Auto merge of #129514 - estebank:default-field-values, r=compiler-errors

Introduce `default_field_values` feature

Initial implementation of `#[feature(default_field_values]`, proposed in https://github.com/rust-lang/rfcs/pull/3681.

We now parse const expressions after a `=` in a field definition, to specify a `struct` field default value.

We now allow `Struct { field, .. }` where there's no base after `..`.

`#[derive(Default)]` now uses the default value if present, continuing to use `Default::default()` if not.

```rust
#[derive(Debug)]
pub struct S;

#[derive(Debug, Default)]
pub struct Foo {
    pub bar: S = S,
    pub baz: i32 = 42 + 3,
}

fn main () {
    let x = Foo { .. };
    let y = Foo::default();
    let z = Foo { baz: 1, .. };

    assert_eq!(45, x.baz);
    assert_eq!(45, y.baz);
    assert_eq!(1, z.baz);
}
```

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 3a9e9b480ec..eeb83a85e59 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/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() {