructTailExpr::None | StructTailExpr::DefaultFields(_) => None,
+                };
                 Some(fields.iter().map(|f| &f.expr).chain(base).map(Deref::deref).collect())
             }
         },
diff --git a/src/tools/clippy/clippy_lints/src/single_range_in_vec_init.rs b/src/tools/clippy/clippy_lints/src/single_range_in_vec_init.rs
index 44e585953bf..bb11daecc07 100644
--- a/src/tools/clippy/clippy_lints/src/single_range_in_vec_init.rs
+++ b/src/tools/clippy/clippy_lints/src/single_range_in_vec_init.rs
@@ -6,7 +6,7 @@ use clippy_utils::source::SpanRangeExt;
 use clippy_utils::ty::implements_trait;
 use rustc_ast::{LitIntType, LitKind, UintTy};
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind, LangItem, QPath};
+use rustc_hir::{Expr, ExprKind, LangItem, QPath, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 use std::fmt::{self, Display, Formatter};
@@ -86,7 +86,7 @@ impl LateLintPass<'_> for SingleRangeInVecInit {
             return;
         };
 
-        let ExprKind::Struct(QPath::LangItem(lang_item, ..), [start, end], None) = inner_expr.kind else {
+        let ExprKind::Struct(QPath::LangItem(lang_item, ..), [start, end], StructTailExpr::None) = inner_expr.kind else {
             return;
         };
 
diff --git a/src/tools/clippy/clippy_lints/src/unnecessary_struct_initialization.rs b/src/tools/clippy/clippy_lints/src/unnecessary_struct_initialization.rs
index afdd3505cdd..0a90d31db7e 100644
--- a/src/tools/clippy/clippy_lints/src/unnecessary_struct_initialization.rs
+++ b/src/tools/clippy/clippy_lints/src/unnecessary_struct_initialization.rs
@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
 use clippy_utils::ty::is_copy;
 use clippy_utils::{get_parent_expr, path_to_local};
-use rustc_hir::{BindingMode, Expr, ExprField, ExprKind, Node, PatKind, Path, QPath, UnOp};
+use rustc_hir::{BindingMode, Expr, ExprField, ExprKind, Node, PatKind, Path, QPath, UnOp, StructTailExpr};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 
@@ -59,15 +59,15 @@ impl LateLintPass<'_> for UnnecessaryStruct {
         let field_path = same_path_in_all_fields(cx, expr, fields);
 
         let sugg = match (field_path, base) {
-            (Some(&path), None) => {
+            (Some(&path), StructTailExpr::None | StructTailExpr::DefaultFields(_)) => {
                 // all fields match, no base given
                 path.span
             },
-            (Some(path), Some(base)) if base_is_suitable(cx, expr, base) && path_matches_base(path, base) => {
+            (Some(path), StructTailExpr::Base(base)) if base_is_suitable(cx, expr, base) && path_matches_base(path, base) => {
                 // all fields match, has base: ensure that the path of the base matches
                 base.span
             },
-            (None, Some(base)) if fields.is_empty() && base_is_suitable(cx, expr, base) => {
+            (None, StructTailExpr::Base(base)) if fields.is_empty() && base_is_suitable(cx, expr, base) => {
                 // just the base, no explicit fields
                 base.span
             },
diff --git a/src/tools/clippy/clippy_lints/src/utils/author.rs b/src/tools/clippy/clippy_lints/src/utils/author.rs
index 51001d374b4..311ed427cb9 100644
--- a/src/tools/clippy/clippy_lints/src/utils/author.rs
+++ b/src/tools/clippy/clippy_lints/src/utils/author.rs
@@ -4,7 +4,7 @@ use rustc_ast::ast::{LitFloatType, LitKind};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::{
     self as hir, BindingMode, CaptureBy, Closure, ClosureKind, ConstArg, ConstArgKind, CoroutineKind,
-    ExprKind, FnRetTy, HirId, Lit, PatKind, QPath, StmtKind, TyKind,
+    ExprKind, FnRetTy, HirId, Lit, PatKind, QPath, StmtKind, TyKind, StructTailExpr,
 };
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_session::declare_lint_pass;
@@ -598,7 +598,10 @@ impl<'a, 'tcx> PrintVisitor<'a, 'tcx> {
             },
             ExprKind::Struct(qpath, fields, base) => {
                 bind!(self, qpath, fields);
-                opt_bind!(self, base);
+                let base = OptionPat::new(match base {
+                    StructTailExpr::Base(base) => Some(self.bind("base", base)),
+                    StructTailExpr::None | StructTailExpr::DefaultFields(_) => None,
+                });
                 kind!("Struct({qpath}, {fields}, {base})");
                 self.qpath(qpath);
                 self.slice(fields, |field| {
diff --git a/src/tools/clippy/clippy_utils/src/higher.rs b/src/tools/clippy/clippy_utils/src/higher.rs
index 11bbe734844..d216879cbd2 100644
--- a/src/tools/clippy/clippy_utils/src/higher.rs
+++ b/src/tools/clippy/clippy_utils/src/higher.rs
@@ -8,7 +8,7 @@ use crate::ty::is_type_diagnostic_item;
 
 use rustc_ast::ast;
 use rustc_hir as hir;
-use rustc_hir::{Arm, Block, Expr, ExprKind, HirId, LoopSource, MatchSource, Node, Pat, QPath};
+use rustc_hir::{Arm, Block, Expr, ExprKind, StructTailExpr, HirId, LoopSource, MatchSource, Node, Pat, QPath};
 use rustc_lint::LateContext;
 use rustc_span::{Span, sym, symbol};
 
@@ -236,7 +236,7 @@ impl<'a> Range<'a> {
                     limits: ast::RangeLimits::Closed,
                 })
             },
-            ExprKind::Struct(path, fields, None) => match (path, fields) {
+            ExprKind::Struct(path, fields, StructTailExpr::None) => match (path, fields) {
                 (QPath::LangItem(hir::LangItem::RangeFull, ..), []) => Some(Range {
                     start: None,
                     end: None,
diff --git a/src/tools/clippy/clippy_utils/src/hir_utils.rs b/src/tools/clippy/clippy_utils/src/hir_utils.rs
index 7f3e331e7f6..4be4340862d 100644
--- a/src/tools/clippy/clippy_utils/src/hir_utils.rs
+++ b/src/tools/clippy/clippy_utils/src/hir_utils.rs
@@ -10,7 +10,7 @@ use rustc_hir::{
     AssocItemConstraint, BinOpKind, BindingMode, Block, BodyId, Closure, ConstArg, ConstArgKind, Expr,
     ExprField, ExprKind, FnRetTy, GenericArg, GenericArgs, HirId, HirIdMap, InlineAsmOperand, LetExpr, Lifetime,
     LifetimeName, Pat, PatField, PatKind, Path, PathSegment, PrimTy, QPath, Stmt, StmtKind, TraitBoundModifiers, Ty,
-    TyKind,
+    TyKind, StructTailExpr,
 };
 use rustc_lexer::{TokenKind, tokenize};
 use rustc_lint::LateContext;
@@ -380,7 +380,12 @@ impl HirEqInterExpr<'_, '_, '_> {
             (ExprKind::Ret(l), ExprKind::Ret(r)) => both(l.as_ref(), r.as_ref(), |l, r| self.eq_expr(l, r)),
             (&ExprKind::Struct(l_path, lf, ref lo), &ExprKind::Struct(r_path, rf, ref ro)) => {
                 self.eq_qpath(l_path, r_path)
-                    && both(lo.as_ref(), ro.as_ref(), |l, r| self.eq_expr(l, r))
+                    && match (lo, ro) {
+                        (StructTailExpr::Base(l),StructTailExpr::Base(r)) => self.eq_expr(l, r),
+                        (StructTailExpr::None, StructTailExpr::None) => true,
+                        (StructTailExpr::DefaultFields(_), StructTailExpr::DefaultFields(_)) => true,
+                        _ => false,
+                    }
                     && over(lf, rf, |l, r| self.eq_expr_field(l, r))
             },
             (&ExprKind::Tup(l_tup), &ExprKind::Tup(r_tup)) => self.eq_exprs(l_tup, r_tup),
@@ -1017,7 +1022,7 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
                     self.hash_expr(f.expr);
                 }
 
-                if let Some(e) = *expr {
+                if let StructTailExpr::Base(e) = *expr {
                     self.hash_expr(e);
                 }
             },
diff --git a/src/tools/clippy/clippy_utils/src/visitors.rs b/src/tools/clippy/clippy_utils/src/visitors.rs
index a79be5ca7d4..351e619d7b1 100644
--- a/src/tools/clippy/clippy_utils/src/visitors.rs
+++ b/src/tools/clippy/clippy_utils/src/visitors.rs
@@ -7,7 +7,7 @@ use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::intravisit::{self, Visitor, walk_block, walk_expr};
 use rustc_hir::{
     AnonConst, Arm, Block, BlockCheckMode, Body, BodyId, Expr, ExprKind, HirId, ItemId, ItemKind, LetExpr, Pat, QPath,
-    Safety, Stmt, UnOp, UnsafeSource,
+    Safety, Stmt, UnOp, UnsafeSource, StructTailExpr,
 };
 use rustc_lint::LateContext;
 use rustc_middle::hir::nested_filter;
@@ -663,7 +663,7 @@ pub fn for_each_unconsumed_temporary<'tcx, B>(
                 for field in fields {
                     helper(typeck, true, field.expr, f)?;
                 }
-                if let Some(default) = default {
+                if let StructTailExpr::Base(default) = default {
                     helper(typeck, false, default, f)?;
                 }
             },