Auto merge of #97705 - compiler-errors:guide-inference, r=lcnr

Fix inference issues with unconstrained base expr in `type_changing_struct_update`

Use fresh infer vars to guide inference along in `type_changing_struct_update`.

Fixes #96878

src/test/ui/rfcs/rfc-2528-type-changing-struct-update/issue-96878.rs

@@ -0,0 +1,31 @@
+// check-pass
+
+#![feature(type_changing_struct_update)]
+#![allow(incomplete_features)]
+
+use std::borrow::Cow;
+use std::marker::PhantomData;
+
+#[derive(Default)]
+struct NonGeneric {
+    field1: usize,
+}
+
+#[derive(Default)]
+struct Generic<T, U> {
+    field1: T,
+    field2: U,
+}
+
+#[derive(Default)]
+struct MoreGeneric<'a, const N: usize> {
+    // If only `for<const N: usize> [u32; N]: Default`...
+    field1: PhantomData<[u32; N]>,
+    field2: Cow<'a, str>,
+}
+
+fn main() {
+    let default1 = NonGeneric { ..Default::default() };
+    let default2: Generic<i32, f32> = Generic { ..Default::default() };
+    let default3: MoreGeneric<'static, 12> = MoreGeneric { ..Default::default() };
+}