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.

1 files changed, 0 insertions, 8 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 14f2dd32e92..4c4e03cdfa3 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -3067,14 +3067,6 @@ pub(crate) struct SingleColonStructType {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_equals_struct_default)]
-pub(crate) struct EqualsStructDefault {
-    #[primary_span]
-    #[suggestion(code = "", applicability = "machine-applicable", style = "verbose")]
-    pub span: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(parse_macro_rules_missing_bang)]
 pub(crate) struct MacroRulesMissingBang {
     #[primary_span]