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.

3 files changed, 6 insertions, 2 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 69ba78282f9..650525a2f52 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/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,
 }
 
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index 3a4a8ce266e..2c09059fe19 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/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);
 }
 
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index 0b000c8cef8..a7f7c37693a 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/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()
 }