Rollup merge of #78836 - fanzier:struct-and-slice-destructuring, r=petrochenkov

Implement destructuring assignment for structs and slices

This is the second step towards implementing destructuring assignment (RFC: rust-lang/rfcs#2909, tracking issue: #71126). This PR is the second part of #71156, which was split up to allow for easier review.

Note that the first PR (#78748) is not merged yet, so it is included as the first commit in this one. I thought this would allow the review to start earlier because I have some time this weekend to respond to reviews. If ``@petrochenkov`` prefers to wait until the first PR is merged, I totally understand, of course.

This PR implements destructuring assignment for (tuple) structs and slices. In order to do this, the following *parser change* was necessary: struct expressions are not required to have a base expression, i.e. `Struct { a: 1, .. }` becomes legal (in order to act like a struct pattern).

Unfortunately, this PR slightly regresses the diagnostics implemented in #77283. However, it is only a missing help message in `src/test/ui/issues/issue-77218.rs`. Other instances of this diagnostic are not affected. Since I don't exactly understand how this help message works and how to fix it yet, I was hoping it's OK to regress this temporarily and fix it in a follow-up PR.

Thanks to ``@varkor`` who helped with the implementation, particularly around the struct rest changes.

r? ``@petrochenkov``

compiler/rustc_hir/src/def.rs

@@ -484,4 +484,9 @@ impl<Id> Res<Id> {
     pub fn matches_ns(&self, ns: Namespace) -> bool {
         self.ns().map_or(true, |actual_ns| actual_ns == ns)
     }
+
+    /// Returns whether such a resolved path can occur in a tuple struct/variant pattern
+    pub fn expected_in_tuple_struct_pat(&self) -> bool {
+        matches!(self, Res::Def(DefKind::Ctor(_, CtorKind::Fn), _) | Res::SelfCtor(..))
+    }
 }