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``

clippy_lints/src/utils/ast_utils.rs

@@ -107,6 +107,15 @@ pub fn eq_expr_opt(l: &Option<P<Expr>>, r: &Option<P<Expr>>) -> bool {
     both(l, r, |l, r| eq_expr(l, r))
 }
 
+pub fn eq_struct_rest(l: &StructRest, r: &StructRest) -> bool {
+    match (l, r) {
+        (StructRest::Base(lb), StructRest::Base(rb)) => eq_expr(lb, rb),
+        (StructRest::Rest(_), StructRest::Rest(_)) => true,
+        (StructRest::None, StructRest::None) => true,
+        _ => false,
+    }
+}
+
 pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
     use ExprKind::*;
     if !over(&l.attrs, &r.attrs, |l, r| eq_attr(l, r)) {
@@ -150,7 +159,7 @@ pub fn eq_expr(l: &Expr, r: &Expr) -> bool {
         (Path(lq, lp), Path(rq, rp)) => both(lq, rq, |l, r| eq_qself(l, r)) && eq_path(lp, rp),
         (MacCall(l), MacCall(r)) => eq_mac_call(l, r),
         (Struct(lp, lfs, lb), Struct(rp, rfs, rb)) => {
-            eq_path(lp, rp) && eq_expr_opt(lb, rb) && unordered_over(lfs, rfs, |l, r| eq_field(l, r))
+            eq_path(lp, rp) && eq_struct_rest(lb, rb) && unordered_over(lfs, rfs, |l, r| eq_field(l, r))
         },
         _ => false,
     }

tests/ui/crashes/ice-6250.stderr

@@ -1,3 +1,14 @@
+error[E0658]: destructuring assignments are unstable
+  --> $DIR/ice-6250.rs:12:25
+   |
+LL |         Some(reference) = cache.data.get(key) {
+   |         --------------- ^
+   |         |
+   |         cannot assign to this expression
+   |
+   = note: see issue #71126 <https://github.com/rust-lang/rust/issues/71126> for more information
+   = help: add `#![feature(destructuring_assignment)]` to the crate attributes to enable
+
 error[E0601]: `main` function not found in crate `ice_6250`
   --> $DIR/ice-6250.rs:4:1
    |
@@ -10,18 +21,22 @@ LL | |     }
 LL | | }
    | |_^ consider adding a `main` function to `$DIR/ice-6250.rs`
 
+error[E0308]: mismatched types
+  --> $DIR/ice-6250.rs:12:14
+   |
+LL |         Some(reference) = cache.data.get(key) {
+   |              ^^^^^^^^^
+   |              |
+   |              expected integer, found `&i32`
+   |              help: consider dereferencing the borrow: `*reference`
+
 error[E0308]: mismatched types
   --> $DIR/ice-6250.rs:12:9
    |
 LL |         Some(reference) = cache.data.get(key) {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
-   |
-help: you might have meant to use pattern matching
-   |
-LL |         let Some(reference) = cache.data.get(key) {
-   |         ^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0308, E0601.
+Some errors have detailed explanations: E0308, E0601, E0658.
 For more information about an error, try `rustc --explain E0308`.