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.

compiler/rustc_parse/messages.ftl

@@ -169,9 +169,6 @@ parse_enum_struct_mutually_exclusive = `enum` and `struct` are mutually exclusiv
 parse_eq_field_init = expected `:`, found `=`
     .suggestion = replace equals symbol with a colon
 
-parse_equals_struct_default = default values on `struct` fields aren't supported
-    .suggestion = remove this unsupported default value
-
 parse_escape_only_char = {$byte ->
     [true] byte
     *[false] character

compiler/rustc_parse/src/errors.rs

@@ -3066,14 +3066,6 @@ pub(crate) struct SingleColonStructType {
     pub span: Span,
 }
 
-#[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 {

compiler/rustc_parse/src/parser/expr.rs

@@ -3533,7 +3533,7 @@ impl<'a> Parser<'a> {
                 let exp_span = self.prev_token.span;
                 // We permit `.. }` on the left-hand side of a destructuring assignment.
                 if self.check(&token::CloseDelim(close_delim)) {
-                    base = ast::StructRest::Rest(self.prev_token.span.shrink_to_hi());
+                    base = ast::StructRest::Rest(self.prev_token.span);
                     break;
                 }
                 match self.parse_expr() {

compiler/rustc_parse/src/parser/item.rs

@@ -1845,6 +1845,7 @@ impl<'a> Parser<'a> {
                         ident: None,
                         id: DUMMY_NODE_ID,
                         ty,
+                        default: None,
                         attrs,
                         is_placeholder: false,
                     },
@@ -2024,12 +2025,15 @@ impl<'a> Parser<'a> {
         if self.token == token::Colon && self.look_ahead(1, |t| *t != token::Colon) {
             self.dcx().emit_err(errors::SingleColonStructType { span: self.token.span });
         }
-        if self.token == token::Eq {
+        let default = if self.token == token::Eq {
             self.bump();
             let const_expr = self.parse_expr_anon_const()?;
             let sp = ty.span.shrink_to_hi().to(const_expr.value.span);
-            self.dcx().emit_err(errors::EqualsStructDefault { span: sp });
-        }
+            self.psess.gated_spans.gate(sym::default_field_values, sp);
+            Some(const_expr)
+        } else {
+            None
+        };
         Ok(FieldDef {
             span: lo.to(self.prev_token.span),
             ident: Some(name),
@@ -2037,6 +2041,7 @@ impl<'a> Parser<'a> {
             safety,
             id: DUMMY_NODE_ID,
             ty,
+            default,
             attrs,
             is_placeholder: false,
         })