Rollup merge of #90035 - SparrowLii:rfc2528, r=jackh726

implement rfc-2528 type_changing-struct-update

This PR implement rfc2528-type_changing-struct-update.
The main change process is as follows:
1. Move the processing part of `base_expr` into `check_expr_struct_fields` to avoid returning `remaining_fields` (a relatively complex hash table)
2. Before performing the type consistency check(`check_expr_has_type_or_error`), if the `type_changing_struct_update` feature is set, enter a different processing flow, otherwise keep the original flow
3. In the case of the same structure definition, check each field in `remaining_fields`. If the field in `base_expr` is not the suptype of the field in `adt_ty`, an error(`FeildMisMatch`) will be reported.

The MIR part does not need to be changed, because only the items contained in `remaining_fields` will be extracted from `base_expr` when MIR is generated. This means that fields with different types in `base_expr` will not be used
Updates #86618
cc `@nikomatsakis`

1 files changed, 33 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/language-features/type-changing-struct-update.md b/src/doc/unstable-book/src/language-features/type-changing-struct-update.md
new file mode 100644
index 00000000000..9909cf35b5b
--- /dev/null
+++ b/src/doc/unstable-book/src/language-features/type-changing-struct-update.md
@@ -0,0 +1,33 @@
+# `type_changing_struct_update`
+
+The tracking issue for this feature is: [#86555]
+
+[#86555]: https://github.com/rust-lang/rust/issues/86555
+
+------------------------
+
+This implements [RFC2528]. When turned on, you can create instances of the same struct
+that have different generic type or lifetime parameters.
+
+[RFC2528]: https://github.com/rust-lang/rfcs/blob/master/text/2528-type-changing-struct-update-syntax.md
+
+```rust
+#![allow(unused_variables, dead_code)]
+#![feature(type_changing_struct_update)]
+
+fn main () {
+    struct Foo<T, U> {
+        field1: T,
+        field2: U,
+    }
+
+    let base: Foo<String, i32> = Foo {
+        field1: String::from("hello"),
+        field2: 1234,
+    };
+    let updated: Foo<f64, i32> = Foo {
+        field1: 3.14,
+        ..base
+    };
+}
+```